in a Permian San Andres Reservoir, Waddell Field, … · We integrated core, log, 3D seismic, and...
1
The gross producing interval in the “high volume area” has been identified as the porous zone between the base of the tight, anhydritic karst and an informal stratigraphic marker we have labeled the “x” marker, which lies approximately 150 ft (46 m) below the top of the San Andres Formation. This reservoir interval is composed of shoal-water, oolitic, fusulinid, skeletal grainstones and packstones, characterized by biomoldic and oomoldic porosity with scattered vugs and fractures. The pore space is partly occluded by gypsum, making distinction of true pore space difficult due to the low bulk density of gypsum and its waters of hydration. Log compositional analysis was used to discriminate actual pore space. Compositional analysis of the cored interval of the San Andres Formation in well #1261 was made using the density, neutron porosity, and bulk photoelectric factor logs to solve for proportions of dolomite, anhydrite, gypsum, and porosity. The sonic log is a good measure of interparticle porosity, but is largely insensitive to larger pores that occur as vugs or oomolds. The compositional solution for the San Andres Formation in well #1261 was used to compute a variable matrix transit time, using weights that matched the proportions of dolomite, anhydrite, and gypsum. This estimate of sonic- derived porosity is shown below left, together with the porosity from the compositional analysis and core measurements of porosity. The porosity estimated from compositional analysis represents a volumetric measure of porosities of all kinds and shows a good concordance with core data. The sonic porosity is a close match with compositional porosity in zones where all the pore space is probably interparticle, but shows distinctive deviations in higher porosity zones where part of the pore space is vuggy or oomoldic. In the karst zone, the sonic porosity suggests that the low pore volumes are dominated by vugs. In the oolite shoal, the higher porosity developments appear to be about equally divided by interparticle and oomoldic pore space. The high content of anhydrite and low porosity of the karst zone was strongly differentiated from the more porous section below the karst which also appeared to be more gypsiferous.Two log variables were used independently to estimate the base of the anhydrite zone: the sonic log and the anhydrite content estimated from the density, neutron porosity, and photoelectric factor curves. The lower boundary of the karst zone was calculated by a zonation program which locates zone boundaries such that variability is maximized between the zones while minimizing variability within the zones (Bohling et al., 1998). POROSITY FROM LOG DATA An approximately 2 square mile (5 square km) area of Waddell Field, in the Permian Basin of west Texas, is characterized by Operator-interpreted tracer and pressure data indicate a highly compartmentalized reservoir with an active water drive. Reservoir heterogeneity appears to be related to stratigraphy and diagenesis, as well as anhydrite-cemented karst features associated with the subaerial exposure surface developed on the top of the Permian San Andres Formation. The present study aims to characterize the variability within the San Andres reservoir by making use of a variety of geological, variable fluid production, with overall fluid production an order of magnitude greater than in surrounding areas of the field. Recoveries of oil and gas for wells in and adjoining this “high volume area” range between 100 M BOE and 1 MM BOE per well. Overall, nearly 50% of the wells produce 250 M BOE or less. In the high volume area, production is typically 300 M BOE and more per well. petrophysical, and geophysical analyses. In particular, Multi-mineral models are essential for estimating effective porosities that provide good matches with core. Statistical zonation of well logs provides a consistent method for identifying the base of karst. Seismic horizon mapping provides details on the configuration of the reservoir interval. Seismic impedance allows us to determine interwell porosity variation. BVW profile analysis is useful for assessing spatial continuity of the reservoir. Multi-trace seismic attributes, such as volumetric curvature, provide added detail in our interpretation of reservoir compartmentalization. i i i i i i A significant number of carbonate reservoirs worldwide have been modified by karst. Karst can impact reservoir performance either by enhancing or destroying porosity/permeability. We integrated core, log, 3D seismic, and production data to identify the nature, distribution, and impact of karst in a Permian San Andres carbonate reservoir in Waddell Field, west