The geology of the Flat Rock Oil Field, Upton County, Texas

Item Type text; Thesis-Reproduction (electronic)

Authors Samii, Cyrus, 1934-

Publisher The University of Arizona.

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Cyrus Samli

1959,

STATEMENT BY AUTHOR

This thesis has been submitted in partial fulfillment of re ­quirements for an advanced degree at the University of Arizona and is deposited in the University Library to be made available to bor­rowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in their judgment the proposed use of the material is in the interests of scholar­ship. In all other instances, however, permission must be obtained from the author.

SIGNED:__ _______

APPROVAL BY THESIS DIRECTOR

This thesis has been approved on the date shown below:

[LLARDD. Professor of Geology

THE GEOLOGY OF' THE. FLAT BOCK. OIL FIEI®?HPTOm COUNTYg TEXAJS. ' ■

' • by .

Cyrus Samii

; " : ABSTBACT

The Flat Rock Oil Field is located in the southern part of the

Midland Basin? about eight miles east of the. town of Ranking southeast

Upton. County,. Texas.

." ■ The discovery well; Cities Service Oil Company No. 1 Univer­

sity was,drilled ih 1051 and was completed in a dolomitic zone

in the Elleiiburger group of Upper Cambrian and Lower Ordovician age.

Later in the same yearsandstone horizons, of the Bpra b erry formation

of Lower Permian age were discovered to contain commercial oil.

Oil is trapped in the Ellenburger group by a combination of

folded and faulted structures... In the Bpraberry formation^ a simple

convex fold, with a n east-West trend, is responsible for oil accumula­

tion. The Bpraberry trap is bounded also to the east by a decrease in

permeability. :

Presently^ 11 wells produce in the field which to. date have

yielded a cumulative production of approximately one million barrels

Of "oil.; this amount of production approximately 53; percent has '

come from the 'Ellenburger dolomite and 47 percent from the Spraberry

. sandstone. The,#llOnburger pay horizon Contains about 310 proved

acres and the Spraberry pay horizon about 560 acres.

. MEJltimate primary recovery from the.Ellenburger payds esti- ..

mated as 5805 000 barrels; from the Spraberry^ as 1 070,000 barrels.

:v:The; Ellenbur ger' producing zone does not appear to be .amenable to - '

secondary recovery but secondary recovery from the Spraberry pay

. may bepossinie:-;:;;;;. :. " ' ; '4

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Method andJDate of Discovery »©« .. . © . © „ ©.. © / - 44 •Producing and Possible Producing Horizons . . . . . . . . . . . . . 48Geology of the Reservoir and Source of Reservoir ^Energy „ 49.Drilling; and .Completion Methods . . . . . . . . .©'... =. .,...'©.. ©. © . ,50Production History . © © © © «.;©©.... ©. © ©.. ©. ©. © © 51Characteristics of Oil and Gas ©■.......©. ©,.©. ©■*:........© 52.Future- Production.Forecast . . .....»... © © . . . . . . . . . . . . . . . . . . ©. • 55

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Core Analysis © @ . o . . 80Amount of ..Oil Per-Acre . . © ©. . . . © . . . . . . . . . . . . . . . ©. 60

- ' : Flstimation-of e ^ r y e s . . ' . . o . . ' . . . . . o ^66 ;

Secondary Recovery . 68

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V ' LIST OF PLATES. '

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- LISTOF-FIGIIEES ,: - '

Figure . ' ’ Page

1, :Index map showing-location of the.field » »■. © . 00. . - 2'

2. Relationship of Flat Rock Field to major structuralfeatures, of West Texas and .southeastern .New Mexico © : ■ 4

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41 Spraberry .cor e :ahalysls; © ©. ©©©©..'.©. ©. © . ,©-© © © © © . ©, ©.•« 61

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Table

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:Ellenbmger eore analysis . » « . 6 4

:in tbo d ijctio .n

V: The. Fiat Rock Oil Field is located■ eight miles due- east of

Rankin,, southeast Upton County Texas (Fig. 1). The field is readily

' accessible sineeTJ- S.» Highway 67 passes: through it ih such a way .

that five of the wells ih the field are on the south side of the highway,,

while all the Other wells ar e located on the north side. . The var ious

wells can be reached by lease roads which braach off Highway 67.

The low relief makes the lease roads easy to construct and maintain.

The neareSt railrbad i$. parallel to the highway- some 1/4 of a mile to

the south.

Physiographically? the subject area is located in a. lowland

separating the Edwards Plateau to the southeast from the High Plains

of northwest Texas which are found to the northwest. The Edwards

; Plateau i s a r egional highland composed of Cr etaceous r ocks having a

southeast regional dip. The High Plains are underlain by Tertiary rocks

with a regional eastward, dip. ' Rocks:of. Cretaceous age form the bed- •

rock of the area. They either outcrop on the surface or are covered by

a layer of soil only a few feet thick.

, The Flat Rock Oil Field lies in the Permian Basin of West

Texas and southeastern New Mexico. This basin, which partially over-

- laps the areas of the Edwards Plateau andThe pgh;, Plains phyeiographie

2

2 5 0 m i l e s

UPTONCOUNTY

F I o t R o c k

R a n k i nF i e l d

2 5 m i l e s

. INDEX MAP

Figure 1. —Index map showing location of the field.

provinces^ is divided into two structural basins and one platform: These

, . .are the Belaware Bahi%; ’ the: Midland Basing - :'wA the Central Basin F la t- ;

form (Fig. 2). The Flat Rock Field, is located in the southern part of

• the.Midland Basin. .

:The Flat Rock FieM waS discover ed :in 1951by well No, 1918

(Table 1 well data), Cities'Service Oil Company No. 1 University nAH,f5

, ■ which: was completed as 'a producing; well, in; the Ellenburger group of -

Upper.; Cambrian and Lower Ordovician age. Later in the same year?

the Spraberry pay sone was; discovered by well. No. I-996? Cities Service

: Cli Cpmpany No. 1 -University The Bllehburger discovery well

Was plugged back to the Spraberry formation in October" of 1951, due to ..

: water encroachment. The field now has 11 producing wells. Four of -

. these are producing from the 'Ellenburger horizon at a depth of abont

10? 750, and seven from the Spraberry pay zone at a depth of about

f eet : 't;, :: ,/. - : . ' y - ' . , ■

The average elevation of the area in which the Flat Rock Field

is found is about 2? 735: feet above sea level. Relief is low, although in

piaces interMittent streams have cut into the Cretaceous limestone bed­

rock. y • : \ ' ; .

Formations of the. Fredericksburg group of Lower;Cretaceous .

age are the only rocks outcropping in the area.

Climate and vegetation ar e those of a semi-arid r egion. NO

. permanent streams exist in the Flat Rock area. The radial pattern,Of-"

4

V

m

nUpton

O Z O N A A R C H

Figure 2, —Relationship of Flat Rock Field to major structural tea tures of West Texas and southeastern New M exico0

some intermittent streams -indicates a higher topographical position for

:the.subject area.. -' '

.. : Ac.knoTyledgments .

, ■ , - Acknowledgment, is gratefully made to. Br. Willard, D.' Pye of

the Department of Geology^ Univer sity of Arizona for invaluable help

during the pr eparation of this paper . The author also wishes to ac­

knowledge the-assistance of many of the Pan American Petroleum

Corporation personnel of the Midland."District,. Mr. Walter, !,. Ammon

and Mr. Daniel V. Yarbrough of that organization were especially gen­

erous With their' aid and consultation. - : -

Sincere appreciation^ too is expressed to the personnel of

Cities Service Dil Company and:Smmay-Midcontinent. Oil Company for

their kindness in allowing the writer the use of their records.

'STRATlGRAPHr

In the area, the oldest exposed rocks are formations of the

Fnedericksbiirg gronp; of Upper Cretaceous age„ The deepest well has

penetrated the Ellenburger group of Cambro-Ordovician age, Data on

these subsurface formations are derived en tire ly from wells, Plate II

gives the stratigraphic section in the Flat Rock Field. ■

Precambrian ' '

Precambrian rocks'have been exposed in three localities in

the region. In the Franklin Mountains, north of El Paso, Texas, the

' Precambrian.rocks ;inelnde; three units-i— (I) the- Red Bluff granite, (2) '

the Lanoria quartzite, and (3) the overlying red rhyolite porphyry.

The duartsite and the porphyry are; I, BOO and. 1, 500 feet thick respec-

” tively. In. the ¥an' Horn r egion, comprising parts of Culberson,

Hudspeth, and Jeff Davis .Counties, Texas, Precambrian units consist

• of the. Carrmo Mountain, metamorphbaed-elastics and. volcanics, the . :--

Allamoore limestone, and. the Hazel sandstone having a total thickness

,. of 11, 000 feet. In the Central Mineral or Llano region of Texas, the

Precambrian rocks include .the Packsaddle schist and the Valley Spring

.■ gneiss. " ' ' .; " .

. In the Midland BaiSinj, an angular unconformity separates the.

.Precambrian from the overlying Upper Cambrian rocks.

Rocks, of Pr ecambrian age are generally non-productive of oil

and gas.' and cannot be considered, as being of. potential oil or gas sig­

nificance in this region.

; ' ' : Paieoaoic " ' ' ; -

. ■ " , ' Cambrian System . - ■ _

Information derived from outside the area indicates that the - -

Power and Middle Cambrian formations are absent in the region. ■ Of

the Upper Cambrian rocks, only the ‘Wilberns formation and the Cambro-

; Ordovician Rllenburger group are present in the.Midland Basin.

.' Wilberns formation. --The..type locality of the Wilberns form a-.

' tion is WilbernS. Glen in Llano County, Texas, where the thickness of ■

the formation varies fr om a thin stratum to 2:20 feet.. The unit consists ■;

of limestone, dolomite, sandstone, and shale. The formation is divided

into four members which from the oldest to the youngest are; the Welge -

Sandstone, the Hogan Creek dolomite, the Point Peak shale, and the

Can Saba limestone. Glauconite is a common mineral in all the members.

'In; the -Flat Bock Field, no well is deep enough to encounter this forma- ‘ /

tion. . ' ' - t V. ; ■

: The Wilberns formation overlies the Precambrian rocks with; '

, -an angular raicofl£ormity# and ,it.4isc;onfor,mably underlies 'the Ellenburger

group. Mp major field has yet produeed any oil from the Wilberns for­

mation in West Texas, . •'

Ellenburger group. --The Cambrian portion of the Ellenburger

, group is disoussedunder the,Ordovician System. ' / " v'

Ordovician System '>

Ellenburger group. --The Ellenburger group was named in 1912

after .exposures in the Ellenburger Hills of southeastern San Saba County

in central Texas. It also includes a limestone series exposed in the

Central Mineral region of Texas. The Ellenburger is generally pres- ' :

'. ent throughout "West Texas and is a major oil producing horizon.

