--> Abstract: Sedimentary Hydrocarbon And Asphalt Gradients Through The Top Of Overpressure - Organic Geochemical Evidence For Dynamic Creation And Maintenance Of Pressure Seals, by J. K. Whelan, L. B. Eglinton, J. S. Seewald, J. M. Hunt, J. Sosha, L. M. Chathles, and D. D. Darby; #90928 (1999).

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WHELAN, JEAN K.1, L. B. EGLINTON1, J. S. SEEWALD1, J. M. HUNT1, J. SOSHA2, L. M. CHATHLES2, and D. D. DARBY3
1Department of Marine Chemistry & Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543-1543
2Department of Geology & Geophysics, Cornell University, Ithaca, NY 144853-1504
3Gas Research and Technology Centre, Leicestershire, U.K. LEI I 3GR

Abstract: Sedimentary Hydrocarbon And Asphalt Gradients Through The Top Of Overpressure - Organic Geochemical Evidence For Dynamic Creation And Maintenance Of Pressure Seals

Recent research in our laboratory has focused on organic matter associated with subsurface pressure compartments out of pressure equilibrium with surrounding sediments. Work has focused on wells where reliable measurements are available to precisely define the boundaries of these pressure compartments in wells from: a) the young actively subsiding U.S. Gulf Coast and b) the much older Cretaceous offshore Canadian Venture Field. We describe three types of data consistent with upward fluid flow through sediments within and above these pressure compartments in both areas:

1) gradients in the ratios of polar (asphaltenes) to less polar (alkanes and aromatic hydrocarbons) with asphaltenes typically being focused at the top of the pressure transition zone.
2) depth gradients of individual hydrocarbons which show no apparent relationship to lithology and with the sharpest gradients within the pressure transition zone.
3) pyrolysis data from pressure seals and transition zones from numerous wells which show poor source rock, but are consistent with migrated hydrocarbons. A dynamic mechanism of pressure seal formation and maintenance is proposed which involves upward fluid flow driven by gas migrating from depth.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas