Understanding growth-faulted, AUTHORS L. Frank Brown Jr. Bureau of Economic Geol- intraslope subbasins by ogy, John A. and Katherine G. Jackson School of Geo- sciences, University of Texas at Austin, University Station Box X, Austin, Texas 78713-8924; applying sequence-stratigraphic [email protected] Frank Brown received his B.S. degree in geology and principles: Examples from chemistry from Baylor University in 1951 and his M.S. degree and his Ph.D. from the University of Wisconsin, Madison, in 1953 and 1955, respectively. Frank worked the south Texas Oligocene for Standard Oil of Texas (Chevron) in 1955–1957, the Bureau of Economic Geology (BEG) in 1957–1960 Frio Formation and 1966–1989, and as an international consultant in 1989–1999. From 1960 to 1966, he was associate L. Frank Brown Jr., Robert G. Loucks, Ramo´ n H. Trevin˜o, professor at Baylor University. He was professor of geo- logical sciences at the University of Texas at Austin and Ursula Hammes in 1971–1989 and emeritus professor in 1989–1999. Since 1999, he has been a research professor at BEG, where he continues his studies of the sequence stra- tigraphy of the Gulf Coast of Texas and Mexico. ABSTRACT Robert G. Loucks Bureau of Economic Geology, A detailed analysis of Oligocene Frio Formation intraslope, growth- John A. and Katherine G. Jackson School of Geosciences, faulted subbasins in the Corpus Christi, Texas, area indicates that University of Texas at Austin, University Station Box X, Austin, Texas 78713-8924; [email protected] deposition during relative lowstands of sea level was the main ini- tiator, or trigger, of growth faulting. Lowstand depocenters on the Robert Loucks is a senior research scientist at the Bureau of Economic Geology, working on siliciclastic low-gradient, upper continental slope comprising basin-floor fan and carbonate reservoir characterization. He was facies, slope-fan systems, and prograding lowstand delta systems the recipient of the 1999 AAPG Wallace E. Pratt Me- exerted sufficient gravity stress to trigger major sections of outer morial Award for Best Paper, the 1982 SEPM Excel- shelf and upper slope strata to fail and move basinward. The faults lence of Presentation Award, and the 1991 SEPM Excellence of Poster Presentation Award. Bob served sole out deep in the basin, and rotation of hanging-wall blocks mo- as the Mideast AAPG Dean A. McGee International bilized deep-water muds and forced the mud basinward and upward Distinguished Lecturer in 1999. to form mud (shale) ridges that constitute the basinward flank of Ramo´ n H. Trevin˜o intraslope subbasins overlying footwall fault blocks. Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geo- Sedimentation associated with third-order relative falls of sea sciences, University of Texas at Austin, University level produced load stress that triggered a major regional syndeposi- Station Box X, Austin, Texas 78713-8924; tional growth-fault system. Subbasins on the downthrown side of [email protected] each arcuate fault segment that constitute a regional fault system are Ramon Trevin˜o received his B.S. degree in geology filled during the lowstands of sea level. Consequently, genetically (Texas A&I University, 1983) and his M.S. degree in similar but noncontemporaneous lowstand depositional systems geology (University of Texas at Arlington, 1988). He worked for Mobil from 1988 to 1992 and received filled each successive growth-faulted subbasin trend. The subbasin an M.B.A. degree from the University of Oklahoma stratigraphy becomes younger basinward because the subbasin de- in 1994. Since 1995, he has worked on sequence- velopment and fill process extended the Frio shelf edge stepwise stratigraphic reservoir characterization at the Bureau into the Oligocene Gulf of Mexico Basin, coinciding with relative of Economic Geology. third-order sea level cycles. Ursula Hammes Bureau of Economic Geology, The subbasins have been prolific petroleum targets for decades John A. and Katherine G. Jackson School of Geo- and are now the focus of prospecting for deep gas. Lowstand sand- sciences, University of Texas at Austin, University Station Box X, Austin, Texas 78713-8924; stones are principal reservoirs, and synsedimentary tectonics produced [email protected] Ursula Hammes obtained her diploma in geology from the University of Erlangen, Germany, in 1987, and her Ph.D. from the University of Colorado at Boulder in Copyright #2004. The American Association of Petroleum Geologists. All rights reserved. 