Santa Fe Srlydcr- Corporntion, Horrston, T X , Terry W. Belsher, James M. Medlin, John L. Stubbs, Robert £? Wright, Chevrorz U.S.A. Product io~ Cotnparry, Housto?z, TX, and Paul M. (Mitch j Harris, Chevrorl Pe t ro l~wr~~ TechoIogy Company, Houston. T S

New exploration plays for Edwards and Sligo Cretaceous margins: untested opportunities in onshore Texas

Abstract

W e examine the petroleum potential in onshore Texas o f

the most prolific reservoirs found to date in the Gulf of Mexico Basin, Cretaceous carbonates, particularly the Edwards and Sligo Formations. Combining 2-D and 3-D seismic data with lithologic and biostratigraphic information enabled the generation of a detailed sequence stratigraphic framework that led to a concentrated effort in Lavaca County, a redefinition of the Edwards shelf margin, and confirmation of a major sequence boundary in the S l i p Formation. Extension of the favorable stratigraphy of the Edwards Formation beyond the commonly recognized margin presents abundant opportunities for future exploration in the Edwards. O u r work also shows the Sligo Formation to he a well-defined aggradational margin that underwent a major period of exposure, resulting in deposition of a series of downslope debris wedges. To our knowledge, the deposits have not been drilled in the United States and present exciting exploratory possibilities.

Introduction O u r analysis of exploration concepts focused o n the Early Cretaceous Edwards and Sligo margins in an area of east central Texas proximal to the San Marcos Arch (Figure la). The area was chosen for its structural configuration, which could poten- tially focus petroleum. Margins of the Edwards are coincident in a few areas but diverge repeatedly along the Sligo trend (Figure lb) . Although the Edwards margin is commonly mapped as coincident with that of the Sligo in the study area, our work clearly demonstrates that the margin actually progrades seaward (southeast) 3 or more miles (>5 km) bcyond the Sligo margin.

An interpreted 2-D seismic line illustrates the architecture of the Cretaceous shelf and margin in the study area (Figure 2). Nine

hequences occur i n the interval f r o m t h e C o t t o n Valley Formation to the top of the Edwards. The sequence boundary at the top of our Sequence 5 in the upper Sligo Formation is equiv- alent to the 112 MY sequence boundary of Goldhammer et dl. (1991) and marks a key period of erosion and possible dcposi- tion of coarse debris downslope. The boundary between Sequences 6 (uppermost Sligo) and 7 (Pearsall) is interpreted to be a drowning unconformity. At the top of the Edwards, the Sequence 9 boundary coincides with the 98 MY sequence boundary of Goldhammer et al. (1991) and is interpreted to be a time of dissolution of coarse backreef grainstones.

The Edwards - An Underdrilled Opportunity An arbitrary line through a 3-D seismic volume and a corre- sponding geologic cross-section show the progradational nature of the Edwards Formation (Figures 3,4). Three wells are includ- cd (Figure 3): the Mobil Kahanek PI (Bebout and Kupecz, 1985) is projected 13 miles (21 km) along depositional strike from brord Field, while the Chevron Coby #I and Exxon Joe Zaruba -%I are on the arbitrary line. Sonic o r density logs and synthetic seismlc traces tied thc wells to the seismic data. Four seismic

reflectors, labeled 1-4, are interpreted as sequence boundaries from stratal geometries. Three Edwards sequences defined by the four boundaries occur within Sequence 9 on the 2-D seismic line (Figure 2 ) .

Four key seismic reflectors in the Edwards Fm. (Figure 3) were used to assist in correlating the wells on the cross-section (Figure 4). Reflector : is a high-amplitude event downdip that dimin- ishes in strength updip (Figure 3). It occurs at the top of a sec- t ion o f deeper water argi l laceous wackestones ( U p p e r Ihmaulipas) and is equivalent to the sequence boundary +

HGS Dinner Meeting Monday, October 11 Westchase Hilton, 9999 Westheimer Social 5:30 p.m., Dinner 6:30 p.m.

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E D W A R D S SHELF TREND I

Figure 1. (a) Regional map of Gulfof Mexico with Cretaceous Edwards and Sligo margins in the northern portion and an undzfereniiated margin in the south. Intrashelfbasins and the San Marcos Arch paleohigh are shown in the northern Currim, as are two major isolated carbonateplatforrns in the south. (b) Detailed map of northern rim of the Guyof Mexico with the trend of the Cretaceous Edwards and Sligo shelfmargins, the study area in Lavaca County, Texas, and Cretaceous production around the study area.

at the top of Sequence Y (Figure 2). Reflector 1 is immediately overlain by a prograding reef and bank complex of the Edwards margin as observed in cores from the Kahanek well and distal slope wackestones described from cuttings of the Coby well (Figure 1).

