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Abstract

(Begin page 2089)

AAPG Bulletin, V. 85, No. 12 (December 2001), P. 2089-2110.

Copyright ©2001. The American Association of Petroleum Geologists. All rights reserved.

Potential salinity-driven free convection in a shale-rich sedimentary basin: Example from the Gulf of Mexico basin in south Texas

John M. Sharp Jr.,1 Thomas R. Fenstemaker,2 Craig T. Simmons,3 Thomas E. McKenna,4 Jennifer K. Dickinson5

1Department of Geological Sciences, University of Texas at Austin, Austin, Texas, 78712-1101; email: jmsharp@mail.utexas.edu.
2Department of Hydrological Sciences/175, University of Nevada, Reno, Reno, Nevada, 89557-0180; email: tomf@unr.edu
3School of Chemistry, Physics, and Earth Sciences, Flinders University of South Australia, GPO Box 2100, Adelaide, SA 5001, Australia; email: craig.simmons@es.flinders.edu.au
4Delaware Geological Survey, University of Delaware, Newark, Delaware, 19716-7501; email: mckennat@udel.edu
5Conoco, Inc., Houston, Texas; email: jennifer.k.dickinson@conoco.com

AUTHORS

John M. (Jack) Sharp Jr. is the Chevron Centennial Professor of Geology at the University of Texas at Austin. His present research involves processes in sedimentary basins; resource sustainability issues; urban hydrogeology; characterization of fluvial, fractured, and karstic aquifer systems and reservoirs; and hydrogeology of desert basins. Jack is editor of Environmental and Engineering Geoscience. He is also currently an AT&T Industrial Ecology Fellow.

Thomas R. Fenstemaker holds B.S. and M.S. degrees in geological sciences from the University of Texas at Austin. He is currently pursuing a Ph.D. in hydrogeological sciences at the University of Nevada, Reno. Tom's current research interests include the geochemistry of ore deposits, the circulation patterns of variable-density fluid in sedimentary basins, and improving geophysical methods to detect fluid migration patterns in the shallow surface.

Craig T. Simmons is a lecturer in hydrogeology at the Flinders University of South Australia and manager of the Centre for Groundwater Studies in Australia. His research interests focus on simulation of ground-water flow and solute transport phenomena, investigation of ground-water processes under variable-density flow conditions, fractured rock flow and transport, heterogeneity characterization, and stochastic surface hydrology. Craig is an associate editor of the Hydrogeology Journal, the official journal of the International Association of Hydrogeologists (IAH).

Thomas E. McKenna is a hydrogeologist with the Delaware Geological Survey at the University of Delaware. His research interests include regional hydrogeology, aquifer/reservoir characterization, Geographic Information System spatial analysis, submarine ground-water discharge, evolution of fluid flow in sedimentary basins, and numerical modeling of ground-water flow and transport. Tom's current projects include thermal imaging of ground-water discharge (and associated nutrients) to estuaries, characterization of the Potomac aquifer in Delaware, and the investigation of shallow-depth faults in the Atlantic coastal plain.

Jennifer K. Dickinson was raised in San Antonio, Texas. In 2000, she was one of the first to receive a B.S. degree from the University of Texas at Austin in geosystems engineering and hydrogeology, a joint degree between the Department of Geological Sciences and the Department of Geosystems and Petroleum Engineering. Jen is currently employed as a petroleum engineer with Conoco, Inc. in Houston.

ACKNOWLEDGMENTS

The United States Department of Energy, Geosciences Research Program, provided support for this research under grant number DE-FG03-97ER14772. The Owen-Coates Fund of the University of Texas Geology Foundation provided support for drafting and manuscript preparation. Jeff Horowitz and Matt Uliana helped draft several of the figures. We appreciate reviews by Chris Neuzil, Jeffrey Nunn, Lori Summa, Leo Lynch, Kitty Milliken, and Karen Mohr.

ABSTRACT

We investigate the plausibility of salinity-driven free (thermohaline) convection in sedimentary rocks of the south Texas part of the Gulf of Mexico basin using salinity data, Rayleigh number calculations, and numerical models. Previous studies speculated that free convection could account for high fluxes evidently required for diagenesis in the basin, but low-permeability shales are calculated to be a barrier to free convection. In the study area, salinity inversions occur either above or within the transition zone from hydropressures to overpressures. The positioning of brines over less saline fluids provides a significant buoyancy force. Rayleigh number calculations and numerical simulations suggest that homogeneous shaly systems are unstable near the high end of the expected ranges of shale permeability (10-15-10-16 m2). Numerical simulations show that the influx of brine into the permeable layers and permeability heterogeneity in the shales are both conducive to free convection. Simulations indicate that salinity-driven free convection can occur at lower permeabilities (10-16-10-18 m2) that may approximate the permeabilities of shales in the zone of extreme overpressures over geologic time. Simple Rayleigh numbers are inadequate to predict the occurrence of free convection in heterogeneous systems. Salinity-driven free convection at depths in some large sedimentary basins, such as the Gulf of Mexico, may be more common than expected.

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