The group is primarily a fine!- to coarse-grained dolomite con­

taining a considerable amount of lithographic limestone. It is generally

■" white; tan;'- Or light brown in color. In yarious localities some chert is

■ present in the group, Some scattered sand grains may be observed in ,

. well cuttings. Fractures and vugs throughout the rock unit provide the

, porosity necessary for the reservoir rocks Secondary crystallization

'. and dolomitization are easily observed-in the. cores. Much of the frac- '

turing is vertical which allows extensive communication of fluids.

' In the Flat Rock Field; although the porosity of the Ellenburger

iS'loW; ’ the. effective porosity and permeability a re high enough for a

r eservoir rock. .It is generally believed (Galley, .1958) that the Ellen-

burger group had no source beds, and that the oil accumulated in the

reservoir unit by migration from probably younger source strata* The

group, is slightly fossiliferous containing several species: of: mollusks.

The thickness of the Ellenburger is not easily determined be­

cause only a few se lls in the region have been drilled through the unit

to the underlying Cambrian, or Pr.ecambrian rocks. ’ .Regional isopach

maps of the Ellenburger group Indicate a thickness of about 1, 400 feet

■ in the Flat Rock Field. ': This’ thickness is substantiated by well No..

857 . Phillips Petroleum Company No* 1 University "ZZ", in Reagan

County which is. about 10 miles southeast of the. Flat Rock Field. This

well penetrated to the Precambrian complex. The Ellenburger thickens

to the South from the Flat Rock area and thins to the north and north­

west. '.Its total thickness is about '50.0 f eet'in southeastern New Mexico.

The Ellenburger group lies disconforrnably on the Wilberns

formation below and is unconformably overlain by the Simpson group .

above. It has been suggested that there is a hiatus within the Ellen­

burger group. This break has been used to divide the group into upper

and lower Ellenburger units* Samples and residues from the two units

indicate that the upper part has more carbonates while the lower part

contains more chert and oolites.

The'upper E.llenbu.rger is of Lower Ordovician (Canadian) age.

The. -age of the. lower Ellenburger is .contr oversial and many geologists

believe it occupies a transitional position between the Cambrian and

-Ordovician periods*.:: It is. often designated, as. of ,6Oambro-Ordovician,r :

age,

Simpson group, —Outcrops of the Simpson group are present - .

in the Baylor Mountains area, Culberson County Texas. The Simpson .

group is subdivided from the oldest to the youngest as follows & The

•Joins formation, the,Oil Creek formation with the Connell.sand mem­

ber, the McLish formation with'the Wadell sand member, the Tulip '

Creek formation with the McKee sand member, and the Bromide for­

mation,. All the formation names have been adopted from the Oklahoma

section of the Simpson group.

The group in the region consists of 5 percent sandstone, 55

: percent shale, commonly green, and..about 40 percent carbonate rocks

mainly of a limestone character (Galley, 1958), The Joins formation

consists mostly of gray limestone and dolomite. The Oil Creek forma­

tion is mainly green shale,but a t the base consists of the .medium- to

coarse-graihedCohneli sandstone member. The McLish formation

consists mostly of: brown Or gray sandstone and green shale. The Tulip

Creek..and: the .Bromide formations are mostly interbedded light-colored

limestone and green shale with trac es. of sandstone and siltstone. Phos­

phate nodules have been reported in the Simpson group. The sandstone

members* as. reservoir rocks, are important oil and gas reservoirs m .

" ■ ■ / ' : : ' ' 11 the gr oup . ' ,

The thickness of the Simpson group in the Flat Rock'Field is

about '300 feeto. The group thins ip an easterly, .direction* toward the

Texas Pehinsula. ■ ■' " ; ' ■

The unit is believed to have been deposited in a broad sea in

-the.'regiOBu,' andrthe lithologie"cMraeteristics;are uniform over a vast ,

. 'area.. ' . ' . ; • . v; .. / ; .. :

.. ”,j n.* uneoniprunity' Separ ates the.- iSilenhurger and .the j^inipson

groups. Ther e is also a disconformity separating the Simpson below

from the overlying Montoya formation., ■ This' is evidenced by a faunal

hiatus as well as the presence of sand grains in the base of the Montoya.

These sand grains, are characteristically Simpson type. . VTrilobites and graptolites ar e the main fossils in the sediments.

In well cuttings, fragments of brachiopods have been observed.

■ - The gr oup is of Middle ©r dovician'age. ' • ■

The Simpson group is considered to be a good producing zone

:in. the.Permian Basin: but does hot produce in the Flat Rock Field.nor .

have any of the wells carried a show Of oil in this horizon.

Montoya formation. —The Montoya formation was named in

1909 from its outcrops in the Franklin Mountains. Although it is present

in most areas of the Permian .Basin? .' in the Flat Rock area the Sylvan

shale replaces it.

'Where the Montoya formation is ■present, it consists mostly of

carbonate rocks and chert* especially in the .upper two-thirds of its sec­

tion. 3h- southeastern Hew'Meiico? the Montoya formation is. more

dolomitiCj while to the southeast in Texas, it is mostly limestone. Re­

worked sands. and .pebbles derived .from Simpson .beds are present in a .' -

sandy and conglomeratic zone at the base of the Montoya formation.

The Montoya formation is bounded by disconformities below it

and above it. ./It iS'pf Bpperv.& ; . . . t ;■

Sylvan shale. --The type section of the Sylvan shale is in the

Arbuckle. Mountains of Oklahoma,. The Sylvan shale is present only

locally in the Midland Basin. Its equivalent in the 'Eastern Shelf area

is called the Stray shale. .

The formation consists of an o liv e -g r e en shale which is slightly

. calcareous and is dolomitic in part. The thickness, of the Sylvan shale

in the Flat Rock area is about 50 feet.

. Graptolites. a r e- abundant in the. formation. The unit either

inter tongues with, or slightly overlies the Montoya formation. It is of

Upper . Ordovician age.

. ;■ ' v .' - . ! : Silurian System

Busselman formation.--The type locality of the Fusselman

formation..is in the Franklin Mountains, north of 'El Paso^ Texas. The

; V , " ■ . 13formation is present over the entire Permian. Basin. . .

The Fusselman formation is a light-colored dolomite with some

limestone and chert. Some glauconite is present in the limestone por­

tion of the formation. Light-pink and. orange^ me.dium to coarse calcite

crystals are abundant throughout the formation. Some oolites also have

been observed at the base of the formation in certain areas. In the

: Flat .Rock. Fields the Fusselman .formation contains mostly limestone.

The, formation is 960 feet thick in the Franklin Mountains, hi

the Flat Rock Field, the average thickness is only 50 feet.

' A disconformity separates the Fusselman formation from the ,

underlying Ordovician beds. : The Fusselman formation is of Alexandrian

.'age, •' . ; . - ;, y :.

. Some shale beds are present at the top of the Fusselman for- .

mation. These are called, the ;’’.Upper Silurian Shale. ” Monograptus

vomerinus and other graptolites indicate that these shale beds are of

,Magaran.age. Thus the narae,”Hpper Silurian” is a. misnomer. The

total thickness , of these shale; beds Is about 40 f eet in the Flat Rock

area. ' ; ' .

■ ' : ;:; ;\ >■ 35eyonian..System .• - ', : ■

Rocks of Devonian age ar e distributed thr oughout the Permian

Basin. Fxcept for the Woodf ord shale^. these rocks have not been named

in the area. / ' " .

- ' - ! ■ 1 4section consists mostly of a.

gray calcar eous chert interbedded with a. buff to tan siliceous limestone.

White chert and white, crystalline limestone are also common, .These ;

beds ar e of Lower Devonian age. ;

This unnamed, Lower Devonian section has been, subdivided

into four zones’ through inso&ble .residue s tu # e s ., Bone .1 i s the youngest

; and is characterized by smooth cherts. Zone 2 residues consist of

white and light-colored, fossiliferous, granular or chalky chert. Fos­

sil molds, are common. Because of numerous stratigraphic pinchouts, •.

this is the most promising oil-bearing zone. In. Zone 3. dark-gray and.

, brown' smooth chert Is abundant. Zone 4 includes both siliceous shale :

and granular chert.

. The thickness of.the unnamedDevonian section is about 560 ' : :

feet in the vSubjeet;area,' The.unit has: its thickest section in' Crane

County. . ' V "

. - The ' Lo wer Devonian unnam ed sediments, disconf ormably over -

lie the Silurian beds and are separated by a disconformity from the

Woodford shale above. . ..

• The. Lower-'Devonian is an oil-producing unit in many parts of .

’ the Permian Basin,

Woodford shale. .—The Woodford shale is present throughout

the Permian. Basin. It is predominantly a black and very dark-brown

. ' • . : ' ■ . ■ -15

shale with some chert. Occasionally some argillaceous^ sandy, light-

colored limestone is present. Glauconite has been observed in samples.-'

The Woodford shale is a spore-bearing formation especially in Crane,

Winkler, andMctor.Gomiies.: 'Most of the spores are.Sporangite

huronenSis. '• Conodonts are also common.;;

1 The.thickness of the unit in'the. Flat;Bock .'Field is about 60

feet.: It thickens westward into Winkler County, reaching a maximum

thickness of 81(3: feet. ; ■ ' ' . /

' ■ Paleontological studies show that the upper- part of the Wood­

ford shale is/of Lower Missidsippian age while the lower part is of

Upper Devonian, age. The formation occupies a transitional position

-between the tW periods^: ,v :

The Woodford shale :is not a reservoir rock but probably was

a- source bed, ;and. .furnished the oil trapped'ih the underlying Lower

BevWan rocks., : : t: . v . - x ; >

' ..... " Mississippian System

; Woodford shale. - - The Woodford shale, the upper part of which

■is Mississippian in age, has, already been discussed under the. Devonian

System. :■ : ^

Barnett, shale.'—The type locality: of the Barnett shale is at

Barnett Spring, San Saba County, Texas, where the average thickness

of the formation is 40 feet. This shale is present in most areas in West

Texas. ' : .

, The formation consists mainly 'of a. calcareous and siliceous^

black or dark-brown shale. Northward the formation becomes increas­

ingly calcar eouss while southward the litiiology is shale,

- in well'NOo 1956 in the Elat Rock Fieid^; which is north of the

fault and on its down-dropped side^ the- Barnett shale is about 380 feet .

thick. In the other wells in'the field^ so far as is known5- the Barnett '

shale is only about 50 feet thick* pr esumably because of post=deposi~

tional erosion on the upthrown side of the fault.