Texas. The reservoir in our study area is highly compartmentalized and fluid production is extremely variable. Reservoir heterogeneity appears to be related to stratigraphy and diagenesis, as well as karst features associated with a subaerial exposure surface at the top of the San Andres Formation. Core data indicate that the uppermost San Andres consists of sporadically porous “macro” karst, characterized by intense chaotic brecciation and anhydrite replacement, followed by isolated, late stage anhydrite dissolution. The karst overprints high frequency sequences composed of gypsiferous oolitic, fusulinid, and skeletal packstone-grainstone reservoir rock with moldic, vug, and fracture porosity. We used wireline log petrophysical solutions to quantitatively discriminate the anhydritic karst from the underlying packstone-grainstone strata in uncored wells. The base of the karst zone corresponds to a sharp decrease in seismic impedance, and 3D seismic data are used to map the base of karst between wells. The karst zone exhibits high variability in thickness; however, the zone is generally thicker on higher portions of a SE-trending anticline that runs through the study area, suggesting a structural control on karst development. Seismic data show that the karst zone truncates the base of the porous reservoir in some areas and seismic attributes reveal potential reservoir compartment boundaries. Better understanding of local karst control on fluid flow in this reservoir can improve reservoir management decisions. ABSTRACT Petrophysical and Geophysical Characterization of Karst in a Permian San Andres Reservoir, Waddell Field, West Texas Susan E. Nissen , John H. Doveton , and W. Lynn Watney 1 2 2 1 2 Geophysical Consultant, McLouth, KS; Kansas Geological Survey, University of Kansas, Lawrence, KS BACKGROUND AND PURPOSE OF STUDY The sequence framework for the Upper Permian (Guadalupian) San Andres Formation is based on the study of surface exposures in the Guadalupe Mountains of West Texas. The dolomitic reservoir in Waddell Field is comprised of cyclic oolitic grainstones, specifically G8 and G9 high frequency sequences (HFS) believed to be 4th-order (0.1-1 Ma) periodicity (French and Kerans, 2004). In the “high volume area”, the regional stratal boundary that separates G8 from G9 has not been precisely located and tied into the regional stratigraphic framework. The G8 and G9 HFS reservoir interval overlies a regional bypass surface corresponding to the top of the Brushy Canyon in the outcrop (also equivalent to the top of G4 HFS). Interpretations of surface exposures of these strata indicate that the inner and middle ramp lithofacies during the G8 HFS underwent backstepping, while the G9 HFS was involved in forward stepping, reflecting relative fall in sea level (French and Kerans, 2004). These observations, coupled with seismic interpretations on the eastern edge of the Central Basin Platform (Qifeng Dou, pers. comm.), suggest that the succession of oolitic shoals in Waddell Field are comprised of a retrogradational to aggradational to progradational stratal architecture resulting in southeastward lateral accretion of a shallow shelf through Waddell Field toward the eastern margin of the Central Basin Platform. STRATIGRAPHIC ANALYSIS AND PETROPHYSICAL CHARACTERIZATION Stratigraphic column for upper Leonardian and lower Guadalupian section in the Permian Basin. After Ruppel and Bebout (2001). GEOLOGICAL SETTING B B' A time structure map of the top of the Grayburg from a 3D seismic survey over Waddell Field shows the regional structural configuration. The black box highlights the “high volume area” study area. Simplified geological map of the San Andres Formation in the Permian Basin showing locations of major oil fields (Scholle, http://geoinfo.nmt.edu/staff/scholle/guadalupe.html; after Ward et al.,1986). Our study area is in Waddell Field, northern Crane County, TX, located on the east central flank of the Central Basin Platform. Study Area Delaware Basin Crane Central Basin Platform Midland Basin 1 mi A A’ Subsea depth map of the top of the San Andres Formation for the “high volume area” of Waddell Field. Structurally, the “high volume area” of Waddell Field is located on the east central flank of the Central Basin Platform. The northwest portion of the “high volume area” is structurally higher than areas to the south and east, although the highest structure does not correspond to the highest production. A southeast-trending plunging anticline approximately two miles long crosscuts the study area, showing a relief of approximately 75 ft (23 m) and a width of approximately one mile (1.6 km). A shallow saddle in the central mapped area exhibits elevated gas and total fluid production. Seismic profile across the shelf