1992. She spent 10 years in industry and joined the Manuscript received February 23, 2004; provisional acceptance May 6, 2004; revised manuscript Bureau of Economic Geology in 2002 as a research received June 11, 2004; final acceptance July 1, 2004. AAPG Bulletin, v. 88, no. 11 (November 2004), pp. 1501–1522 1501 associate. Her main research focus is in clastic and anticlinal and fault traps and associated stratigraphic pinch-out carbonate sequence stratigraphy, depositional sys- traps on the flanks of the structures. Understanding the origin of tems, and image analysis. the faulted subbasins and their chronostratigraphic relationships and depositional processes provides a perspective that can improve ACKNOWLEDGEMENTS deep gas exploration. The State of Texas Advanced Resource Recovery Pro- gram supported this research. Patricia Montoya and Randy Remington, geophysical associates at the Bu- INTRODUCTION reau of Economic Geology, contributed to the work on this project. Mike Pawelek of IBC Petroleum, Inc., Deltaic and other associated coastal reservoirs in the study area Gary Biesiedecki and Gary Miller of SABCO Oil and Gas Corporation, and Matt Hammer of Royal Explo- (Figure 1) exhibit closures provided by rollover anticlines and shale ration Company, Inc., provided industrial support. ridges inferred to have been activated by sediment discharge of river We especially extend our gratitude to WesternGeco systems and consequential progradation of deltaic systems. Complex for use of seismic data. We thank David Jennette, extensional fault systems associated with these anticlines also pro- Susann Doenges, and David Stephens, respectively, for critical reviews, technical editing, and graphics vide abundant fault-trap opportunities. support during manuscript preparation. Sediment composing major global delta systems was supplied Major credit for ideas expressed in this paper are by the world’s principal rivers, which, during the Tertiary, discharged due to Robert M. Mitchum Jr., John Sangree, and along thinned continental crustal margins (e.g., Niger, Amazon, and John C. Van Wagoner and their former Exxon ancestral Mississippi). Tertiary reservoirs and synsedimentary struc- associates, including Peter R. Vail and Henry W. Posamentier, for their original pioneering research tures are the principal hydrocarbon components in the Niger Delta in the field of siliciclastic sequence stratigraphy. and northern Gulf of Mexico Basin. Many smaller fluvial systems These workers all contributed significantly not only also entered the northwestern margin of the Gulf of Mexico Basin to the entire field, but also to the valid, original throughout the Tertiary. Most structures associated with these del- static sequence model. taic systems are syndepositional, but postdepositional reactivation In addition, although conceptual differences exist be- of faults also produced traps for younger, onshelf, deltaic, and coastal tween our sequence ideas and inferred processes pro- posed earlier by William E. Galloway and his associates facies. The term ‘‘onshelf’’ is used herein instead of simply ‘‘shelf’’ and students, we are deeply indebted to them for pro- to avoid connoting sediment deposited under actual shelf environ- viding a regional perspective of the character and ments (i.e., below maximum storm-wave base). Wire-line log and distribution of Frio depositional systems. Our differ- various vintages of two-dimensional (2-D) seismic data guided most ing views are based on data and concepts unavailable of the early exploration directed toward identifying shallow-buried to them and other prior workers who concentrated on the Tertiary systems of the Texas Gulf Coast Basin. shelf structures. To all of the numerous earlier contributors to this vital Long-term oil and gas production in the Gulf of Mexico Basin petroleum region, we extend our professional grati- has significantly depleted reserves in traditional onshore and offshore tude for their research efforts. For example, early work fields producing from shallow to medium depth ranges (i.e., 6000– by both Merle C. Israelsky beginning in 1935 and 12,000 ft [2000–4000 m]) associated with regional growth-fault S. W. Loman in 1949 contributed documentation of the systems (Figures 1–3). Nonetheless, deep gas reserves exist, and cyclicity and deltaic facies characterizing the Tertiary rocks of the Gulf of Mexico Basin. These two geolo- because of increased demand and improved gas economics, these gists, among many others, such as pioneer micropa- deep gas reserves (i.e., >12,000 ft [>4000 m]) in lowstand sand- leontologists Helen Jean Plummer (Bureau of Economic stone reservoirs are currently being explored. However, finding Geology), Julia A. Gardner, Alva C. Ellisor, and Ester deep reserves requires modification of earlier conventional inter- R. Applin (former University of Texas students of J. A. pretation techniques and concepts. Robust application of sequence- Udden, director, Bureau of Economic Geology, 1915– 1932),