, The Barnett, shale disconfdrmably overlies the "Woodford, shale

and is overlain with an angular unconformity by r ocks of the Fennsyl- ■

.vanian age. . The post-Mississippian erosional period and the resulting

iincoiiformity is of major importance. This is the. time .equivalent to

the emergence and erosion of. the Central Basin .Platform.

The pr esence: of the goniatite^ Glyphiocerasg and the bivalve^

Caneyllaa and also its stratigraphie position^ indicate that the formation

is of'Meraniecian age.

• Tn well No. 1956 a calcareous shale interval lies between the

typically Barnett shale and the unconformity. This may, be- of Chesteran

IT

age. . . ' . . ,

Pennsylvanian System

• Atoka formation. --Lower PetoisylvaMaii rocks .are absent in

the. Elat.Rock Field; the Middle Pennsylvanian Atoka formation is found

in most parts-of the Midland Basin but is absent in the field,

v i gtrawn series, -•“‘The gtrawn series of'Middle .Pennsylvanian

age'is not' subdivided. It is an important reef-building unit in the region

with large, amounts of limestone,, Bahdstone, .red shale, conglomerate,

and coal likewise. are present, .■ v . '

■ In the Flat Rock Field,, the Strawn series is about 175 feet

thick,: In places .where the series attains a. consider able thickness^

. Jones' (1953) believes that this does not indicate a great depth of water

but merely continuous subsidence^; beeause,. of the occurrence of con­

glomerate, coal, and red shale in the section,

: . .. The Strawn series overlies. Mississippian beds , with an angular

unconformity, and is disconformably overlain by Permian, beds,

: ErOmiSusuliBids such as: Fusulina found in the Strawn series,

the age of this unit has been determined as Besmoinesian (Middle

Pennsylvanian). \ . '

• Canyoh and Cisco series, —The Canyon and Cisco 'series of

Upper Pennsylvanian age are absent in the Flat. Rock Field; however.

" : ■ ' ■ ' . : ' - ■ ■ : ' 18 '

they ar e present in most-parts of the Midland Basin. '

- _ : ; -.. 'Permian.System v;::. , - . // ^

Wolfcamp series. --The Wolfcamp series is present through-

out West Texas and is named from the'Wolfcamp Hiils? in the Glass

Mountains of Texas where the matt consists of mostly limestone and

■shale. ■ ' ' ' ' ' ' ' 'V ^ - ; - ' > - ' -

On the Central Basin Platform and the Diablo Platform^ the

Series consists of carbonate rocks which are mostly limestone and

shale-. Around the edges7- of. the Central Basin. Platforrns there are ;

some Wolf Camp reefs which are of major economic importance. Thin

beds of limestone in association with dark shales and other clastic rocks

predominate in the Wolf camp: series in the deposits deeper in the basin.

The light-gray to' brown. - fine-gr ained.Dean sandstone is a for­

mation in the middle of the upper' Wolfcamp series. Its occurrence is

restricted to the Midland: Basin. : ' .

' The. Wolfcamp series is over iOs 000 feet thick in the Delaware

, Basi% : While on the Central' Basin .Platform it is 'less tba.n l , 000 feet - ■ - -

thick. In. the Flat Rock Field,., the thickness of the series is about 2, 200 ,

feet. : , - ' . - '.■- ; J- ' ' ' ' ' ' . ' ' ■ . ' '

The Wolfcamp series disconformably overlies the Pennsylvanian

beds and is. conformably overlain by the Leonard series.

Oil commonly is found in the limestone of the series. The

; - " . ' ■ ' ' i s

Bean sandstone is productive in some places. The traps are either

anticlinal'structnre or stratigraphiCo

Age is given by the "Wolfcampian fusilinids. such as Schwagerina0

• Bpraberry formation. --TheBpraberry: formation: was named in-

1911 from its type locality in the Spraberry field in eastern Bawson

CountyTexas. The .Spraberry formation is present only in the-Mid*-

land Basin, although .equivalent formations ar e present in the whole

Permian Basin, '

The formation is primarily a grayish, silt-size sandstone con­

taining some calcareous material. The"Spraberry, formation in its type

Section is .divided; into two sandstone members, by a. member of brown. .

limestone with some.calcareous.'shale. In the pay section, the sand­

stone is. less calcar eous and. shaly. Some pyrite is pr esent,

m the Flat. Rock Field the thickness. of the Spraberry formation

is about 930: feet. Well cuttings, consist of brown dolomitic sand and

pale-green shale. - The., formation is divided into'an upper and lower

sandstone member, although no limestone.or::shale member is present

between, them ,. l i .the field,. the pay; section occurs in the lower sand­

stone member of the formation and is from 29 to SB feet thick. The

reservoir rock‘has porosity ranging from .1. 6 percent to 17.2 percent. .

The interstitial permeability is very low. The permeability ranges

from 0.0 to 5.0 millidarcies, although 5.0 millidarcies is an excep­

tionally high value. However, owing to the development of a system

, of yertical fractures, in the pay section, commercial oil .production from

- : the formation has been possible.

The Spraberre formation in the area ia conformable with both

the Volf camp series below and the Clear. Fork group above. The for='

mation is lower Leonardiati, in age.

The Spraberry formation is one of the important oil-producing

\ sectlons. of V ■ ■ ■

■Clear Fork group, —The: group was named from the Clear Fork

• branch of the Brazos Biverb in the Delaware Basin# the upper part of

the Bone Spring limestone is equivalent to the Clear.- Fork group.

The unit primarily consists of shale, sandstone, and dolomite.

. . In the Flat Bock area the litholbgy is mostly'sandy, calcar eous, and

dolomitic shale with a clark-brown.. color . . ■ /

■ The thickness'.of the. Clear Fork group in the; Flat-Bock ar ea

is approximately 1, 450 feet. - -

The unit is conformable with both the Spraberry formation be­

low and the San Andres group above. It is of upper Leonardian age.

San Andres,group. —The type locality of the San;Andres group

is. the San Andres Mountains.of Hew Mexico. . The group, is present in

the Midland Basin, on the Central Basin Platform,. and in the Eastern

• Shelf area. The Cherry Canyon and/the Brushy Canyon formations in

the Delaware Basin are equivalents of the San .Andres. The.San Andres

group- in tbeSastern Shelf area aod the Central Basin. Platform has two

' basal memberB; ealled the San Angelo and the Glorieta respectively. '■

. The group has gray or buff,, dolomitic dense beds at the base?

but. it is mostly limestone in the upper part. Northward a. facies change

in the San Andres group introduces precipitated dolomite and evaporites.

- such as anhydrite and salt, M the $iat Bock 'Field, the group contains

dolomite, limestone, and caicarebus and shaly sandstone. ■' '

M the Flat Rock area the San Andres group is about 1,200 feet

.thick: / ' ■ : / : ’ ;

The Sari Andres group conformably overlies the. Leonard series

and it is: also conformably overlain by the Whitehorse group. It is of

lower/Guadalupian: age.

; Whitehorse:group. --The name 'Whitehorse" is restricted to

the-Eastern Shelf area, the Midland. Basin, and the, Central Basin Plat­

form. In, the marginal reef area, of the.ZDelaware Basin, Gapitan is the

equivalent name, while in the Siela#are:: Basin, the name .Bell Canyon is

used for the upper .Guadalupian rockS.

The Whitehorse group consists Of five formations which from

the oldest to the youngest are: The Grayburg formation, the Queen for­

mation, , the Seven..Rivers .formation, the' Yates sandstone, - and the

Tartsill. formation. -' The. Grayburg formation was named in lMQ from

the Grayburg field in New Mexico. It mainly consists of a pink or gray.

dolomite::# f i tlio Flat Bock Fields the lith-

ology of the Grayburg formation is mainly dolomitic anhydrite. The'

Queen formation :is a sandstone which was named from its outcrop some

30. miles: southwest of Carlsbad, New. Mexico. In the Flat Rock Field,

the .Queen sandstone is anhydritic. , The. Seven River s formation con­

sists of mostly anhydrite^. dolomite,; sandstone, and shale. In the Flat

-BOck Field# salt is predominant .in this formation. The Yates sandstone

was named from the Yates field in Pec os. County, Texas, The lithology

is mostly gray sandstone interbedded with dolomite. Frosted'quartz

•grains ahd'red shale are common, In the Flat Rock area, the formation

is mainly sandstone, ■ The Tansill formation is a dolomite with a few .

beds of sandstone and siltstone in the .Flat Bock area. The dolomite in

the formation is a.facies change, Elsewhere anhydrite and salt are ■

present instead, \ ;.

; • ■ hi the Flat Bock.area, the total thickness of the "Whitehorse

group is about 1# 700 feet,. The Whitehorse'group rests conformably

' upon: the; San:'Andr:es: group? but it u n # r l# s the .Ochoa series, discon-. .

' : f o r m a b l y , v

. . The- 'Whitehorse group is of upper Guadalupiau age.

. ..' Galley (1058). has' indicated that about half of the oil found so

far in.the Permian Basin comes, from the Guadalupe series, although

the volume of the rocks does not exceed ST# 000 cubic miles. This is

in comparison to a total of 190,000 cubic miles for the volume of the

r ;. ■ . ; , 23

. ■ :P9l@ozoiG sectioii itt the, Permian Basim: Becamse of its shallow depths

Permian series. i3 gr eatly responsible for early oil discoveries:

aM later development of oil fields in West'Texas.' Accumulation of oil .

in the Guadalupe carbonate rocks and sandstone occurs, in both struc- , -

tural and stratigrapMc traps, Matty reefs, one of which forms El ’

■ Capitan Mountain, are of Guadalupian age, and those, strata indicate a

considerable facies, change in the region,

Ochoa series^ —The Ochoa series was. named after, exposures

. near, the Ochoa post, office in Lea Oo%m% Hew Mexico, It is divided

. into three formations whiph in ascending order, aret The Salado forma- ,

tion, the Rustler formation, and theDewey Lake formation.. All three

formations are present in'the Midland Basin, The Salado formation was

named from a. well near. Salado W ash in -Loving County. The formation

is composed mostly of salt and anhydrite with, smaller amounts of dolo-

- mite and .magnesite. Bylvite is mined from the Salado formation in Eddy

.; County - Mew Mexico, The. Rustler formationwas named:from the Rustler

M ils in IMlbersoh.0 ' Texas.. The formation is. a goodmarker on _

. - the electric. logs.. . The unit, is primarily a: white to gray dolomite and :

anhydrite. ,Borne sandy beds and conglomerates' are present at the base

of the formation. In the: Flat Rock Field, •'the. Rustler formation is mostly

anhydrite.' The Dewey Lake formation was named from .Dewey. Lake in

. Glasscock County, Texas, where it has a thickness of 280 feet. It is

primarily a red, .fine sandstone and silt associated with anhydrite. In

the Flat Bock-Field, the Dewey Lake formation is a shaly sandstone.