margin of the Central Basin Platform showing reflector configurations that lead to the interpretation of the G4 to top San Andres as a transgressive systems tract and the top San Andres to Grayurg as a highstand systems tract. A-A’ Grayburg Top of San Andres G4 unconformity G3 unconformity HST TST HST TST A’ A Cumulative oil and gas (left), cumulative total fluid (oil+gas+water) (center), and cumulative gas (right) maps in barrels of oil equivalent (BOE) showing . A conversion factor of 5.7 MCF gas to one equivalent bbl of oil was used in the conversion to BOE. the “high volume area” of Waddell Field Depth contours for the top of the San Andres Formation are superimposed. LITHOFACIES Cores of the upper San Andres Formation were taken from two wells within the “high volume area”, W. N. Waddel #1204 and W. N. Waddell #1261. Core descriptions provided by the operator indicate an upper karst interval in these wells involving macroscopic collapse and chaotic brecciation and extensive anhydrite replacement of gypsum in the upper San Andres Formation. In contrast, the “porous karst zone” and “karsted and bioturbated fusulinid shoal” below the main karst zone in well #1204 is less intensely karsted, without macro-scale chaotic brecciation and anhydrite replacement. This cm-scale dissolution and brecciation is recognized here as “micro” karst where the matrix is essentially intact. The matrix properties in the microkarst intervals are also probably dominant in terms of fluid flow. Thus, the lower karsted zones in well #1204 are placed in the in situ bedded carbonate reservoir of the San Andres Formation. An example of the results of the karstic zone methodology is shown for the cored well #1261 well and two neighboring wells. In well #1261, the core description subdivision of the upper San Andres Formation between the karst zone and porous, oolitic shoal facies is closely matched by the zonation picks based on the sonic and estimated-anhydrite logs. Note the significant variation in thickness of the interpreted karst zone in the three wells. Estimates of the depth of the base of the low-porosity zone and anhydrite phase at the top of the San Andres Formation were compiled for all wells with density, neutron, photoelectric factor, and sonic logs. In wells where only a sonic log was available, the estimate was restricted to the base of the low-porosity section. Generally, the thickness of the low porosity Karst zone is greatest on the southeast- trending anticline that cuts through the “high volume area”. However, this zone shows high spatial variability, punctuated by anomalies where the "karstic" interval is either markedly thick or thin. In a few wells, there was no evidence of a low-porosity, anhydritic zone at the top of the San Andres and this was interpreted as an absence of karst development. 1207 1228 1204 1261 B’ B Isopach map of upper tight zone of San Andres Formation interpreted as karst, with top of San Andres Formation subsea depth contours superimposed.
Transcript of in a Permian San Andres Reservoir, Waddell Field, … · We integrated core, log, 3D seismic, and...
The g
ross p
roducin
g in
terv
al in
the
“hig
h v
olu
me a
rea
”has b
een id
entifie
d a
s th
e p
oro
us z
one b
etw
een th
e b
ase o
f the tig
ht,
anhydritic
kars
t an
d a
n in
form
al s
tratig
raphic
mark
er w
e h
ave la
bele
d th
e“x
”m
ark
er, w
hic
h lie
s a
ppro
xim
ate
ly 1
50 ft (4
6 m
)belo
w th
e to
p o
f the S
an
Andre
s F
orm
atio
n.T
his
reserv
oir in
terv
al is
com
po
sed o
f shoal-w
ate
r, oolitic
, fusulin
id, s
kele
tal
gra
insto
nes a
nd p
acksto
nes, c
hara
cte
rized b
y b
iom
old
ic a
nd o
om
old
ic p
oro
sity
with
scatte
red v
ugs a
nd
fractu
res.T
he p
ore
space is
partly
occlu
ded b
y g
ypsum
, makin
g d
istin
ctio
n o
f true p
ore
space d
ifficult d
ue to
the lo
w b
ulk
de
nsity
of g
ypsum
and its
wate
rs o
f hydra
tion. L
og c
om
positio
nal a
naly
sis
was u
sed to
dis
crim
inate
actu
al p
ore
space.
Com
positio
nal a
naly
sis
of th
e c
ore
d in
terv
al o
f the S
an
Andre
s F
orm
atio
n in
well #
1261
was m
ade u
sin
g th
e d
ensity, n
eutro
n p
oro
sity, a
nd b
ulk
photo
ele
ctric
facto
r logs to
solv
e fo
r pro
portio
ns o
f dolo
mite
, anhydrite
, gypsum
, and p
oro
sity.
The s
on
ic lo
g is
agood m
easure
of in
terp
artic
le p
oro
sity, b
ut is
larg
ely
insensitiv
e to
larg
er p
ore
s th
at
occur a
s v
ugs o
r oom
old
s.
The c
om
positio
nal s
olu
tion fo
r the S
an
Andre
s F
orm
atio
n in
well #
1261 w
as u
sed to
com
pute
a v
aria
ble
matrix
transit tim
e, u
sin
g w
eig
hts
that
matc
hed th
e p
rop
ortio
ns o
f dolo
mite
, anhydrite
, and g
ypsum
.T
his
estim
ate
of s
onic
-deriv
ed p
oro
sity
is s
how
n b
elo
w le
ft, togeth
er w
ith th
e p
oro
sity
from
the c
om
positio
nal
analy
sis
and c
ore
measure
ments
of p
oro
sity.
