October, 1999
HGS Meetings

HGS Dinner Meeting

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

Abstract:

We examine the petroleum potential in onshore Texas of 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 Sligo Formation. Extension of the favorable stratigraphy of the Edwards Formation beyond the commonly recognized margin presents abundant opportunities for future exploration in the Edwards. Our work also shows the Sligo Formation to be 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

Our analysis of exploration concepts focused on the Early Cretaceous Edwards and Sligo margins in an area of east central Texas proximal to the San Marcos Arch (Figure 1a). The area was chosen for its structural configuration, which could potentially focus petroleum. Margins of the Edwards are coincident in a few areas but diverge repeatedly along the Sligo trend (Figure 1b). 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) beyond 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 sequences occur in the interval from the Cotton Valley Formation to the top of the Edwards. The sequence boundary at the top of our Sequence 5 in the upper Sligo Formation is equivalent to the 112 MY sequence boundary of Goldhammer et al. (1991) and marks a key period of erosion and possible deposition 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 corresponding geologic cross-section show the progradational nature of the Edwards Formation (Figures 3, 4). Three wells are included (Figure 3): the Mobil Kahanek #1 (Bebout and Kupecz, 1985) is projected 13 miles (21 km) along depositional strike from Word Field, while the Chevron Coby #1 and Exxon Joe Zaruba #1 are on the arbitrary line. Sonic or density logs and synthetic seismic traces tied the 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 1 is a high-amplitude event downdip that diminishes in strength updip (Figure 3). It occurs at the top of a section of deeper water argillaceous wackestones (Upper Tamaulipas) and is equivalent to the sequence boundary ' at the top of Sequence 8 (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 4).

Reflector 2 (Figure 3) is a weak event that ties lagoonal packstone/grainstones in the Kahanek cores to a reef and bank complex 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 wackestone/packstones of the Kahanek cores to reef and backreef grainstones in the Coby cuttings (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 packstone/wackestones 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 ultimately culminates seaward of the Zaruba well. Correlations on figures 3 and 4 delineate prospective 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 recognized in the upper part of the Sligo in outcrops in Mexico (Goldhammer et al., 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 1a) 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 migration 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 for part of a downslope debris wedge in excess 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-grainstone 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 drilled in Louisiana, e.g. Union #1 Kirby (Tyrrell 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 Mexico and has yet to be tested.

' References Cited:

Bebout, D. G. and J. A. Kupecz, 1985, Lower Cretaceous Stuart City trend facies and environments, Mobil No. 1 Kahanek core, Lavaca County, Texas, in Bebout, D. G. and D. Ratcliff, (eds.), Lower Cretaceous depositional environments from shoreline to slope — a core workshop: Austin, Gulf Coast Association Geological Societies Annual Meeting, p. 55-63.

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

Fritz, D. A., T. W. Belsher, J. M. Medlin, J. L. Stubbs, R. P. Wright, and P. M. Harris, New Exploration Concepts for the Edwards and Sligo Margins, Cretaceous of Onshore Texas: American Association of Petroleum Geologists Bulletin, in press.

Goldhammer, R. K., P. J. Lehmann, R. G. Todd, J. L. Wilson, J. L., W. C. Ward, 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, in Bally, A. W., (ed.), Atlas of seismic stratigraphy, AAPG studies in geology, no. 27. v. 3: Tulsa, American Association of Petroleum Geologists, p. 11-17.

Figures

Note: Graphics for figures will be added to this web page when available

Figure 1. (a) Regional map of Gulf of Mexico with Cretaceous Edwards and Sligo margins in the northern portion and an undifferentiated margin in the south. Intrashelf basins and the San Marcos Arch paleohigh are shown in the northern Gulf rim, as are two major isolated carbonate platforms in the south. (b) Detailed map of northern rim of the Gulf of Mexico with the trend of the Cretaceous Edwards and Sligo shelf margins, the study area in Lavaca County, Texas, and Cretaceous production around the study area.