Reflector 2 (Figure 3) is a weak event that ties lagoonal pack- stonelgrainstones in the Kahanek cores to a reef and bank com- plex in the Coby cuttings (Figure 4). We infer that this reflector ties to forereef and slope deposits in the Zaruba well.

Reflector 3 (Figure 3) ties backreef wackestonelpackstones of the Kahanek cores to reef and backreef grainstones in the Coby cut- tings (Figure 4). We interpret from the logs of the Zaruba well that a reef and minor forereef succession occur in the interval.

Reflector 4 (Figure 3) ties backreef deposits in the Kahanek well (inferred from logs and observed in the equivalent interval in other Word Field cores) to backreef packstone1wackestones in cuttings from the Coby well. The same deposits are correlated to reef and backreef grainstones in cores from the Zaruba well. Reflector 4 is equivalent to the sequence boundary at the top of Sequence 9 (Figure 2). Clearly, the top Edwards interval between reflectors 3 and 4 represents a progradational package that ulti-

mately culminates seaward of the Zaruba well. Correlations on figures 3 and 4 delineate prospecti.ve grainstone packages 3 miles (4.8 km) or more seaward of the published Edwards margin.

The impact of our findings is considerable. We have defined a virtually unprospected area of the Edwards with significant potential for new gas reserves. The highly progradational nature of the Edwards margin places prospective backreef and reef grainstones well seaward of the recognized margin. Comparison of the 2-D seismic expression of Edwards Sequence 9 (Figure 2) with the three Edwards sequences identified on the 3-D data (Figure 3) allowed us to develop a high-resolution sequence stratigraphic framework that reveals the location of favorable facies. Combining these data with the likelihood that faulting (Figure 3) can create avenues for development of secondary porosity in the Edwards and charge the system with petroleum from deeper source rocks (Fritz et al., in press), it is evident that numerous opportunities for exploratory drilling exist. In Lavaca County alone, the new fairway of opportunity is over 3 miles (4.8 km) wide and 25 miles (40 km) long.

Sligo Forereef - An Untested Opportunity A Sligo debris play is based on a sequence boundary recognizec in the upper part of the Sligo in outcrops in Mexicc

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(Goldhammer et ai., 1991). We interpret the same sequence boundary on seismic data and recognize a wedge geometry downslope from the Sligo margin (Fritz et al., in press). Base- level change about the sequence boundary would have caused coarse carbonate debris- and grain-flow deposition seaward of the Sligo shelf margin. Data from analogous reservoirs confirm that downslope carbonates can retain reservoir-quality porosity, e.g., Poza Rica Field in east-central Mexico (Figure la) described by Enos (1985). Facies variation and slump-faulting on the foreslope create potential for trapping in proximity to deepwater carbonates, setting up a petroleum source and migra- tion pathway.

Figure 5 is a 3-D seismic line illustrating the major sequence boundary in the Sligo margin. The same boundary is identified on 2-D (sequence boundary at top of Sequence 5, Figure 2) and 3-D seismic lines (Figure 3). Several events on the seaward side of the margin have onlap and downlap reflector terminations (Figure 5) and display the proper architecture tor part of a downslope debris wedge in wess of 1000 ft (300 m) thick The Sligo debris wedge (Figure 5) is also visible on the 2-D data (Figure 3; downslope portion of sequence 6).

Existence of a Sligo downslope wedge between the sequence boundary and the overlying Pearsall Shale does not guarantee the presence of coarse-grained material. We postulate there were two primary sources of carbonate debris production during the

sea level lowstand associated with the sequence boundary that would have supplied coarse sediment to the wedge: erosional retreat of the shelf margin, and in-place growth of reef-grain- stone environments on the slope. The lowstand created a period of instability, causing transport of coarse breccia and grainstone farther downslope by debris flows and sediment gravity flow, resulting in the formation of a debris wedge at the base of slope. During the subsequent transgression and relative highstand, the Sligo shelf margin kept up with sea level rise and continued to contribute grainstone debris downslope (included in Sequence 6, Figure 2). Uppermost Sligo deposition corresponds to the Cupido Formation of northern Mexico (Goldhammer et al., 1991). Rapid deposition of the downslope carbonates may have helped preserve primary porosity by limiting the amount of marine cementation. Eventually, the Sligo shelf margin was flooded by a major transgression represented by the Pearsall, which could provide a petroleum seal.

3-D seismic is critical in defining and properly testing a target of this type. A few wells driUed in Louisiana, e.g. Union #I Kirby ('If.rrell and Scott, 1989), may have penetrated the distal end of the Sligo debris wedge, but the debris wedge itself apparently has not been drilled. The Sligo forereef and slope play is regional in extent throughout the northern rim of the Gulf of Mexiw and has yet to be tested.