The p
oro
sity
estim
ate
d fro
mcom
positio
nal a
na
lysis
repre
sents
a v
olu
metric
measure
of p
oro
sitie
s o
f all k
inds a
nd
show
s a
good c
on
cord
ance w
ith c
ore
data
.T
he s
onic
poro
sity
is a
clo
se m
atc
h w
ithcom
positio
nal p
oro
sity
in z
ones w
here
all th
e p
ore
space is
pro
bably
inte
rpartic
le, b
ut
show
s d
istin
ctiv
e d
evia
tions in
hig
her p
oro
sity
zones w
here
part o
f the p
ore
space is
vuggy o
r oom
old
ic. In
the k
ars
t zone, th
e s
onic
poro
sity
suggests
that th
e lo
w p
ore
volu
mes a
re d
om
inate
d b
y v
ugs. In
the o
olite
shoal, th
e h
igher p
oro
sity
de
velo
pm
ents
appear to
be a
bou
t equally
div
ided b
y in
terp
artic
le a
nd o
om
old
ic p
ore
space.
The h
igh c
onte
nt o
f anhydrite
and lo
w p
oro
sity
of th
e k
ars
t zone w
as s
trong
ly d
iffere
ntia
ted fro
m th
e m
ore
poro
us s
ectio
n b
elo
wth
e k
ars
t whic
h a
lso a
ppeare
d to
be m
ore
gyp
sife
rous.T
wo lo
g v
aria
ble
s w
ere
used in
dependently
to e
stim
ate
the b
ase o
f the
anhydrite
zone: th
e s
onic
log a
nd th
e a
nhydrite
conte
nt e
stim
ate
d fro
m th
e d
ensity, n
eutro
n p
oro
sity, a
nd p
hoto
ele
ctric
facto
rcurv
es.T
he lo
wer b
oundary
of th
e k
ars
t zone w
as c
alc
ula
ted b
y a
zonatio
n p
rogra
m w
hic
h lo
cate
s z
one
boundarie
s s
uch th
at
varia
bility
is m
axim
ized b
etw
een th
e z
ones w
hile
min
imiz
ing v
aria
bility
with
in th
e z
ones (B
ohlin
g e
t al., 1
998).
PO
RO
SIT
YF
RO
M L
OG
DA
TA
An
app
roxim
ate
ly 2
sq
ua
re m
ile (5
sq
ua
re k
m) a
rea
of W
addell F
ield
, in th
e P
erm
ian
Basin
of w
estTexas, is
chara
cte
rize
d b
y
Op
era
tor-in
terp
rete
d tra
ce
r an
d p
ressu
re d
ata
ind
icate
a h
ighly
co
mp
artm
en
taliz
ed
rese
rvo
irw
ith a
n a
ctiv
e w
ate
r driv
e. R
ese
rvo
irh
ete
roge
ne
ity a
ppe
ars
to b
e re
late
d to
stra
tigra
ph
y a
nd d
iag
ene
sis
, as w
ell a
sa
nh
ydrite
-cem
en
ted k
ars
t fea
ture
s a
sso
cia
ted
with
the s
ub
aeria
l expo
su
re s
urfa
ce
de
velo
pe
d o
n th
e to
p o
f the P
erm
ian
Sa
nA
ndre
s F
orm
atio
n.
The
pre
sen
t stu
dy a
ims to
ch
ara
cte
rize
the
varia
bility
with
in th
e S
an
An
dre
s re
se
rvo
ir by
ma
kin
g u
se
of a
va
riety
of g
eo
log
ica
l,
varia
ble
fluid
pro
du
ctio
n, w
ith o
ve
rall flu
id p
rod
uctio
n a
n o
rder o
f magn
itude g
reate
r than in
surro
un
din
g a
reas o
f the fie
ld.
Re
coverie
s o
f oil a
nd
ga
s fo
r we
lls in
an
d a
djo
inin
g th
is“h
igh v
olu
me a
rea
”ra
nge b
etw
een 1
00 M
BO
E a
nd 1
MM
BO
E p
er w
ell.
Overa
ll, nea
rly 5
0%
of th
e w
ells
pro
du
ce
25
0 M
BO
E o
r less. In
the h
igh v
olu
me a
rea, p
rod
uctio
n is
typic
ally
300 M
BO
E a
nd
mo
re p
er w
ell.
pe
trop
hysic
al, a
nd
geo
physic
al a
na
lyse
s.
