AAPG Bulletin, V. 82 (1998), No. 5B (May 1998 Part B), P. 1083-1112.

The Effect of Allochthonous Salt on the Petroleum Systems of Northern Green Canyon and Ewing Bank (Offshore Louisiana), Northern Gulf of Mexico¹

Barry C. McBride,² Paul Weimer,³ and Mark G. Rowan³

©Copyright 1998.  The American Association of Petroleum Geologists.  All Rights Reserved

¹Manuscript received September 30, 1996; revised manuscript received October 1, 1997; final acceptance November 19, 1997.
²Energy and Minerals Applied Research Center, Department of Geological Sciences, Campus Box 399, University of Colorado, Boulder, Colorado 80309-0250. Present address: HS Resources Inc., 1999 Broadway, Suite 3600, Denver, Colorado 80202.
³Energy and Minerals Applied Research Center, Department of Geological Sciences, Campus Box 399, University of Colorado, Boulder, Colorado 80309-0250.

We thank the following companies and their representatives for their tremendous support and input as part of the Gulf of Mexico research consortium at the University of Colorado: AGIP, Amoco, Anadarko, BHP, BP Exploration, Burlington Resources, Canadian Occidental, Chevron USA, CNG, Conoco, Enterprise, Exxon, Marathon, Mobil, Occidental, PanCanadian, Pennzoil, Petrobras, Phillips Petroleum, Shell Oil Company, Texaco, Total, Union Pacific, and Unocal. We extend special thanks to Halliburton Geophysical (now Western Geophysical), specifically to Bob Graebner, John Anderson, and Gary White, for the use of seismic data used in the project. We are indebted to PaleoData Inc. and Art Waterman for use of biostratigraphic data. Software donations from GeoQuest, CogniSeis Development Inc., Landmark, Platte River Associates, and Zeh Graphics are essential to the success of our program and very much appreciated. Special thanks are in order to Bob Ratliff, Jay Leonard, China Leonard, Doug Waples, Fred Meissner, Al Koch, Thane McCulloh, Andy Pulham, Tom McKenna, and George Gail for their support, advice, discussions, training, and input during the development of this research over the years. We appreciate reviews by Mike Lewan, Bill Galloway, an anonymous reviewer, and especially Chris Travis for his particularly thorough review. We thank Jonathan Irick, Jennifer Crews, and Ryan Crow for drafting. Acknowledgment is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support for this research. 

ABSTRACT

The northern Green Canyon/Ewing Bank region, northern Gulf of Mexico basin, contains the Oxfordian-Neogene (.), Tithonian-Neogene (.), Albian-Neogene (.), Turonian-Neogene (.), and Eocene-Neogene (.) petroleum systems. The systems encompass 42 fields or discoveries in the study area and include four subsalt discoveries. Essential elements of the systems include source shales of Oxfordian, Tithonian, Albian, Turonian, and Eocene age; Neogene siliciclastic turbidite reservoirs; allochthonous salt; and overburden strata ranging in age from Jurassic to Quaternary.

The petroleum systems of the area are significantly affected by the evolution of allochthonous salt. The high thermal conductivity of salt retards the thermal maturation of subsalt petroleum source rocks and causes late generation and migration from them. Most traps were formed during the Pliocene-Pleistocene, and the generation-migration-accumulation of petroleum ranges from early Miocene to the Holocene. The critical moment of peak oil generation for each source varies spatially and temporally as a function of the overlying sediments and allochthonous salt evolution.

The impermeability of salt prevents vertical petroleum migration and causes migration pathways to be deflected laterally up the dip of base salt. Where salt welds form, petroleum migration is unimpeded and continues vertically. By integrating predictions of potential source rocks, structural restorations, thermal maturation modeling, regional salt maps, and petroleum systems logic, we can determine petroleum migration pathways and zones of concentration. All 42 fields or discoveries within the study area are associated with predicted zones of paleosubsalt petroleum concentration. Present-day salt geometries do not delineate many of these zones because of salt weld formation during the Pleistocene. This generation, migration, and accumulation technique enables geoscientists to focus their exploration efforts toward areas with a greater probability of success.