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-re 2. 2 -0 seismic line with gross matigraphic surfaces and intervals. Colored reflectors show the nature of the boundingstratigraphic surfaces. Zlw plays are in the Edwards Sequence 9 and Sligo Sequence 6.

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4. Cross-s&n of Edwards fa& relationships between three wells. Line of section is the same as that of the seismic line of Figure 3. The Mobil Kahanek #I well SP log is kft of the depth track (in feet) and a resistivity log is to the right. Conventional core was describedfiom 13,670-14,240fi Rock types in the core are summarized in the right column; boundstone and packstones, fiom approximately 14,la)ft to the bottom of the core, are overlain by packstones and grainstones. Gamma ray (left) and sonic (right) logs are shown for the Charon Coby #I well Facies o b s m d in drill cuttings between 14,200-16,000ft in the Edwards Formation are in the right column. For the Earrm Zaruba #1, thegamma ray is on the kft with resistivity on the right. Core descriptionfiom 14,280-14,500fr is in the right column; grainstones and packstones overlie boundstones at the base of the core.

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F&we 5. 3-0 seismic line showing nature of exploration opportunity in Sligoforereefand slope. Downslope debris intervals seaward of the Sligo margin are between a squence boundary in the upper Sligo and the top P e a d l boundary. Slope debris displays onlap and downlap terminations. Compare the same squence boundary and Sligo debtis with Figures 2 and 3.

B i i i l Sketches Speaker Dale A. Fritz Dale Fritz is a Senior Staff Earth Scientist in the Gulf Division of Santa Fe Snyder Corporation of Houston, Texas. He received a B.S. and M.S. in geology from University of Wkconsin - M h u k e e in 1980 and 1982, respectively. he joined Chevron for 17 years. During his exploration career with Chevron, he spent ten years working the offshore Gulf of Mexico, several years on the EkpIoration VP's staff and the last five years working onshore plays in Texas. He recently accepted a position with the Santa Fe Snyder Corporation. He is keenly interested in the pursuit and develop- ment of high NPV prospeds from concept through production.

Poster S-ion Terry W. Belsher is a Senior Earth Scientist in the Midcontinent Business Unit of Chevron North America Exploration and Production Company in Houston, Texas. He joined Chevron in 1980 after receiving a B. S. degree in geology from Stephen F. Austin State University. Assignments have included exploration and development projects focused on Ordovician through Permian carbonate reservoirs of the Permian Basin, and the Jurassic and Cretaceous sections of the Texas and Mississippi Gulf Coast regions.

Paul M. (Mitch) Harris Mitch Harris is a Senior Staff Research Geologist with Chevron Petroleum Technology Company in Houston, Texas. He does carbonate technical support projects, consulting, and training for the various operating units of Chevron. His work during the last

22 years has centered on facies-related, stratigraphic, and diage- netic problems that pertain to carbonate reservoirs and explo- ration plays in most carbonate basins worldwide. Mitch received his B. S. and M. S. degrees from West Virginia University and Ph.D. from the University of Miami, Florida. He has published numerous papers, edited several books, and is active in AAPG and SEPM.

Bebouf R G. and J. A. Kupecz, 1985, Lower Cretaceous Stuart City trend hcies and environments, Mobil No. 1 Kahanek core, Lama County, Texas, in Bebout, D. G. and D. Ratdiff, (eds.), Lower Cretaceous depositional environments From shoreline to slope - a core worhhop: Austin, Gulf Coast Association Geological h i e t i e s Annual Meeting. p. 55-63.

Enos, Paul, 1985, Cretaceous debris reservoirs, Poza Rica field, Veracruz, Mexico, Roehl, P. 0. and P. W. Choquette, (eds.), Carbonate petroleum reservoirs: New York, Springer-Verlag, p. 457-469.

%t~,IX&T.W.&bha,J.M.M&J.LStubbg.RP.Wright,andP.M.Hnrriq New Exploration Concepts for the Edwards and Sligo Margins, Cretaceous of Onshore Texas: American Association of Petroleum Geologists Bulletin, in press.

Goldhamn#r,RK.,P.J.LchmsnnRG.Todd,J.LWhn,J.L,W.CWard, and C. R Johnson, 1991, Sequence stratigraphy and cyclostratigraphy of the Mesozoic of the Sierra Madre Oriental, northeast Mexico: Houston, Gulf Coast Section Society of Economic Paleontologists and Mineralogists Foundation, field trip guidebook, 86 p.

Tyrrell, W. W., Jr. and R. W. Scott, 1989, Early Cretaceous shelf margins, Vernon Parish, Louisiana, h Bdy, k W., (ed.), Atlas of seismic stratigraphy, AAPG studies in geology, no. 27. v. 3: Tulsa, American Association of Petroleum Geologists, p. 1 1 - 17. 0

October 1999