In p
artic
ula
r,
Mu
lti-min
era
l mo
de
ls a
re e
sse
ntia
l for e
stim
atin
g e
ffectiv
e p
oro
sitie
s th
at p
rovid
e g
ood m
atc
hes w
ith c
ore
.
Sta
tistic
al z
on
atio
n o
f we
ll log
s p
rovid
es a
co
nsis
ten
t meth
od fo
r identify
ing th
e b
ase o
f kars
t.
Seis
mic
horiz
on
map
pin
g p
rovid
es d
eta
ils o
n th
e c
on
figura
tion o
f the re
se
rvoir in
terv
al.
Seis
mic
imp
eda
nce
allo
ws u
s to
de
term
ine
inte
rwe
ll po
rosity
varia
tion.
BV
W p
rofile
an
aly
sis
is u
se
ful fo
r asse
ssin
g s
pa
tial c
on
tinu
ity o
f the re
serv
oir.
Mu
lti-trace
se
ism
ic a
ttribu
tes, s
uch
as v
olu
me
tric c
urv
atu
re, p
rovid
e a
dde
d d
eta
il in o
ur in
terp
reta
tion o
f reserv
oir
com
pa
rtme
nta
lizatio
n.
iiiiii Asig
nific
an
t nu
mb
er o
f ca
rbo
na
te re
se
rvo
irs w
orld
wid
e h
ave b
een m
odifie
d b
y k
ars
t. Kars
t can im
pact re
serv
oir p
erfo
rmance
eith
er b
y e
nh
an
cin
g o
r de
stro
yin
g p
oro
sity
/pe
rme
ab
ility.
We in
tegra
ted
co
re, lo
g, 3
D s
eis
mic
, an
d p
rod
uctio
n d
ata
to id
entify
the n
atu
re, d
istrib
utio
n, a
nd im
pact o
f kars
t in a
Perm
ian
San
An
dre
s c
arb
ona
te re
se
rvo
ir in W
ad
de
ll Fie
ld, w
estTe
xa
s.T
he re
serv
oir in
our s
tudy a
rea is
hig
hly
com
partm
enta
lized a
nd flu
idp
rod
uctio
n is
extre
mely
va
riab
le. R
ese
rvo
ir he
tero
ge
ne
ity a
ppe
ars
to b
e re
late
d to
stra
tigra
phy a
nd
dia
gen
esis
, as w
ell a
s k
ars
tfe
atu
res a
sso
cia
ted w
ith a
su
ba
eria
l exp
osu
re s
urfa
ce
at th
e to
p o
f the
San
Andre
s F
orm
atio
n. C
ore
data
indic
ate
that th
eu
pp
erm
ost S
an
An
dre
s c
onsis
ts o
f sp
ora
dic
ally
po
rou
s“m
acro
”kars
t, chara
cte
rized b
y in
ten
se c
ha
otic
bre
ccia
tion a
nd a
nhydrite
rep
lace
men
t, follo
we
d b
y is
ola
ted
, late
sta
ge
an
hyd
rite d
issolu
tion.T
he k
ars
t overp
rints
hig
h fre
que
ncy s
equence
s c
om
pose
d o
fg
yp
sife
rous o
olitic
, fusulin
id, a
nd
ske
leta
l pa
cksto
ne
-gra
insto
ne
reserv
oir ro
ck w
ith m
old
ic, v
ug, a
nd
fractu
re p
oro
sity. W
e u
sed
wire
line lo
g p
etro
physic
al s
olu
tion
s to
qu
an
titativ
ely
dis
crim
inate
the a
nhydritic
kars
t from
the u
nde
rlyin
g p
acksto
ne-g
rain
sto
ne
stra
ta in
unco
red
we
lls.T
he b
ase
of th
e k
ars
t zo
ne
co
rrespond
s to
a s
harp
de
cre
ase
in s
eis
mic
imp
edance, a
nd 3
D s
eis
mic
data
are
used to
ma
p th
e b
ase
of k
ars
t be
twe
en
we
lls.