: ' / :In Rock ,are%' the thickness of the Ochoa series is '

abont 1,.058, feet., " . , ;

. The Ochoa series disconfdrmably overlies the 'Whitehorse

group; and it is overlain with an angular unconformity by Upper TriaSsic

rocks.

: : - Mesojraic . ■ , ' -

Since the •Mesozoic and Cenozoic systems, are not considered

to be of petroliferous importance, they will be dealt with only briefly.

Triassic System . ,

Lower Triassic rocks are absent from V est Texas.

’• Bocks of .Upper Triassic age are present as the Bockum group

named from .Dickens 'County,. Texas. • The. gr oup has been divided into

three formations which are, from the oldest to the youngest? The

Teeovas formation, the .Santa Rosa, formation, and the Chinle forma­

tion. The Teeovas and the Chinle formations ar e absent from the

, Belaware S a s in ., -- The' Tecoyas formation alsd is, absent from the Flat

Bock Field. ■ The Santa- Bosa f ormation is composed mainly of sandstone

and conglomerate beds. The sandstone is arkosic and gray in color with

a calcareous cement. The pebbles are subrounded chert, quartzite.

and limestone. In'the ..Flat Rock ar ea, the Santa Rosa formation is

mostly.composed of sandstone. The Chinle formation is a red shale.

Minor amounts of micaceons sandstone and limestone also are present,.

/ \ , .Theflldclpm: group is believed to he-a continenW deposit. Pet-m

' ’rified. wodd is common throughout the group, • Locally lignite is present

in the unit. Dinosaurs, amphibians (Stegocephalia), and fish are the

main vertebrates. '

In the Flat Rock Field, the Dockum group is about 725 feet

thick. • .. : / v - \

.. . The Upper Triassic section, overlies the. Ochoa, series of Per­

mian age with an mguZar linconformityand it is :overlain With an angular

unconform#y by\Lower Cretaceous beds. = .: ,

• . \ . Jurassic .System' ■ • . ’

The.Jurassic system is totally absent,from the. Permian Basin

of West Texas,

. Cretaceous System ,

The Lower Creta.ceous rocksvincludeg from the base to the top,.

. the following units in West Texas: ' The Trinity group, the Fredericks­

burg' group, and the Washita- group, / ' ,

26

Trinity group. --The. Trinity group., from, the oMest.to the

: youngest^ is c l i ^ the' Tracis peak formation (the^inosaur‘ sand)

and the Glen Rose formation. The Travis Peak formation is a sandstone

with some sandy clay 'and conglomerate.. The Glen Rose formation is a

hard, thin- to medium-bedded limestone- ' ■

•. . : In the Flat. Rock Field, the Trinity-group consists mainly of ■

sattdstohe.: ■ The thickness: of the grOnp,in the field, is about, 100 feet. - .

The Trinity group, oyer lies 'Rpper.Triassic beds with. an. angular.

unconformity, and is conformably overlain by the Fredericksburg group.

.Fredericksburg gfoupo--The-Fredericksburg group is divided

Into: four. formatiOnS/whiCh are from the oldest to the youngest: The . , .

'Walnut formation,' the COmanchefPeak formation, the Edwards forma­

tion, and the Kiamichi formation. The Walnut formation consists of

limestone and clay, and is a neritic facies of the. group. Shells of

. Exogyra. texana: and Gryphea: marcoui are abundant in this formation.

The Gomancke. Peak. fOrmation is a cherty limestone. . Its thickness

never exceeds 156 feet. It is gradational into the Walnut formation be- ;

low and the-Edwards formation above. The formation has been used for

cement and road metal. The Edwards formation consists of sandstone,

limestone, raarly-Mmestone, and reef limestone. The sandstone parts .

' .exhibit, crossbeddihg. ■ SiliCified woodis present in the formation. The

Kiamichi formation in the north is composed of clay and shale while

so^thwardv i't ehanges iBto anhydritic limestone-. -

The bedrock outcrops in. the Flat Rock area ..consist of forma- .

■ tions of the Fredericksburg group. An-exposure of the group lies about

4-1/2 mii.es. east of Rankin^ • along the. north side of Highway 67. . This,

outcrop is about 75 yards ..long and-exposes between two and six feet of

the section* The outcrop, is composed of a buff-tan^ fine crystalline

limestone which has1 some bigger. crystals, of calcite.- : Yellowish- specks ■

are abundant on the surface. • Fossil fragments may be present. The

weathered- surface is tan, mottled brown and black. No indication of.

structure was in evidence.. ■ The bedding planes.are found to be nearly

horizontal,: although.regional, mapping indicates that these -Cretaceous

beds must have a regional dip of about 50 feet per mile toward the south-

east. : ■ ' . ■ -

. - Another exposure,occurs in a railroad cut on the south Side of

the highways about 3-1/2 miles from Rankin. This outcrop has more

or-less'the same characteristics.',; No other outcrops were observed in

..the adjacent areas. : ' -

The thickness of the: Fredericksburg group in the Flat Rock

F ield lsabout 106 feet. - ,

' The' Fredericksburg group conformably overlies the Trinity .

group, t .. : - / ;: - . -

■ - Washita group. --The Washita group is the uppermost unit of

the .Lower :€r etaeeous section. Where pr esent, it is subdivided from

the. oldest to .the. youngest into the Georgetown formation^ the Del Rio ■

formation^ and the. Buda. formation. The -Georgetown formation is a

soft to dense limestone^ the Del Rio formation consists of shale,, and

the Buda formation is a light*- color ed, dense limestone. The -Washita '

group is absent through erosion from the ,Flat Bock Field. .

" .In most parts, of the Midland Basin, the Washita group uncon- '

formably over lies the Fredericksburg group and is unconformably over-

■ lain by Cenozoic beds.

Dpper.--Cretaceous rocks are r epr esented by the Gulf series,

but they are entirely absent, in the- Flat Bock area.

- Except for Quaternary sediments, rocks of Pliocene age are

the' Only r epr esentations of the/Cenozoic system in the Midland Basin .

and the Central Basin. Platform areas. The Pliocene sediments, in

the .High'Plains of southeastern Mew. Mexico and West Texas? ar e

divided into the underlying Couch formation and the overlying Bridwell

formation. .These sediments were deposited as an alluvial apron from

the .high mountains of New Mexico. The Couch is a gray, calcareous

sand which is overlain'by the brown sediments of the- Bridwell forma- '

tion. The Pliocene rocks, apparently are absent in the Flat Bock area.

The Quaternary sediments are only a few feet thick in the Flat

, Bock ar ea^; but may have cpMSlder able thickness, els ewher e in the Mid­

land Basin where present. The .Pleistocene rocks are divided from the

■base upwatd- t o formations, the Tuie formation, and the

Tahoka formation.

' BeCent deposits #e: mainly aeolitonan ; . v

■ "Where Cenozoic sediments are present,, as in the Delaware

■ Basin* the; physicai nature of these 'Cenozoic fills is -such as to pr esent

many technical difficulties to seismic exploration.

■ ' : , v ; : sm ucT U E E .

: Most of . the traps in the Miolland.Basin ar e combinations of

■ folded and faulted structures; however; the faults contribute little to

the traps. Many folds trend in northwest-southeast directions. Faults

trend, mainly north-south or: east-west., - A regional fault, part of which

passes through the Flat Rock Field^- iSzpresent in the Midland Basin.

Several fields are located bn the upthrown side of this fault and have

domal structures. Nearly all structures in the region were formed

during two periods^ of movement. One occurred at the end of Chester an

time' apd continued throughout,the post-Mississippian erosional period;

the other , occurred at the end of Besmoinesian time and lasted into

. Wolfcampian time. . : " • ■ -■

Structur e, of the Flat Rock Field

Ellenburger Structure

, The regional dip of the Ellehburger group in the area does, not

exceed a-few degrees, toward the west- and northwest. This is of similar

magnitude to the dips in . other parts , of the Midland Basin; however, the

dips, oh the Central Basin Blatform are much steeper.

' In the Flat Bock Field the accumulation of oil in the Ellenburger

'..group'is the result of KW fold and. a,faultr .The/fold is

asymmetrical and is'arcuate,' being .concave northward (Fig. B)., It is

broken along the north Side by .a. normal fault trending in an almost east-

west direction. The vertical displacement of the fault is about 400 feet

with ah unknown horizontal .displacement. The trap occurs on the up-

thrown, side, of the .fault. The. total closure of the combination of the •

..fold and the Fault is,154 feet.; - \ / •

Well Ho0 195©.indicates an. .abnbrmally gr eat thickness of

' Mississippian: sediments in comparison with that in the. other wells in

the-field. This thick Mississippian section as.contrasted with the much

thinner sections in all othen wells in. the Flat Rock Field can best be

justified by differ ential uplift and erosion rather .than, by ■conditions of ,:

sedimentation. The low structural position of the top. of the 'Ellenburger

in well No. 195© can be ezplained by a fault which passes'between well

.No. 1956 and all other wells, although the exact position and direction

are not confirmed. Even though there is as yet no proof of the fault,

there i s neither any proof of any abrupt increase in dip to account for

the structurally low position of the Ellenburger in well No. 1956. Spe-

: Cificaliy| . correlation' of the-logsi of Well No.. . 1956 and. of well No0 2233. ..

shows that in the latter well the section between the bottom of the Strawn

series and the top of the "Woodford shale is about 435. feet shorter than

that in the former well. This is also true in correlation of well No.

32

Un i v e r s i t y Lonjds S u r v e y

Figure 3. --Structural contour map on top of the Ellenburger group BY

C YR US S A M H

1956 with all the other wells in :the .Flat Rock Field* although the dis­

crepancy in the thickness of the Mississippian system may vary a. little

from well to wello Neither the thicker nor thinner sections are due to

repetition, or omission of strata. The' logs of the.wells give no indica­

tion of having mtersected any faults. As .is shown on ..the cross section •

. (FI, Hlk the east-west fault is.: believed to. be very steep or almost

vertical. ■ ' . ' ; . ' "

Seismic information corroborates the presence of a regional

fault with several hundred feet of vertical displacement trending in an

almost east-west direction. This regional fault cuts the pre-Des- '

moinesiM bedsralong the north side of the Flat Eock structure,,

\ : It is believed that :the fault started at the end: of Mississippian

time, and was active: throughout the erosional period of post-Mississip-

pian and Early Pennsylvanian times. The lesser effect of erosion on

the north side of the structure is due to its lower topographical position,

since that area Is on the downthrown side of the fault,

- ; ' .V Spraberry Structure

: y :. .. A structural'in% '(Fig. 4| drawn on the top of the'Bpraberry.

formation indicates, the presence of a simple anticlinal trap. No fault

structure is. evidenced in association with this fold. This fold is greater

in areal extent than that found at the Fllenburger horizon and the strata

. have gentler dips.. The. fold trends in an east-west direction with an

34

o l UFLAT ROCK

FIELD

B L K 3Un i ve r s i t y Lonjds S u r v e y

BLKo n e m i l e

S p r o b e r r y P r o d u c e r g E l l e n b u r g e r P r o d u c e r

Figure 4 --Structural contour map on top of the Spraberry formation BY

CYR US S A M M50 feet

. . V . ' ■ ; - ■ . . 3.5 .

almost straiglit axis. Amount of closure is 114 feet;

;' A decrease in permeability to the east further limits the trap

in that dir ectioho

::Eigure;5 is the structural boutour. map d ra w on the top of the .

