Shelf Margin Complex in the Downdip Tuscaloosa-Woodbine Trend and Implications for the Assessment of Conventional Gas Resources

Russell F. Dubiel and J. K. Pitman
U. S. Geological Survey, Denver, Colorado

The Upper Cretaceous, Tuscaloosa-Woodbine trend lies downdip of the Lower Cretaceous, carbonate shelf edge, in onshore Texas and Louisiana. The overpressured trend is a major gas-producing area in the Gulf Coast Province, with cumulative production approaching 3 trillion cubic feet from only 20 fields. Current U.S. Geological Survey assessment studies indicate that additional conventional gas fields will be found over the next 30 years. Determining the number- and size-distribution of those undiscovered fields requires a comprehensive geologic and petroleum-system model that characterizes the principal source rocks and reservoirs in the trend.

Sequence- and seismic-stratigraphy document a Cenomanian, shelf-margin delta complex and deep-water clastic system overlain by Turonian marine shale. At least three, and possibly more, fluvial systems provided sediment to the shelf-margin complex following a major lowering of sea-level represented by the mid-Cretaceous unconformity. Coarse clastic sediment was supplied to the shelf edge under generally rising sea level. Both the Tuscaloosa and the Woodbine shelf-margin deltas are composed of several higher-order sequences separated by sequence boundaries and marine flooding surfaces that contain lowstand fan and prograding complexes. Sequence stratigraphy predicts that both the Tuscaloosa and the Woodbine contain a significant number of possible fields in system tracts that occur downdip from known fields.

The Jurassic Louann Salt forms salt ridges and diapirs that influenced sediment accumulation in the deep-water system, and in turn, the distribution of syndepositional growth faults. The salt is thought to have influenced the distribution of minibasins in a pattern that reflects the known producing fields. That pattern of minibasin distribution can be used to predict the future distribution of undiscovered fields.

AAPG Search and Discovery Article #90032©2004 GCAGS 54th Annual Convention, San Antonio, Texas, October 10-12, 2004