Figure 2. 2-D seismic line with gross stratigraphic surfaces and intervals. Colored reflectors show the nature of the bounding stratigraphic surfaces. Two plays are in the Edwards Sequence 9 and Sligo Sequence 6.

Figure 3.3-D seismic arbitrary line of Edwards stratigraphy. Three wells located on the seismic line are correlated in Figure 4. The sequence boundary in the upper Sligo Formation (boundary between Sequences 5 and 6, Figure 2) is picked toward the base of the seismic line. Four sequence boundaries (reflectors 1-4) are identified in the Coby well and correlated across the seismic line. Edwards sequences bounded by the four reflectors occur in Sequence 9 (Figure 2). The four seismic sequence boundaries are the basis for the well correlation in Figure 4.

Figure 4. Cross-section of Edwards facies relationships between three wells. Line of section is the same as that of the seismic line of Figure 3. The Mobil Kahanek #1 well SP log is left of the depth track (in feet) and a resistivity log is to the right. Conventional core was described from 13,670-14,340 ft. Rock types in the core are summarized in the right column; boundstones and packstones, from approximately 14,100 ft to the bottom of the core, are overlain by packstones and grainstones. Gamma ray (left) and sonic (right) logs are shown for the Chevron Coby #1 well. Facies observed in drill cuttings between 14,200-16,000 ft in the Edwards Formation are in the right column. For the Exxon Zaruba #1, the gamma ray is on the left with resistivity on the right. Core description from 14,280-14,500 ft is in the right column; grainstones and packstones overlie boundstones at the base of the core.

Figure 5. 3-D seismic line showing nature of exploration opportunity in Sligo forereef and slope. Downslope debris intervals seaward of the Sligo margin are between a sequence boundary in the upper Sligo and the top Pearsall boundary. Slope debris displays onlap and downlap terminations. Compare the same sequence boundary and Sligo debris with Figures 2 and 3.

Biographical Sketch:

Posters:

Terry W. Belsher, James M. Medlin, John L. Stubbs, Robert P. Wright, Chevron U.S.A. Production Company, Houston, TX,
and Paul M. (Mitch) Harris, Chevron Petroleum Technology Company, Houston, TX

Speaker

Dale A. 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 Wisconsin - Milwaukee 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 Exploration 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 development of high NPV prospects from concept through production. Poster Session

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

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 diagenetic problems that pertain to carbonate reservoirs and exploration 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.

Environmental / Engineering Dinner Meeting

"Evolution of Fuel Systems and their impact on the environment"

Abstract:

The history of fuel systems will be presented from the use of visigauge pumps to the latest hands-free robotic fueling. The technical discussion of the historical design of underground fueling systems will guide a discussion of how governmental regulations were modified based on the design changes. Financial implications will be discussed as the markets have changed from "mom and pop" stores to markets dominated by oil company stations and convenience stores.

Biographical Sketch:

J. Curtis DuPriest has been installing and designing fuel systems for over 50 years. He began in the early 1950s by working with his father on fuel system installations and parts distribution. He left to work with Shell Oil Company as the construction and maintenance supervisor for the southwest region. Under appointment by the Texas governor, Mr. DuPriest spent six years assisting in the design of the Texas Water Commission, which became the Texas Natural Resource Conservation Commission (TNRCC). Currently he is automating the fuel systems for the City of Houston through the use of file servers and host PC computers.

International Dinner Meeting

"Salt-tectonics provinces and superposed deformation across the continental-oceanic boundary in offshore Angola"

Poster Session

Abstract:

The Angolan margin is the type area for raft tectonics. New seismic data reveal the contractional buffer for this thin-skinned extension. A composite section from the Lower Congo Basin and Kwanza Basin illustrates a complex history of superposed deformation caused by (1) progradation of the margin and (2) episodic Tertiary epeirogenic uplift. Late Cretaceous tectonic movement was driven by a gentle slope created by thermal subsidence. Extensional rafting took place updip, contractional thrusting and buckling downdip. Some distal folds were possibly unroofed to form massive salt walls.

