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Book Chapter

Case Histories from North America

January 01, 2002


Widespread oil and gas seepage in the offshore U.S. Gulf of Mexico has allowed the extension of hydrocarbon systems and maturity maps far beyond well control. Analysis of sea-bottom dropcores and imaging of sea-surface slicks have complemented Exxon’s integrated, multidisciplinary study of sources, maturation, and migration pathways. Our approach involved development of a regional geologic framework through interpretation of 2-D and 3-D seismic data, identification and mapping of potential source intervals, and delineation of likely migration pathways to reservoirs and seismic-amplitude anomalies. Hydrocarbon compositions from more than 2000 reservoired oils, 600 reservoired natural gases, and 3000 hydrocarbon-bearing, sea-bottom dropcores help constrain such source-rock characteristics as organic-matter type, depositional facies, level of maturation and, to some extent, age. East of the Mississippi River Delta, the complete stratigraphic section is visible on seismic sections, and wells have penetrated deep source intervals. To the west, correlative organic-rich rocks have been sampled onshore and from sheaths overlying salt diapirs offshore. Integration of these data into a regional geologic framework provides a strong basis for hydrocarbon-systems interpretations.

Major offshore hydrocarbon systems are derived from lower Tertiary (centered on Eocene), Upper Cretaceous (centered on Turonian), and Upper Jurassic (centered on Tithonian) source intervals. All Eocene oil types (marine, intermediate, and terrestrial) have beentied to source rocks and are consistent with paleofacies distributions for the Eocene deltaic systems. Eocene oils and gases are prevalent on the Texas and Louisiana Shelves and extend both onshore and onto the Texas Slope. Turonian oils have been tied to source rocks offshore (east of the Mississippi River Delta) and onshore (e.g., Tuscaloosa and Giddings trends). Based on seismic-image thinning of the interval and disappearance of diagnostic oils, we interpret a basinward loss of this source type. Elevated-sulfur oils and associated (cogenerated) gases on the upper Gulf of Mexico Slope are interpreted to have originated from a Tithonian source. High-maturity, organic-rich calcareous shales of this age have been penetrated in the eastern Gulf of Mexico, and Tithonian oils occur in Cretaceous reservoirs on the Florida Shelf where the Upper Cretaceous and Tertiary sections are immature. Oxfordian carbonate-sourced oils are common across the northern Gulf basin’s rim, and lower-maturity hydrocarbons from this source occur in stains and seeps in the deep central Gulf of Mexico.

Refinements in our understanding of Gulf of Mexico hydrocarbon systems have resulted from this study. They have helped us to improve our exploration methodologies and have provided us with new play concepts.

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AAPG Studies in Geology

Surface Exploration Case Histories: Applications of Geoschemistry, Magnetics, and Remote Sensing

Dietmar Schumacher
Dietmar Schumacher
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Leonard A. LeSchack
Leonard A. LeSchack
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American Association of Petroleum Geologists
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Publication date:
January 01, 2002




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