Th
e k
ars
t zo
ne e
xh
ibits
hig
h v
aria
bility
in th
ickne
ss; h
ow
ever, th
e z
on
e is
ge
nera
lly th
icke
r on h
igh
er p
ortio
ns o
f a S
E-tre
nd
ing
an
ticlin
e th
at ru
ns th
roug
h th
e s
tudy a
rea, s
ug
gestin
g a
stru
ctu
ral c
ontro
l on
ka
rst d
eve
lop
me
nt. S
eis
mic
da
ta s
ho
w th
at th
e k
ars
t zo
ne
trun
cate
s th
e b
ase
of th
e p
oro
us re
serv
oir in
so
me a
reas a
nd
seis
mic
attrib
ute
s re
vea
l po
ten
tial re
se
rvo
ir co
mp
artm
en
t bo
un
da
ries. B
ette
r unders
tandin
g o
f local k
ars
t co
ntro
l on flu
id flo
w in
this
reserv
oir c
an
imp
rove re
serv
oir m
an
ag
em
en
t de
cis
ion
s. A
BS
TR
AC
T
Pe
trop
hy
sic
al a
nd
Ge
op
hy
sic
al C
ha
rac
teriz
atio
n o
f Ka
rst
in a
Pe
rmia
n S
an
An
dre
s R
ese
rvo
ir, Wa
dd
ell F
ield
, We
st T
exas
Su
san
E. N
issen
, Jo
hn
H. D
oveto
n, a
nd
W. L
yn
n W
atn
ey
12
2
12
Ge
op
hysic
al C
on
su
ltan
t, Mc
Lo
uth
, KS
;K
an
sa
s G
eo
log
ical S
urv
ey, U
niv
ers
ity o
f Kan
sa
s, L
aw
ren
ce, K
S
BA
CK
GR
OU
ND
AN
D P
UR
PO
SE
OF
ST
UD
Y
The s
eq
uence fra
mew
ork
for th
e U
pp
er P
erm
ian (G
uadalu
pia
n) S
an
Andre
s F
orm
atio
n is
based o
n th
e s
tudy o
f surfa
ce e
xposure
s in
the G
uadalu
pe M
oun
tain
s o
f WestTexas.T
he
dolo
mitic
reserv
oir in
Waddell F
ield
is c
om
pris
ed o
f cyclic
oolitic
gra
insto
nes, s
pecific
ally
G8 a
nd G
9 h
igh fre
que
ncy s
equence
s (H
FS
) belie
ved to
be 4
th-o
rder (0
.1-1
Ma
)perio
dic
ity (F
rench a
nd
Kera
ns, 2
004
). In th
e“h
igh v
olu
me a
rea
”, the re
gio
nal s
trata
lbounda
ry th
at s
epara
tes G
8 fro
m G
9 h
as n
ot b
een
pre
cis
ely
loca
ted a
nd tie
d in
to th
ere
gio
na
l stra
tigra
phic
fram
ew
ork
.
The G
8 a
nd G
9 H
FS
reserv
oir in
terv
al o
verlie
s a
regio
nal b
ypass s
urfa
ce c
orre
spondin
g to
the to
p o
f the B
rushy C
anyon in
the o
utc
rop (a
lso e
quiv
ale
nt to
the to
p o
f G4 H
FS
).In
terp
reta
tions o
f surfa
ce e
xposure
s o
f these s
trata
indic
ate
that th
e in
ner a
nd m
iddle
ram
plith
ofa
cie
s d
urin
g th
e G
8 H
FS
underw
ent b
ackste
pp
ing, w
hile
the
G9 H
FS
was in
volv
ed in
forw
ard
ste
ppin
g, re
flectin
g re
lativ
e fa
ll in s
ea le
vel (F
rench a
nd K
era
ns, 2
004).
These
observ
atio
ns, c
ouple
d w
ith s
eis
mic
inte
rpre
tatio
ns o
n th
e e
aste
rn e
dge o
f the C
entra
lB
asin
Pla
tform
(Qife
ng
Dou, p
ers
. co
mm
.), suggest th
at th
e s
ucce
ssio
n o
f oolitic
shoals
inW
addell F
ield
are
com
pris
ed o
f a re
trogra
datio
nal to
aggra
datio
na
l to p
rogra
datio
nal s
trata
l arc
hite
ctu
re re
su
lting in
south
eastw
ard
late
ral a
ccre
tion o
f a s
hallo
w s
helf th
rough W
addell F
ield
tow
ard
the e
aste
rn m
arg
in o
f the C
en
tral B
asin
Pla
tform
.
ST
RA
TIG
RA
PH
ICA
NA
LY
SIS
AN
DP
ET
RO
PH
YS
ICA
LC
HA
RA
CT
ER
IZA
TIO
N
Stra
tigra
ph
ic c
olu
mn
for u
pp
er L
eo
na
rdia
n a
nd
low
er
Gu
ad
alu
pia
n s
ec
tion
in th
e P
erm
ian
Ba
sin
.A
fter R
up
pe
la
nd
Be
bo
ut (2
00
1).