Oligocene deformation was triggered by kinking of the Atlantic Hinge Zone as the shelf and coastal plain rose by 2 or 3 km. Uplift stripped Paleogene cover off the shelf, provided space for Miocene progradation, and steepened the continental slope, triggering more extension and buckling.

In Neogene time a subsalt half-graben was inverted, creating keystone faults that may have controlled the Congo Canyon. A thrust duplex of seaward-displaced salt jacked up the former abyssal plain, creating a plateau of salt 3–4 km thick on the present lower slope. The Angola Escarpment may be the toe of the the Angola thrust nappe, in which a largely Cretaceous roof of gently buckled strata may have been transported above the salt duplex as far as 1–20 km.

Biographical Sketch:

Martin Jackson's early career interests included lunar structures, mineral exploration, and Precambrian geology. He received his Ph.D. from the University of Cape Town in 1976. He joined the Bureau of Economic Geology at the University of Texas at Austin in 1980, where he currently directs the Applied Geodynamics Laboratory, funded by a consortium of oil companies. A recipient of AAPG's Sproule, Matson, and Dott awards, he lectured in AAPG's Structural Geology School, was an AAPG Distinguished Lecturer, and served 6 years as Associate Editor for AAPG Bulletin and GSA Bulletin.

Vendor's Corner

Western Geophysical

Samples of new 2D and 3D seismic data over open acreage in hot new areas of Angola
Contact John McTernan 713-689-6812

GETECH (Geophysical Exploration Technology Inc.) DIGs (Dickson International Geosciences)

Latest gravity maps and interpretation from newly available Volume 3-West Africa of the SAMBA (South Atlantic Basin Margins Analysis) Project and new 3D modeling of salt diapir
Contact: Mark Odegard (281) 240-0004

North American Exploration Dinner Meeting

"Deltaic systems: perspectives on facies models and sequence stratigraphy"

Abstract:

Deltas were among the first depositional systems built into widely utilized facies models. Elegant process-response classifications of Holocene examples as river, wave, or tide-dominated provided the building blocks for actualistic models. The advent of sequence stratigraphy has changed this simple view of deltas.

Deposition and preservation of reservoir, source, and seal facies is better viewed in the context of continual deltaic evolution, reflecting both autocyclic and allocyclic forcing factors. During falling base level, delta switching is minimal and shelf-phase deltas prograde rapidly. These deposits typically lack well-developed transgressive phases and may become progressively wave-dominated during the overall fall, as decreasing shelf-width lessens frictional attenuation of wave energy. Fluvial systems erode into and cannibalize the deltas, creating incised valleys.

At eustatic low stand, deltaic systems are fixed at the heads of the valleys, at or near the shelf margin. These shelf-margin deltas can reach significantly greater thicknesses, are greatly modified by mass movement processes, and deliver sediment to deeper water. Rising base level forces deltaic systems to retrograde, producing estuaries and backstepping shelf-phase deltas. Embayments created by transgression can enhance tidal effects, yielding tide-dominated deltas. In more fluvially dominated systems, delta switching becomes more important. Delta complexes, unique in space but not necessarily in time, may develop simultaneously in different areas of the delta plain. Transgressive deposits make up a significant portion of the stratigraphy. Understanding these phases of deltaic evolution has produced better tools for more accurately interpreting the ancient record.

Originally presented as: Suter, J. R., and D. Nummedal, 1999, Thirty years of evolution of deltaic facies models, AAPG-SEPM Annual Meeting, San Antonio, Texas.