•'pay;section, of tiae.Spraberry formation.■ and is very similar to Figure

4. ' ' ; . ' ; ; ■

Structural and'Strafigraphic,Evaluation : ; of the Flat Rock Field ; ■ \

. ; The1 marine history of #0. F began. # th tipper Cambrian;

seas flooding the .area.: From, that time until the close of the Late Mis-

sisaippian^ with minor br eaks at the end of Canadian and Cincinnatian

times?. sedimentation was largely cohtiriuous with carbonate rocks pre- .

dominant. -. Deposition of Mississippian limestone and shale was ended

by the post-MissiSSippian period of emergence and erosion.

. ■ After the deposition.of th e MissisSippian sedimehtsj through-;,

out the. region there was a long period of nondeposition and erosion which

produced, a pronounced unconformity between the. Mississippian and

' Pennsylvanian systems. After the Lower Pennsylvanian and prior to

the DeSmomesian epochy again a. period of uplifting and -erosion existed. ■

These .periods of emergence and erosion are responsible-tor; the partial .

absence, of Late Mississippian and Early Pennsylvanian rocks, both in

the local ar ea and in the r egion. - .

36

z l z

u l u

FLAT ROCK

' J. |r">

-417#

Is Su r ve y

S p r o b e r r y P r o d u c e r §> E H e n b u r g e r P r o d u c e r

Figure 5. --Structural contour map on top of the pay section of the Spraberry formation.

Cc I. - 50 feet

BY.

CYR US S A M I l

in Barly f enngylvmian tlme^ the.d'entral Baam Platform began

to form. The Permian period, is characterized by marine basinal de­

posits. - .Reef-buildiiig around the'Delaware and the.Midland BasinS.con­

tinued, through Late, Pennsylvanian and Permian times.

.The Triassic system is nepresented by continental deposits?

while the Jurassic was a period of nondeposition. • The 'Cretaceous sea$

covered mpist parts of the.region.: lh 'these seas limestone and sand- '

stone'beds were deposited.;, ' ■ i C:-. :'ci,,

■ The Cenozoic is represented by veneers of continental deposits.

The movement which first formed the Ellenburger structure in

the Flat Rock area probably commenced in Middle Ordovician time?

since the; Simpson group (Middle Ordovician) is, thicker on the. flank of ,

:: the: Ellenburger' structure (Fig.; 6).; Similar reiatibnships. 'indicate that .

' in Silurian time there was a possible reoccurrence of movements on

the structure (Fig. 1J. ; ■

It is believed that the fault in the Ellenburger structure was

active throughout the period of erosion during Late Mississippian times.

Rejuvenation‘of the earlier structure occurred, at the' post-

:Mississippian pre-Desmoinesian period of emergence and erosion.

Figures S through '9. Show variations in thickness, of section

for various intervals of timet Figur e 10 shows the total thickness

variation from the top of the Ellenburger group to the top of the.Strewn

series. There is a pronounced thickening off the structure.

38

oFLAT ROCK

• i

411

U n i y r s i t y Lon|ds S u r v e y

Figure 6. --Isopach map of interval between top of the Ellenburger group and top of the Sylvan shale. BY

C YR USC. I. = 10 feet

39

uFLAT ROCK

I t i

Universi ty Londs Sur vey

S p r o b e r r y P r o d u c e r

Figure 7. --Isopach map of interval between top of the Sylvan shale and top of the Silurian rocks. BY:

CYRUS S A M I IC.I. - 10 feet

40

xl z

FLAT ROCK

B L K 3Un i v e r s i t y L o n d s S u r v e y

o n e m i l e

• S p r o b e r r y P r o d u c e r § E H e n b u r g e r P r o d u c e r

Figure 8. --Isopach map of interval between top of the Silurian rocks and top of the Devonian cherty limestone.

BY

CYRUS S A M I IC. I =10 feet

41

oFLAT ROCK

U S

Un i v e r s i t y Lonjds S ur ve y

« S p r o b e r r y P r o d u c e r $ E l l e n b u r g e r P r c d u c e r

Figure 9. --Isopach map of interval between top of the Devonian cherty limestone and top of the Strawn ser ies.

BY:

CYRUS S A MI IC.I. = 50 feet

42

1----

FLAT ROCK

1700

Li*?

Uni vt r t i f y Lonjds S u r v e y

S p r o b e r ry P r o d u c e r P r o d u c e r

Figure 10. --Isopach map of interval between top of the Ellenburger group and top of the Strawn ser ies. BY:

CYRUS SA M IIC. I. * 50 feet

.:y' ■„v:- ■ : , ,, :: - ;

. The: present - structttre of the Spraberry formation Is the. result

. 'of: initial dips:, cfeveloped on the .ancestral Mat Rock sir uc tare at the time

of. deposition; of the JLeomsrd series. Later; the fold was accentuated by

; a series of movements with the greatest one probably oecurring between

Guadalupian md#cho^n time. . ...

. . ; .,Mo structural, changes .are in evidence sinceRefmian time •

other than, r egional uplift and perhaps some r egional tilting.

FLAT ROCK.FIELD:

. : ■ Mtroctoctim,

The Flat Rock .Field lies in sections 2%' 30, 31, and 32 of

Block 3, University Lands. Survey, in southeastern Upton County,

T exas,1 A.total of: li: wellS: have been:#Rled in t her f i e l d^^ ' ^^^^^ .!

four wells produce from the'Ellenburger group and seye# wells pro- -

duce from the Spraberry formation. The field is defined on the north

by Well JSTOo 2889, Cities Service Oil Company No, 1 University "AP” ;

on the southeast by well No„ 961. (Reagan County), Cities Service Oil

Company MOc 1 University ’’AX1; on the southwest by well No. 2689,

Sunray-Midcontinent Oil Company So. 1. University ’’SUB”. The wells -

above were dry and abandoned at total depths of 7, 440, 7, 400, and

%322\feet respectively'. - :: v tA ;

■ ■ ■ Information on the wells, such as the serial number, the name,

. the total depth, the date of completion, and the producing zone of each

.well in the Flat Rock Field and adjacent areas, is given in Table 1. .

, Method and Date of Discovery ' . -'

Subsurface geologic mapping, based upon earlier dry holes

drilled hi the general region, indicated the presence of a favorable

TABLE 1 WELL DATA

Serial numbers and notes (Upton County)

Well Names Totaldepths(feet)

Datescompleted

Ellen-burger

Spra-berry

Initialpotential(BOPD)

Acid(gallons)

Sand-fracturingfluid(gallons)

Hydro-fracturingfluid(gallons)

1918oF*

Cities Service Oil Co No 1 University "AH”

11,281 1/23/5110/5/51

PB

XX

2, 044F 241+14 BOWD

7501,000

1956 o Sunray-Mid­continent Oil Corp. and Slick Urschel Oil Co. No 1 University ”A”

11,740 8/6/51 1951 PB

XX

-

2 000

1996oF*

Cities Service Oil Co. No. 1 University ”AM”

11,097 7/21/51 1951 PB X 103F 4, 500

2233+F

Cities Service Oil Co. No 2 University "AH”

10,747 2/11/52 X 313F

2689dryhole

Sunray-Mid­continent Oil Co. No. 1 University "SUB”

7, 322 8/13/52 P&A

i

TABLE 1— continued

Serial numbers and notes (Upton County)

Well Names Total..depths(feet)

Datescompleted

W en -burger

Spra-berry

Initialpotential(BOPD)

Acid(gallons)

Sand-fracturingfluid(gallons)

Hydro-fracturingfluid(gallons)

2708 oF Cities Service Oil Co. No 3 University "AH"

10, 838 8/27/52

2/28/55PB

X

X

128+78% water F 345+9% water F

10, 000

2889dryhole

Cities Service Oil Co. No. 1 University "AP"

7# 440 7/31/52 P&A

2922 + Cities Service Oil Co. No. 1 University "AU"

10,760 2/1/54 X. 465F

3011 + Cities Service Oil Co.. No 1 University "AQ"

10,751 5/20/53 X 298F 2,000

3095 + Cities Service Oil Co. No. 2 University "AQ"

10, 744 9/6/53 X 545F 2,000 •

3204 oF Cities Service Oil Co. No. 3 University "AQ"

11,086 7/11/54 1954 PB X 725F 4, 000

TABLE 1-—continued

Serial numbers and notes (Upton County)

Well Names Totaldepths(feet)

Datescompleted

Ellen­burger

Spra­berry

Initialpotential(BOPD)

Acid(gallons)

Sand-fracturingfluid(gallons)

Hydro-fracturingfluid(gallons)

3246 oF Cities Service Oil Co. NOc 2 University "AU"

10, 923 10/9/54 1954 PB X 328421%

water F10, 000

3332 o Hanley Oil Co. No. 1 Univer­sity "A"

7y 302 7/8/55 X 125+16BOWD

P

2,000

Reagan County: 961 dry hole

Cities Service Oil Co. No. 1 University "AX"

7,400 1/13/55 P&A

+ Presently producing from the Ellenburger zone (F, flowing), o Presently producing from the Spraberry zone (F, flowing). P&A Plugged and abandoned.PB Plugged back (to the Spraberry zone),* Discovery well.BOPD Barrels of oil per day.BOWD Barrels of water per day.

structure m the area. Seismic studies, followed this geologic lead'anti': '

confirmed it. Well Ho- 1918, Cities Service'Oil Company No, 1 Univer-

. sity "AH", was tirilleti -in January 1951 anti was the firs t exploratory

-hole- . It was completeti as :a commercial: well in the Ellenfourger group '

at a total depth of: i l ?:281;.feht, ^ a S p ra b e rry producing zone was

.tiiscoyersti.in Octoher of the same year by well No0 1996; .Cities/Service

Oil Company Ho. 1 Pniversity.nAl/T", which.was drilled to the Ellen­

fourger at a total depth of 11, 091 feet anti then plugged hack to a depth

of 7,' 13'8 feet to produce from the Spraberry zone. The discovery well,

- Ho- 1918 of the. Ellehburger' zone^ was .also, plugged hack to the Spra- ' ' .

berry formation in' la te '195:1 because'1 of water encroachment- :

Producing and, Possible Producing Horizoiis . ■

Cuttings from wells drilled in the Flat Rock Field have carried

: oil stains: in the indicated formations at the approximate indicated depths: -

llpper Spraberty formation . 6, 8:50 feet t •

* tow er .,%raherry formation: : ' l s. 180 feet ■

Wolfcamp series ■ v'c"' '■ . • Mean sandstone , ; ' - 7,650 feet

Black'calcareous shale 8, 750 feet

. -: ..lleYonian eherty lim este ' V , - . 9,'850: feet -

: ' , :, l0-:800 feet , 1 "

' •- . Plate II indicates the position .of'these shows of oil in the strat-

igraphic column- , :..