GE
OL
OG
ICA
LS
ET
TIN
G
BB
'
Atim
e s
truc
ture
ma
p o
f the
top
of th
e G
ray
bu
rg fro
m a
3D
se
ism
ic s
urv
ey
ov
er W
ad
de
ll Fie
ld s
ho
ws
the
reg
ion
al
stru
ctu
ral c
on
figu
ratio
n. T
he
bla
ck
bo
x h
igh
ligh
ts th
e“h
igh
vo
lum
e a
rea”
stu
dy
are
a.
Sim
plifie
d g
eo
log
ica
l ma
p o
f the
Sa
nA
nd
res
Fo
rma
tion
in th
eP
erm
ian
Ba
sin
sh
ow
ing
loc
atio
ns
of m
ajo
r oil fie
lds
(Sc
ho
lle,
http
://ge
oin
fo.n
mt.e
du
/sta
ff/sc
ho
lle/g
ua
da
lup
e.h
tml; a
fter W
ard
et a
l.,19
86
). Ou
r stu
dy
are
a is
in W
ad
de
ll Fie
ld, n
orth
ern
Cra
ne
Co
un
ty, T
X, lo
ca
ted
on
the
ea
st c
en
tral fla
nk
of th
e C
en
tral
Ba
sin
Pla
tform
.
Stu
dy
Are
a
Delaware Basin
Cra
ne
Central BasinPlatform
Midland Basin
1 m
i
A
A’
Su
bs
ea
de
pth
ma
p o
f the
top
of th
e S
an
An
dre
s F
orm
atio
n fo
r the
“h
igh
vo
lum
ea
rea
”o
f Wa
dd
ell F
ield
.
Stru
ctu
rally, th
e“h
igh v
olu
me a
rea
”of W
addell F
ield
is lo
cate
d o
n th
e e
ast
centra
l flank o
f the C
en
tral B
asin
Pla
tform
.
The n
orth
west p
ortio
n o
f the
“hig
h v
olu
me a
rea
”is
stru
ctu
rally
hig
her th
an
are
as to
the s
outh
and e
ast, a
lthough
the h
ighest s
tructu
re d
oes n
ot
corre
spo
nd to
the h
ighe
st p
roductio
n.A
south
east-tre
ndin
g p
lungin
g a
ntic
line
appro
xim
ate
ly tw
o m
iles lo
ng c
rosscu
ts th
e s
tudy a
rea, s
ho
win
g a
relie
f of
appro
xim
ate
ly 7
5 ft (2
3 m
) and a
wid
th o
f appro
xim
ate
ly o
ne m
ile (1
.6 k
m).
Ashallo
w s
addle
in th
e c
entra
l mapped a
rea e
xhib
its e
levate
d g
as a
nd to
tal
fluid
pro
ductio
n.
Se
ism
ic p
rofile
ac
ros
s th
e s
he
lfm
arg
in o
f the
Ce
ntra
l Ba
sin
Pla
tform
sh
ow
ing
refle
cto
rc
on
figu
ratio
ns
tha
tle
ad
to th
ein
terp
reta
tion
of th
eG
4 to
top
Sa
nA
nd
res
as
atra
ns
gre
ss
ive
sy
ste
ms
trac
t an
dth
e to
p S
an
An
dre
sto
Gra
yu
rg a
s a
hig
hs
tan
d s
ys
tem
stra
ct.
A-A
’
Gra
yb
urg
To
po
fS
an
An
dre
sG
4u
nco
nfo
rmity
G3
un
co
nfo
rmity
HS
T
TS
T
HS
T
TS
T
A’
A
Cu
mu
lativ
e o
il an
d g
as
(left), c
um
ula
tive
tota
l fluid
(oil+
ga
s+
wa
ter) (c
en
ter), a
nd
cu
mu
lativ
e g
as
(righ
t) ma
ps
inb
arre
ls o
f oil e
qu
iva
len
t (BO
E) s
ho
win
g.A
co
nv
ers
ion
fac
tor o
f 5.7
MC
F g
as
to o
ne
eq
uiv
ale
nt b
bl o
f oil w
as
us
ed
in th
e c
on
ve
rsio
n to
BO
E.
the
“h
igh
vo
lum
e a
rea”
of W
ad
de
ll Fie
ldD
ep
th c
on
tou
rs fo
r the
top
of th
e S
an
An
dre
sF
orm
atio
n a
re s
up
erim
po
se
d.