Biographical Sketch:

John R. Suter is a Research Associate with the Predictive Stratigraphy Group of the Integrated Interpretation Center of Conoco, Inc. in Houston, Texas. Suter earned Bachelors' (1977) and Master's (1980) degrees in geology from the University of Texas at Austin, and a Ph.D. (1986) in geology from Louisiana State University. He spent his formative geologic years with the USGS and the Louisiana Geological Survey, working primarily on the effects of sea level fluctuations on the evolution of the continental shelf and shorelines of the northwest Gulf of Mexico and the Mississippi Delta. Following this stint in modern environments and marine geology, Suter took employment in the petroleum industry in 1989 as a sequence stratigrapher with Exxon Production Research, and joined Conoco in 1994. He has published extensively on clastic facies and sequence stratigraphy and taught numerous short courses and field schools on these subjects. Suter received the SEPM Excellence of Presentation Award in 1986 and 1987, GCAGS Best Published Paper Award in 1989, Best of AAPG for SEG in 1985, and the AAPG Jules Braunstein Memorial Award in 1986 and 1995.

HGS Lunch Meeting

"A re-evaluation of the Hackberry - new life for a comatose trend"

Abstract:

The Hackberry trend of southeast Texas and southwest Louisiana has posed one of the Gulf Coast's most perplexing targets for petroleum exploration. Thick pay with outstanding reservoir quality at moderate drilling depths has lured many explorationists to try to unlock its potential. However, reservoir extent has, in the past, been unpredictable, at least in part as a result of multiple unconformities and poor structural and stratigraphic resolution. Economic success was elusive, and the play was all but abandoned during the late 1980s and early 1990s.

A regional re-evaluation of the geologic model arose from the assimilation of well log correlations, dipmeter use and evaluation, interpretation of paleo reports, and the combination of quality 2-D seismic data throughout Jefferson and Orange Counties, Texas, and Calcasieu Parish, Louisiana. From this work it became very apparent that the majority of the trend should be interpreted as a subunconformity/slump block play and not a deep-marine, basin-floor/turbidite sequence. The new model predicted reservoir extent and thickness concisely and logically, not as an artistic contouring exercise.

The history of the basin is one of prograding shallow-marine deposition rising out of the lower Frio Formation through the Nodosaria blanpiedi and Nonion struma zones, followed by thick shallow-water deposition of up to 500 feet of strand plain sands in the upper Frio Formation. Basin collapse, resulting from regional salt withdrawal and development of salt domes caused over-steepening of the sea floor, resulting in slumping within the soft sediments from the Vicksburg Formation through the early strand plain deposits. Wave-base erosion removed the majority of the youngest sand section. It was followed by marine shale deposition containing a diagnostic Hackberry faunal assemblage. As subsidence ceased and the basin finally filled with shale, shallow-marine strand plain deposition continued through deposition of the upper Frio section.

Armed with this geologic model, a group of industry partners partook to gather regional 3D seismic coverage in order to take advantage of greater than 500 undrilled square miles that still existed in the trend. Eventually the effort resulted in 24 successful wells out of 29 attempts. Additional information from over 520 square miles of 3D seismic data and our drilling shows that the model developed earlier has been highly accurate and very helpful in predicting reservoir age and extent and production performance of individual wells. work has renewed the interest in exploration throughout this play and resulted in more than 900 square miles of 3D data acquisition and an additional 40 wells being drilled by other oil and gas companies.

Biographical Sketch:

Michel Puzio is the geologist at Mayne & Mertz, Inc., Houston, Texas. He received his Bachelor of Science degree in geology and attended graduate school at Michigan State University in East Lansing, Michigan.

Mr. Puzio's career started at Amoco in 1980, where he worked on the Lobo trend of the Wilcox and the Vicksburg of the middle Texas coast. From 1984 until 1992 he worked for Bartell Exploration in Houston, where he began his work in the Hackberry trend. He joined Mayne & Mertz when they opened an exploration office in Houston.

LSU Basin Research Institute

"Technology + Creativity = Winning Solutions in E&P"