Of these horizons^ the Spraberry formation,, the Wolf camp ■

series, the Devonian cherty limestone, and the Ellenburger group are .

productive in nearby fields such as the Big Lake Field, about eight

miles, to the east, ■ the Benedum: Field, about six miles to the north, ■ :

and the Block 4 Field, about four miles to the souths At present, the 7-'

. lower Spraberry formation and the Ellenburger group are the only pro­

ducing zones in the Flat Rock Field. The r emaining zones, with the .. .

exception, of the Devonian.cherty limestone, ar e not believed to have

future possibilities hi the Flat Rock Field due to insufficient porosity

and permeability, as inferred from the electric and radioactivity logs.

. Geology of the Reservoir and Source of Reservoir Finergy ■

The lithoiogy and, structure of the reservoir rocks have' already

been diseusSed.m. the. chaptersron 'Stratigraphy; andBtructure. ' ’ ■ '

: Bpraberry -

The thickness of the pay zone is about 31 feet, out of which 16

feet is net productive. The amount of porosity, permeability, .oil-.satura­

tion, and water saturation wili'be discussed later in this chapter.

The Spraberry formation is a volumetric-reservoir, and the

. flowing-wells obtain their energy from the expansion,of the natural gas ■

in solution. : The production .decline is due to the gas pressure decline .

of the reservoir. -. ' ' \ ' v . .

50

, . Ellenburger

The thickness of the pay zone is about 150 feet, out of which

50 feet is net productive. "ThijS is a water-drive reservoir, and the

reservoir pressure has remained fairly constant since discovery. The

production, decline is due to wafer .encrbaehment. - ,

'y''' ' / Drilling and Completion Methods. : : ..

All wells are drilled through completion with rotary tools.

They are spudded with a lT -i/2-m ch bit and a 13-3/8-inch surface pipe

iS: cemented at a depth of about 3 # feet. Wells are usually reduced in

size to 12-1/4 inches and drilled to about 3, 600 feet or into the San

Andres group where 9-5/8-inch casing is cemented with about 1, 600

sacks Of cement. Hole, size again •is r educed to 8-3/4 inches or pos­

sibly 7-7/8 inches into thp Ellenburger. A 5-1/2-inch casing is cemented

.at the tqppf :the:Ellehburger pay - :.:' ■ .

/ v ^ p r a b e r r y

If commercial oil production from the Ellenburger zone of the

deep well is not possible, the well is plugged back to the Spr aberry zone.

A 5-1/2-inch casing is set through the Spraberry pay zone and is ce­

mented with about 450 sacks of cement. The pay section, which has

been determined by means of drill stem tests and electric log charac­

teristics, is then perforated with about 8 shots per foot. The perforated

' ■ ' , ... .. ' ' 51

zone is usually between 30 and 35 feet thick. Hydraulic fracture treat-

mehtj, with 2,000 to 4,000 gallons of fluid, is given to two wells. Gen­

erally, other Spraberry wells have been treated with about 10, 000

gallons, of sand-fracturing fluid,- -

/ V X V; ' ' ' MlleWurger ' . ' - %

The pay zone has been determined by drill stem tests and

electric logs. Casing is set on top of the zone and open-hole comple­

tion is used. The pay section is about 150 feet thick. Two of the wells

have been treated with 2, 000 gallons of acid.

No well has dual completion in the Flat Rock Field. Electric

and radioactivity logs have been run in: all the wells. Some micrologs

. and a few cores are also available.

- ' #roductionHistory •.

: ;-V..V'' . .v:;%raberry■<' . . ' -

Initial potential flow ranged, from to 725. barrels of oil per

day. . One well. No. 3332, Hanley Oil Company No. 1 University "A”,

had to be pumped with initial potential of 125 barrels of oil per day and

16 barrels Of water per day. The water production has slightly increased,

and for different wells it varies between 8 percent and 20 percent of the

pr oduction of that well.

'52

■ . y- ; '.Ellenimrger;: y. %

• . Mitiar potential flow ranged from 128 to 2? 044 barrels of oil

•per.-day... TEater. production haa rapidly increased since production ’ ; ..

Wdm.the field commenced.. / ■ " ' ,

Table S.indicates annual production of, the field.Since discovery^

as =wll.as/the:cumulative"production up to July ;1? T9B9.'. ;•

■ V AH wells In-the: F1M Bock Eield-produce easinghead gas. Com­

plete data on the gas production'are not available.

%19B%..: the annual production Of the casinghead gas was .

.94., 038 MCF (thousand cubic .feet) for the; Spraberry and 44, Oil MCE

for theEllenburgerv- y -' \ " \;y . • y : •

:Ghar aeteristics .of/Oil and Gas

.: Spraberry

' ' - The oil from the- SSprWerry iormatiou;' which is the shallower

sonej, is dark, green in color and heavy .T he present gravity is 37.4

degr ees API, The cal 1 is ■ sweet

and has a low sulphur

Ellehburger

'■The oil from the Ellenburger pay zone ranged from 52 to 62

API gr avities in various' wells when production commenced:. P r esently

TABLE. 2 - 'A HNUAL PRdODUC T10N AHB. CEITOLATI^ PROIBUCTIOHj

. FLAT ROCK. EIELD :' ■ • (Bi Barrels) ' ■

—— . : . ... . . . . . . ' : ■ ■ ; V ;■ 1 ; . : Cumnla-' V - : -. tive pro-'

: ■ .. '•1051 . 1952 /■ 1953;.. '1954 : 195& 1956 - , .195T 1958 daction; " ' ' : . ; ■ ■ •. - , • as of .

'. : v July 1, ;\ •' V. \ . y : -. • 1959 ■

Bpraberry : . - 20,780 52, 61 m 36,360 '47, 584 8$;752 : , 76,944.% 297 :41,799 457,018

Slleriburger 10,163 67,999.106,572 163,081' 71,954. ,33,158'" 26,041 .20,006 ' 511^090

Total 1 1 30,943 120,615 142,932 >310,'645.161,706 110,102 ,93, 338 61,'805 968,108

the average specific gravity is 60,2 degrees AEL . Tables Indicate a

calorific value of 11400 caL /g r. for this OIL The Ellenburger oiL too,

has a low sulphur content and a sweet odor.

t ■ Table- 3 ^indicates 'results of an analysis of the associated gas -

from well NO0 22.33, an Ellenburger producer, in 1952. .

T AB LE. 3. ELLENBIJBEEB GAS ANALYBIS .

' - ; ' Hydrogen sulphide ' Present but. notv. ' : determined;

■ Oxygen : : t 0, 00 p e r o e n t - -

: Carbon''diOxide t ' ' 0„ 00 percent

- ' , • Nitrogen 3„ 85 percent .

Methane- 73. 55 percent

Ethane - 13. 79 percent

.PrOpane , ' ‘ ;V 0 .33 percent: .

- ; ' A : ' . Isobutane ', , 0 .6,3 percent - ; ;

■ ' N-butane ' 1.32 percent

Isopentane 0.18 percent

-- ■ N-pentane. 0 ' : ' 0.18 percent

: H e x a n e s . . ; .0.11 percent ;

1 ' " ,'v : ; Heptane and heavier ' > 0.06 percent - " ' -

55

Future Production Forecast

In this section;, porb^itjr determmation ie made iDy elecirle and

radioaetivily'logs? and resuits are compared with core data. Amount .

of oil in barr els per- acre#' and. :also reserves are. calculated from these

data. - , ■ , 7 'r ... \ / ' '

■. Porosity Determination by Electric and Radioactivity Logs

Electric arid radioactivity logs have been utilized for making

lithologic interpr etationsy bed definition^, correlations^ arid for secur­

ing other subsurface data. - - •

The logs available include the gamma ray# the neutron. curve, •

the spontaneous or self potential curve, and various types of the re­

sistivity curves. Of these only the neutron curve, the microlog, and

the 32=inch .lateral limestdne curve: have been employed in making.

porosity determinations. ■' . ;; ; T

: Limestone lateral curve. --The, 32-inch limestone latera l. curve

is designed prihiarily as a tool to simplify logging and evaluation of

p o ro sity in hard formations such as limestones, dolomites, and hard

sandstones. The limestone lateral curve has a small radius, of investi­

gation and shallow penetration, and therefore is,greatly affected by the

hole size and mud resistivity. The ratio of deflection of the curve op­

posite any given zone to the deflection of the curve opposite a zone of

highest density indicated .by the highest -resistivity, gives: an indication

of the porosity of the zone ' providing' that the mud r esistivity and. the '

hole size remain fairly oonstant.' - ■ - :

• As shown by Figure 11? the deflection opposite the interval

7130-7140 feet in well No 1918 (pay section of the Spratierry forma­

tion) is 35 units: the' maximum: deflection in the same well is SO units,

' Utilizing the Schlurnberger Chart C-16— and the curve for ■ a hole di­

ameter of 8-3/4 inches^ it is found that the porosity index is 14 per­

cent, 7?o this valuer 'a, correction factor. C must be applied. This

.factor is obtained, from Chart C -t 6 utilizing the spontaneous potential ■

r eading opposite the given zone and the kind of the forniation fluid (in ' \ :

this case oil). By using.80- millivolts, for the self potential (SP) reading’;.

.. in comparison with the shale base line and by interpolation in the Oil- -

; bearing formation.ln the table, dhe multiplier is found: to be 1,088,',,

., Therefore^ the:'porosity is : equal to'14 x..t, 088 or 15. percent. This

amount of porosity might be a little higher than the actual value., owing

to some shale content,

Neutron curve. —The porosity also can be determined from the

neutron curve. This curve is produced by bombarding the formations :

1 / ScMumberger Well Surveying Corporation (l959)? ’’Log ■ Interpretation Charts^" Houston, Texas, Chart C-IG,

57

POROSITY DETERMINATION

32* LIMESTONE LATERAL C U R V E

S P R A B E R R Y

F L A T R O C K F I E L D

j r'l*<

r z

# 1 9 1 8ft.

Figure 11. --Porosity determination by the 32-inch limestone lateral - curve.------------------------ .— --------------------------------------------------------

SAN

A

ND

RE

S

SP

RA

BE

RR

Y

: ' " ' . ■ . ' : / ; • • ■ .. ■ '58-

in the hole with a source of neutrons? such as that derived from a com­

bination of a radioactiw m aterial. ahd heryllium® Any .high concentra­

tion of hydrogen ions? in any kinds of formation fluids,, slows down the

movements of the neutrons and keeps them close to the source and away

from the detecting instruments which are spaced a given distance from .

the neutron source. Since the concentr ation of hydrogen ions is dir ectly

related to the.amount of fluid pr esent: in the rock :and this?; in turn? is

dir ectly related to porosity? the neutron curve will indicate a low re ­

sponse .Opposite a porous'vzone. It is found that with a decrease in.