LIT
HO
FA
CIE
SC
ore
s o
f the u
ppe
r San
Andre
s F
orm
atio
n w
ere
taken fro
m tw
o w
ells
with
in th
e“h
igh
volu
me a
rea
”, W. N
. Waddel #
1204 a
nd W
. N. W
addell #
1261. C
ore
descrip
tions
pro
vid
ed b
y th
e o
pera
tor in
dic
ate
an u
pper k
ars
t inte
rval in
these w
ells
invo
lvin
gm
acro
scopic
colla
pse a
nd c
haotic
bre
ccia
tion a
nd e
xte
nsiv
e a
nhydrite
repla
cem
ent o
fgypsum
in th
e u
pper S
an
Andre
s F
orm
atio
n. In
contra
st, th
e“p
oro
us k
ars
t zone
”and
“kars
ted a
nd b
iotu
rbate
d fu
sulin
id s
hoal”
belo
w th
e m
ain
kars
t zone in
well #
1204 is
less
inte
nsely
kars
ted, w
ithout m
acro
-scale
chaotic
bre
ccia
tion a
nd a
nhydrite
repla
cem
ent.
This
cm
-scale
dis
solu
tion a
nd b
reccia
tion is
recogniz
ed h
ere
as
“mic
ro”
kars
t where
the
matrix
is e
ssentia
lly in
tact.
The m
atrix
pro
pertie
s in
the m
icro
kars
t inte
rvals
are
als
opro
bably
dom
inan
t in te
rms o
f fluid
flow
.T
hus, th
e lo
wer k
ars
ted z
ones in
well #
1204 a
repla
ced in
the in
situ
bedded c
arb
onate
reserv
oir o
f the S
an
Andre
s F
orm
atio
n.
An
ex
am
ple
of th
e re
su
lts o
f the
ka
rstic
zo
ne
me
tho
do
log
y is
sh
ow
nfo
r the
co
red
we
ll #1
26
1 w
ell a
nd
two
ne
igh
bo
ring
we
lls. In
we
ll#
12
61
, the
co
re d
es
crip
tion
su
bd
ivis
ion
of th
e u
pp
er S
an
An
dre
sF
orm
atio
n b
etw
ee
n th
e k
ars
t zo
ne
an
d p
oro
us
, oo
litic s
ho
al fa
cie
s is
clo
se
ly m
atc
he
d b
y th
e z
on
atio
n p
ick
s b
as
ed
on
the
so
nic
an
de
stim
ate
d-a
nh
yd
rite lo
gs
. No
te th
e s
ign
ifica
nt v
aria
tion
in th
ick
ne
ss
of th
e in
terp
rete
d k
ars
t zo
ne
in th
e th
ree
we
lls.
Estim
ate
s o
f the d
epth
of th
e b
ase o
f the lo
w-p
oro
sity
zone a
nd a
nhydrite
phase a
t the to
p o
f the S
an
Andre
s F
orm
atio
n w
ere
com
pile
d fo
r all w
ells
with
density, n
eutro
n, p
hoto
ele
ctric
facto
r, and s
onic
logs. In
wells
where
only
a s
onic
log w
as a
vaila
ble
, the e
stim
ate
was re
stric
ted to
the b
ase o
f the
low
-poro
sity
sectio
n.
Genera
lly, the th
ickness o
f the lo
w p
oro
sity
Kars
t zone is
gre
ate
st o
n th
e s
outh
east-
trendin
g a
ntic
line th
at c
uts
thro
ugh th
e“h
igh
volu
me a
rea
”. How
ever, th
is z
one s
how
shig
h s
patia
l varia
bility, p
unctu
ate
dby a
nom
alie
s w
here
the
"kars
tic" in
terv
al is
eith
er m
ark
edly
thic
k o
r thin
. In a
few
wells
,th
ere
was n
o e
vid
ence o
f a lo
w-p
oro
sity,
anhydritic
zone a
t the to
p o
f the S
an
Andre
sand th
is w
as in
terp
rete
d a
s a
n a
bsence o
fkars
t develo
pm
ent.
1207
1228 1
204
1261
B’
B
Iso
pa
ch
ma
p o
f up
pe
r tigh
t zo
ne
of S
an
An
dre
s F
orm
atio
n in
terp
rete
d a
s k
ars
t,w
ith to
p o
f Sa
nA
nd
res
Fo
rma
tion
su
bs
ea
de
pth
co
nto
urs
su
pe
rimp
os
ed
.
