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AAPG Bulletin


AAPG Bulletin, V. 86, No. 8 (August 2002),Copyright ©2002. The American Association of Petroleum Geologists. All rights reserved.

Reservoir fluids and their migration into the South Eugene Island Block 330 reservoirs, offshore Louisiana

Steven Losh,1 Lynn Walter,2 Peter Meulbroek,3 Anna Martini,4 Lawrence Cathles,5 Jean Whelan6

1Department of Geological Sciences, Snee Hall, Cornell University, Ithaca, New York, 14853; email: [email protected]
2University of Michigan, Department of Geological Sciences, 2534 C. C. Little Building, 425 E. University Avenue, Ann Arbor, Michigan, 48109; email: [email protected]
3Materials and Process Simulation Center, California Institute of Technology, 1600 E. California Road, Pasadena, California, 91101; email: [email protected]
4Department of Geology, Campus Box 2238, Amherst College, Amherst, Massachusetts, 01002; email: [email protected]
5Department of Geological Sciences, Snee Hall, Cornell University, Ithaca, New York, 14853; email: [email protected]
6Department of Marine Chemistry and Geochemistry, Fye Laboratory, Woods Hole Oceanographic Institute, Woods Hole, Massachusetts, 02543; email: [email protected]


Steven Losh earned a B.S. degree with high honors in geological engineering at the Colorado School of Mines and a Ph.D. in geology at Yale University. His work focuses on the integration of geochemical and geological data to address problems of fluid flow in a variety of settings.

Lynn M. Walter received her M.S. degree from Louisiana State University (1978) and her Ph.D. from the University of Miami (1983). She was an assistant professor at Washington University in St. Louis until 1988. She then joined the University of Michigan, where she is now a professor of geological sciences and director of the Experimental and Analytical Geochemistry Laboratory. Her research interests focus on the hydrogeochemistry of near-surface and deeper basin environments, with an emphasis on carbon transformations and mineral mass transport.

Peter Meulbroek received a B.S. degree in mathematics from the University of Chicago and a Ph.D. in geology from Cornell University. He is currently working at Caltech at the Molecular and Process Simulation Center. His interests there include modeling macroscopic chemical behavior with equations of state, microscopic behavior using molecular dynamic and quantum mechanical models, and developing data-centric models for all scales.

Anna Martini received her B.A. degree in geology from Colgate University, her M.S. degree in geology from Syracuse University, and her Ph.D. in geology from the University of Michigan. She is currently an assistant professor of geology at Amherst College. Her research to date has focused on unconventional natural gas plays in the Michigan and Illinois basins.

Lawrence Cathles received his A.B. degree and Ph.D. from Princeton University. He was employed in research by Kennecott and later by Chevron and is currently a professor of geological sciences at Cornell University. He has been involved in modeling fluid flow and chemical processes related to both ore deposits and sedimentary basins, with a major focus on three-dimensional finite element-coupled fluid flow and chemical models for petroleum systems in the Gulf of Mexico.

Jean Whelan is a senior research specialist. She became interested in organic geochemistry through her mentor at Woods Hole, John M. Hunt. Her current research focuses on use of organic compounds as indicators of geologic and oceanographic processes, particularly with respect to gas and oil formation, migration, and destruction both in subsurface sediments and in the ocean. Her past research focused on gases and light hydrocarbons in oceanic environments and on microbial transformations in shallow and deep ocean sediments. Whelan earned her bachelor's degree in chemistry from the University of California at Davis and her doctorate in organic chemistry from the Massachusetts Institute of Technology.


Support for this work was provided by Gas Research Institute contract GRI50972603787 to L. Cathles, Cornell University, with a subcontract to J. Whelan at Woods Hole Oceanographic Institute (WHOI). Corporate sponsors of the Global Basins Research Network (GBRN) provided additional support. The analyses reported herein were obtained under U.S. Department of Energy grant DE-FC22-93BC14961 to Roger Anderson, Lamont-Doherty Earth Observatory, with subcontracts to L. Cathles, Cornell University, and J. Whelan, WHOI. We thank Pennzoil Exploration and Production Co. (now Devon Energy) and their partners for their generous cooperation in this project. S. Losh thanks Glen Wilson for helpful discussions. We also thank Bruce Hart, Roger Sassen, Jeff Hanor, and an anonymous reviewer for thoughtful and constructive formal and informal reviews of this article.


This study in the well-documented Pliocene-Pleistocene South Eugene Island Block 330 (SEI330) field, offshore Louisiana, unravels a complex petroleum system by evaluating both the inorganic and organic geochemical characteristics of reservoir fluids. The brines at SEI330 dissolved halite prior to entering the reservoirs and equilibrated with reservoir sediments, exchanging sodium for calcium, magnesium, and other cations. The systematically varying extent of brine sodium depletion in two reservoirs defines south to north flow in those sands. These sands were filled from a fault that bounds the reservoirs on the south. Oil compositional parameters also show north-south variation across these reservoirs. The SEI330 oils and gases each had different sources. In contrast to published Jurassic sources for oil, carbon isotope data indicate that SEI330 hydrocarbon gases probably sourced from early Tertiary or Cretaceous sediments, after oil had migrated through them. Distribution of biodegraded vs. unbiodegraded oils indicates the reservoirs filled much more recently than formation of the salt weld beneath the field. Oil compositions indicate that some SEI330 oils were partially stripped of low-molecular weight compounds by their dissolution in a mobile vapor phase (gas washed) by large volumes of gas several hundred meters below the deepest reservoir. Modeling of this gas-oil interaction aids in identifying deep potential targets in which gas washing occurred.

Hydrocarbon distribution, combined with oil chemistry and reservoir pressures, indicate reservoirs filled from fault systems on both the north and south sides of the field. The fault feeders are wide (>100 m), structurally complex zones that can direct different types of fluids into different reservoirs.

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