.porosity^ the .neutron'curve response logarithmically increases,

As a general rule? the porosity of the formations in West Texas

vary between I percent and- 35 percent,to 40 percent. Assuming .40 per-'

cent porosity of some shale formation^ such as the Sylvan or the Wood-

fprd? and also a 1 percent porosity for a dense formation^ such as parts

of the Peyonian cherty limestone^,. and by connecting these 'values by a

logarithmic scale? one may obtain the porosity of a given zone. As

shown in Figure 12? by this procedure the porosity of the previous

-zone,' 7,120-% 130 feet'in well Ho, 1918,' has been determined to be

.15 percent, . '

Microlog, —A few attempts were made to determine porosity .■

from the microlog. However, due to microlog limitations, such as

thick mud cake and low. values of porosity? unsatisfactory results were -

59

F T

POROSI TY OET E RMI NATl ON

N E U T R O N C U R V E

S P R A B E R R Y

f l a t r o c k f i e l d

f -1

f 1918

Figure 12 .- -P o r o s i ty dfttermina.tiQn by the neutron CUfve .

SP

RA

BE

RR

Y

DE

VO

NIA

N

SY

LV

AN

SM

Al

E

/obtained.'

'''.. Porosity determination by electric and radioactivity logs cannot

be made in the Ellenburger zone ,dae to its fracture-type and low-value

porosity.; / ^

■Spraberry. —Table 4 is a core analysis of the Spraberry zone; :

1 of well No.'-..1996j Cities .Service-'Oil'Company NO. 1 .Nniversity TrAMf'?

: a Spraberry producer.' ' - .

Ellenburger. --Table S is 'a c o re analysis from the Ellenburger

zone, of well No. 2233* Cities Service Oil-Company No. 2 University

■ "AH’* an Ellei^xmger: pr oducery : ■■

Core analyses indicate lower values for porosity than electric

:.and radioactivity logs.' ■ This:is believed to be due to the fact that in.

. eoreanalysiSs: effective■poroSity;is'tireasured.; • , ' .;

: Amount, of ;Oil: PCr Acre ;

The amount of oil per acre can be obtained by the f ollowing

formula? - ' '

barrels' per acre = 7758 x porosity x net pay x .(l.- water saturation) x:/• recovery factor

reservoir volume factor

where/Beservoir Volume Factor- (RVF) is

61

■ . : - i M m . 4 , . -

; - ; ; : : : : V :' - S$SABBBKY O TlB .

Septh ' ■POTOSity Permeability- ' Oil satpration Water saturation:(feet) - 0erGeiitil '(millidareieA) : 0ere.emt) .■ (percent)

^ i i 9 -s,:i : 18, 6 . w . i

% 120 10.9 dp.

ig 120.6 v • 7.4 y- : v: dp. ;; - :: ; . 2 ; . :: . 22. 7

% 121.-5 ' .6. 8 do. ' 28.0 35.1

7 , m ia .9 : ■ ; . ; :

7,123 10.4 do. : ;

% 123.5 y :; ; 71,8

7,124' ' 16.6 : - do.;; ■ ■■

7.124.5 5.3' , do. '' : ' , ; 1514 % - ' 67.3.

7,125. 5 7. 6 . ; . do. '

7,; 128.5 '1.8 " do. ' '46.3 53.7

7,12 7 1. 6 , . ' do , . : , r ' . ' ' '

. 7,128 10o 7 ' ;do.. .

7,129: io.% do. • : : ; : '■

7.129.5 ' Blofeeit ; . ■ . V ; • .

' 7,130 7.4 do. .

7.131 5.8 „ do.

.7,131.5 1.6 do» 36.6 63.4

7.132 : 5.1, : do. V ' '

7.133 .. 9.6 do. ; . 1 . ;

TABIjE, 4— continued

62

■Porosity1 . Permeability ■■ Oil saturation -(percent) (millidarcies) percent)

"Water saturation (percent)

: : 3 .3 ,

7j 134. 5 Broken

:13« 6- V

7.136

7.137

% 137. 5

7,140

7, i40a. 5:

8. 3.

l o t

' 2 . 6

:i2o R

11.5

7,141, 5 9. 5 -

7,142. 5 Broken

■ 4.:9 ■

2.1

■',4.6.; ;

5.0

'1.0

1.3

7,144.5

7^145.5

7,146. 5

7,147.5

' 7, 148. 5

■7,1

7,151

■do.

do,.

do,

do,'

do.

do.

do.

do.

,do.:

do,

do.

:dd;:

do.

0.6

do,

do.

29. 5 70.5

10. 6 82.0

6.3

;.T^I^'-4.^cQntintt-6d'

r Pwosity : (pereeat|:

PermeaMMty(milliiareies)

•Oil satnr atidn (percent)

Water saMration (percent) v

' -' ; b( 8 i ' do.

7,153 7^9. / : do#'' ' '

7yl54 9.0

7r 155 :; ' do#

7^156 ; 1,3 : V ; do..

7,157 & o ; ' do.

7,15 0 - B.rO: ■

7,159 y : : # ' V ' 'do#' : ' ■"

7,160 7.0 vdo# v v, ■ .

7,161 : 6,6 / # ' : ..

d o ,'. • .

7,163 /0 ,9 - , :do.: ;; - ; „ r " ■ " ,J ■■ ' ■ '

7,164 0 .3 ” do.'

7,165 ' Broken'

B 'I ';

*Perraeabilities smaller than CL 1 millidarcy<.

M

D e p th P o ro s ity Permeability;' Oil saturation Water Saturation: ^eroent) (mlllidarciesi! (percent) , .(percent)

10j 713: , ' 4:8.4

lds,7 l4 : 5.5 No flow*

10 .7 1 8 ;; ; 7o '3: 0 ,2 7 , 7

10, f l5 . i ' 11.3: ' : . No flow* # . p _ 23.8

l.p?7l6„:.|; /5° 5 0,1

lp ,:7 m # : ; 0o7 ‘ No flow* ' .

10, 71B0;S: . 5, 5 doe .

10s 719.5 5 ,0 / do. ; : ;

lO gi# .#: 2 ,9 .

i0 ,m i: M f . . ■ :dp y 10/4; :v : ; 13.5

.. 7. 5 ; '

10, 723 : '' 11.2 : . 0,1 • 28,0 . 23.2

10, 724 : v 3 ,5 ' '' ' . 1, 4.' :. .; \

10, 725: ' ;:;;:0 ,2 : y . y

10, 720 . ' . o . i : , 0 /0

10,72.7 ,8 .7 0 .2 .

10, 728 % : 5 .0 , No flow*

:10,7#;;:^:'f:0,4 ;

10 720;. 8 .0 :; do. \

10,7311 7.1 , do.

' ' ; TABDRB^continued

. 65

Depth(feet)

Porosity : (percent) '

' Permeability; (millidareies)

Oil saturation , (percent)

Water saturation (percent)

lOj: 733 ,4.1 : do. '

10; 733' 4.0 : -. do.

10, 734 : 3.1 do.V

10; 735 . 5.6 ' do.

10; 73.6. : :8,4" ; 0. 2 :; 14.o 1 : 11.6 •

10; 737» 2.;/ 7. 2

. * Permeabilities, smaller than 0.1 .millidarcy.

volume of reservDir oil (at reservoir- pressure and temperature)stock tank oil

and Recovery Factor is the fraction of the oil in place that is recovered.

Sprabeyry* --The follovbng assumptions^ mainly based on core

analysis have been used in this section,

. Porosity s 80. S'percent

\ . M ;,pay ~ 16'feet .•

■ Water saturation “ .30 percent

. Recovery factor s 20 percent

' : RVF = 1.8 .

Using these values the primary r ecovery of oil is computed as

being 850 barrels of oil per acre.'

■ ■ 66

Ellenbnrgero --The following assumptions^ mainly based on

core analysis; have been used in tMs section.

, Porosity s 6,7 percent

• 1 : .' ''^'^y■:Netpay;s ■BA■f0et ■'■■'■: V::■ .

Water saturation s M percent

; ' v : ■ Recovery factor - 80 percent

"■ BVF'a: lo 2

Using these values the primary recovery of oil is computed as

tDmBg'^:900:barrelSPf oil per acreJ j ■

. Estimation of Heserves

. Bpraberry. —The - decline curve method , for estimation of the

volume of oil in place.in. a volumetric reservoir (Spraberry)? has been

.chosen due to simplicity and.correctness* - It is noteworthy that pro- '

rated and curtailed production affects the accuracy of this method to

; some extent. - ' - - ' V

For a better indication of the production performance of the

■; field, a decline curve has been drawn by plotting the annual producing

rates in barr els against the time in years on a semi-log paper (PL 1)«

The. production .from the Spraberry zone shows a peak in 1955, By

extrapolation of an average line indicating the production decline, one

may obtain the anticipated economic life of the field and also the amount

of the recoverable oil which can be expected from the sevenBpraberry

producing wells through primary recovery methods. • A total of 5 110

the Spraberry formation, will last until the middle of the year 1971.

. One may use the following formula, for •computation of the

amount of the recoyerable oil iby primary methods: still left in place.

: E s' amount of: r ecover ahle n ii still in. the'gr ound, In harr els for the ' seyen wells. ' Y. ’ ' -: : ‘ ^'1.. ' ?

T = remaining economic life of the field, in years (here 12 years).

