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The AAPG/Datapages Combined Publications Database

Houston Geological Society Bulletin


Houston Geological Society Bulletin, Volume 42, No. 1, September 1999. Pages 15-15.

Abstract: Subsalt Exploration in the Deepwater Foldbelts of the Gulf of Mexico: Regional Analysis of a Giant Petroleum System


Michael J. Roberts and Thomas W. Hall
Chevron North America Exploration and Production Company, Deepwater Business Unit, New Orleans, Louisiana

The geologic setting of the distal U.S. waters is characterized by a complex assemblage of salt-related contractional features. In the U.S. western Gulf, the Perdido Foldbelt is a north-south trending zone of shortened Mesozoic through Miocene strata covering an area of over 30,000 square kilometers. In the deep east central U.S. Gulf, the Mississippi Fan Foldbelt is an arcuate trend of folded/thrusted Mesozoic through Pliocene strata encompassing over 25,000 square kilometers. Approximately 80% of each trend is in the subsalt environment. The difficult seismic imaging of these subsalt trends necessitates a regional approach to understand the development and prospect potential in these vast areas.

Our regional methodology employs source rock analyses, potential field and basement mapping, regional structural mapping of key horizons including the top and base of salt, regional cross sections, stratigraphic studies and depth-migrated seismic imaging. Basement-controlled thickness and distribution of the Louann Salt and subsequent sediment loading history are the primary controls on the location, distribution, and character of the foldbelts. These primary controls are responsible for observed changes along the regional strike and dip of the trends that include structural inversion. Contraction in both areas has lifted objective intervals up to 4 km above regional elevation. Folding accommodates only a small portion of the total regional extension, with the emplacement of the Sigsbee salt canopy serving as the primary balancing mechanism. Understanding the linkage between contraction of the objective intervals and the emplacement of the overlying salt canopy is the key to successful exploration in these trends. Because emplacement of the canopy is partially synchronous with folding, a correlation exists between the structural grain of the subsalt folds and the base of allochthonous salt.

Primary reservoir objectives in both foldbelt trends are Tertiary-age turbidites, although a higher-risk poorly understood Mesozoic section is present. Hydrocarbon sources for both trends arc from Upper Jurassic and Lower Cretaceous marls, with the Perdido Foldbelt also accessing potentially mature Paleogene intervals. The combination of large structural traps, rich source rocks, potentially excellent reservoirs, and a regional top seal places the subsalt foldbelts in the forefront of future Gulf of Mexico exploration.

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