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SWLGS Luncheon Topics

Updated August 05, 2003

April 2000

Subsalt Exploration In The Deepwater Foldbelts Of The Gulf Of Mexico: Regional Analysis Of A Giant Petroleum System
Roberts, Michael J., and Thomas W. Hall, Chevron North America Exploration and Production Company, New Orleans, LA


Abstract: 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 involves Mesozoic through Miocene strata and covers an area of 20,000 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 both trends are in the subsalt environment. The difficult seismic image in 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. The 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. Contraction in both areas has lifted objective intervals up to 4km above regional elevation. The 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 allocthonous salt.

The primary reservoir objectives in both foldbelt trends are Tertiary aged turbidites although a higher risk and poorly understood Mesozoic section is present. Hydrocarbon source in both trends is likely from the Upper Jurassic with the Perdido Foldbelt also accessing potentially mature Cretaceous and Paleogene intervals. The combination of large structural traps, rich source rocks, potentially excellent reservoirs and a regional top seal place the subsalt Foldbelts in the forefront of future Gulf of Mexico exploration. 

Biographical Sketch: Mike Roberts received a Bachelor's degree in Geology from Bowling Green State University (Ohio) in 1982 and his Master's degree in Geology from the University of Cincinnati in 1984. He has been employed by Chevron since 1984 and has spent these 15 years working in Chevron's New Orleans office. Mike has had numerous assignments across the GOM and has spent the last 10 years working in the deepwater. Mike is a member of NOGS, AAPG and SEPM.