. - present yearly production^ in barrels per year (here-43,000. - I barrels).^ , . ' v:'' -i' ■ : " ; , : : •

P'g - annual production at the end of the economic life of the field, in 'barrels per year ■ (here 5? 110). : ,

Using these figures: in the formula, the.:recoverable. oil still in the ground

. in the Spraberry formation, from the seven wells in the Mat Rock Field:

is 220,000 barrels. To date a total 01457,018 barrels have been pro-

' duCedfrom .this pay .zone. : Therefore, a total ultimate .recovery' by :. ■

primary means of Spraberry oil from this horizon by the seven wells

will be approximately 680,000 barrels.

barrels of oil per year by p r imar y methods : has been assumed as the

minimum economic annual production of the field from the seven Spra^

berry pr oducing1 wells. This amounts to two barrels per. well per ■

calendar day. .CM this basis, the economic production of the field, for

"Where:ioge.P i/P 2

■ ; :': Because of the rather low porosity and permeability of the

Spraberry zone,. it is probable that each of the seven wells will not

/ . : . v, 68 :

drain more than the 40 acres upon which they are drilled. The final r e ­

covery will be from 11 wells,, since all the Ellenbur ger wells eventually

will be plugged back to the Spraberry. Therefore:

Total ultimate production from the Spraberry can be estimated

as 680,000 x 11/7 or about 1,070, 000 barrels of oil.

_Ellenburger. --Estimation of reserves of a water-drive re se r­

voir (Ellenbur ger) ismot as •simple. Tedious methods, which include

drawing flood-out curves and obtaining the position of the water table

for each year since discovery, are not within the scope of this paper.

:: It is assumed that' after plugging back two of the four Ellen­

bur ger wellsv in the immediate future'(eee section on Economic Consider-

atiohs). the. economic life of the other two wells will last until the year

. '1,964. By that time^ it is: estimated that the Ellenbur ger zone, in the

Flat Rock Field, will have ..produced 680,000 barrels of oil.

Casinghead gas. --Bata on production of the casinghead gas are

insufficient. The estimation for the cumulative production of the gas as

of July 1, 1959, based on the pr oduction in 1958, is 1, 200,000 MC'F.

. Secondary- Recovery

Ho secondary recovery project has yet been planned.

: 69

■ - Sprabegry0 ---.A waterflood pr oject can .be useful in- the Spra-

berry zone; however^ a ready source of water is unknown in the Flat

Rock, area, : :

Fllenburger- --'Secondary recovery is not considered to be

economically feasible in the Ellenburger zone.- Primary production is

essentially a water drive. A ;

. Economic'-Considerations

The Flat Rock # il Field, in comparison with the other oil fields

in West Texas? is not of great importance. It is a Small oil field produc­

ing from two zones; the Ellenburger group and the Spraberry formation.

Fourteen wells* including three dry holes* have been drilled in the field.

Of these*. 10: wells. are deep with ah average depth of 11* 000 feet and 4 ' /

wells are shallower with an average depth of 7* 400 feet.

The transportation of oil is done by a pipeline. This method

has some advantage in comparison With transportation by truck* since

there is not any loss in volume or decrease in API gravity. Transporta­

tion by truck sometimes has up to 8 percent loss-in the volume and 2. 5 -

to 3 API degrees m. gravity.

■ ' Cities:Service .Pipeline Company is the-main crude purchaser. :

Univ. of Arizona Lfbrif^

;v;;Xv: ;v ; " ■ X^rajDerry, " .

■ Cost of dr illing' anti completion, --Estimated cost for drilling

anti completion of eacli Spraberry well (about 400 feet deep) -is

'$110,000* _ : : X;

Cost, of operation, —Estimateti cost of operation of each Spra-

• berry- producing well per month (average for flowing or pumping) is

$150, ■ : . . ; '

Income. —.Price of each barr el of the crude oil* after taxes

anti transportation, costs, throughout the economic life of the field is

estimateti to be '$%. 75., gGf each barrel ot oil, only 0* 875 of a barrel

belongs to the producing company since the royalty must be detiucteti,

, The average mpniMy; prdductiM per: well throughout the payout time is

'lylSO'batrelS of olM-X" X X ’ • X XX

Monthly income per well is 1,150 barrels x $2* 75/barrel x.- - ./XX :X'X, '":, X . X / 0„ 875. - $2/760

Cost of. operation .per month is .f l 50

Net. income per weli pev month is .$2, 610

Payout. —PayOut is computed for a well drilled only to the

Spraberry.

Pftyffli* pgr.-’nwii - cost of well drilled to the Spraberry _= estimated net monthly income

71

$110;, 000__ - 42 months or 3-1/2 years.$2.'610/m6nth ■ ^

' , Cost of :Ellenbnrger wells plugged back and completed in Bpra-; ,

berry. --The estimated cost of plugging' back of each well is $% 7005 :

which .includes 20? 000 gallons' of sand-fracturing fluid for treatment of

the lower Spraberry sone,.. / ' . .

.Cost of plugging back, and abandoning:wells, --At th e ;end.of

economic life of the.Spraberry horizon^ . 'each well will have to be plugged.

Estimated cost..of plugging, the 7j 400-foot hole is $700.

Summary of Spraberry costs,, profits^.. arid losses. '

; ' : ■ Drilling' 4; wells, to .Spraberry . 1.. » . . « . » . . . . . . .$ 440 ,000

:...;Dluggiaig:.bach lO'.W^li§;frbm. th^'Blienburger 37$. 000

.Jinal plugging and- abandoning' 14 wells . . . . . . . . . . . $ 9$ 800 -

Cost of operation at $150/well month:: To date-504 well m o n t h s o . . . . . . . . . $ 75$ 600.

From discovery to abandonment / r : ' T?.968'Well months . « ■ ' . . . . . . » . . . . . $ 295$ 000 ■

. • ; ■ TotaJ costs. to abandonment . . . . . . . . . . . . . . . . . . $. 907$ 400

Income to present: ■ ■ ■457$ 018 b a r re ls i :$2*75/barrel s;0 .875 „ .. . . . . . . ,$1$ 096$ 848

Income from discovery to abandonment of field I ;p rim ary pf oduction):

:; 1$ 070$ 00.0 barrels'X $2.75/barrel x 0.875 ... . . . . . . . $2$ 568$ 000

Net profit from Spraberry to abandonment of the ' fields exclusive- of depreciation^ inter est ’, charges^ salvage^. and. so forth , . . . . . . . . = . . . . . . $1, 660$ 600

r : '. ; . , ; ■ ; : .y ■ , 72

; ; ' ' : ' ; .Ellenbtirger ■ ' ' '

Cost of drilling and completion. °°Estimate€l cost for drilling

and- completion lo r each, of the. two .deep . discovery wells is $200s 000o

Estimated cost for. drilling and completion of each deep well is $1B0$ OOOe

; ' ' " ■' .:-Codt pf;qpdratibn. --^tiniated-~cost of operation of each Ellen-

burger producing well per month ’(average for flowing or pumping) is

$300. „ , V \ ' : 1

Income, -r-Income is computed for four Ellenburger producing

wells. Average: monthly production of each well to present is about

ylg:B50harr;elsof.oiL; ’ ; y

Monthly income per well is l s 250 barrels x.$2.75/barrel x . ' . 0o 8m - ##000 •

Cost of operation per month is $300

. Eet income per well per month, i s . 000 - $300 s $2^700

--Payout is computed for four wells producing from :

the Ellenburger- zone. '

Payout g $180? 000 _ 61 months or about 5=1/2 years.^ $2,700/month

: Sumhiar'y of Ellenburger costsy profits* and losses

;Srilling two discovery wells ® . .«■. . . . . . . , , . . . . . . . . ,4005 000

"-Drilling 8 more wells to Ellenburger . ^ . .$1, 440; 000

73

Cost of operation at $300/well month:.• To date Si2■ well months » . . . . . . $ . 93? 600: ' From: .discovery to' abandoBment .. -432 well months . . . . , . . . . . . . . . . . . . . . . . . . . $ 130s 000 ,

Total costs to abandonment . . . . . . . . . . . . . . . . . . . . . . . $1? 970 000

Income to present: ' ...;'511^090 barrels, x. $2. 75/barrel,x0., 875, ».. = . . . . $1* 22% 616 ,

Income front discovery to abandonment of field* .580? 000, barrels x |2 .75 /barre l x:0o 875 . = . . . . . . 400? 000 .

Net loss from E-llenburger exclusive of de= . ,preciatiori? interest charges,, salvage^, and.so

$ 570j 000

. . -.Return, on lnvestment and.Net. Profit

From diSGOvery to - abandonment of field. .

' • . ■ ■ ;' ” Total drilling' and completion, costs:). $44Q5 00Q+$1# 840? 000- •. ■« /: I $2,280,000

■ Total plAgging costs: |8 7 5.000+|9j 800496, 800 '

• Total operational costs: $295? 000+$130? 000=$425? 000 ..

' Total gross income: $2 568,000+$lj 400 000=$3? 968,000 ;

' MQf (rAbirn oin i-nvAStmAnt) „ GrOSS income-total COStS _, ■ ' "C;...: Costs of drilling and com--.'

: ' - pleting . .. ' .

■ $3$9683000-$2s:801,800 _ $1$;166?200 =. m' ^ 280,000;. ' , " f 2,280,0W °.

• •" Net profit,.exclusive.of depreciation, interest charges, salvage, : and soiorthi . $1? 166,200

It is noteworthy to mention that the income from the casinghead

gas has hot been included in the calculations above. Since the estimated

cumulative gas production is X? 200 000 MCF#. therefore* the gas in-

:come to present^; at 9 Mll'jbe $X08*0009 . ’ ' '

BIBLIOGRAPHY

Adams*. John: Emery (195% nMid-Paleozoic Paleogeography of Central , Texas,n. San Angeolo Geological Society Guidebook^ Ganabrian Field. - / Trip> ,pp. 70-73» v ^

. Beayer* Paul Jo' (1957)*’"Elat.Rock'Field* Upton;Comty* Texas*n .Bureau' of Ecdnomie Geology* Texas University^ Publication No. 5716,ppo 120-122o ■ , ■ , ■

• Galley*' John E. (1958)* '!tOil and Geology in the Permian Basin of Texas and FTew Mexico* ” in Habit of .Oil* American Association of Petroleum Geologists, ppo 395-446.

Jones* T. S. (1957)* "Stratigraphy of the Permian. Basin of "West Texas* n . West Texas Geological Society* pp. 1-45.

Lane-Wells Company (1958), "Radioactivity 'Well Logging Handbook, n . Hpnston, Texas, pp. ,8Q-%'

Levor sen, ’ A. L % "Geology of Petroleum, " W .. H.' Freeman.Company* San Francisco* California*. 703 pp.

'.Post*. To Go (195.9)* . "A General Report Covering the Permian. Basin of West Texas and Southeast Mew Mexico,n North Basin Pools Engineering ' Committee, 'Midland* Texas* Table T-JCC

Schlurnberger Well Surveying Corporation (1959), "Log Interpretation C harts," .Houston, Texas, Chart C lGo"

, Sellards*:' R ,' H. * .Adkins* . W. So * and Plummer, F, B; (1.94,7), "The '' Geolbgy of Texas* Texas .Uniyersity .Bulletin No. 3232* second print­ing* Volume I* Stratigraphyr 10O7 ppo ' ■ V '

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