ABSTRACT: A Geologic/Petroleum-System Model of the Downdip Tuscaloosa-Woodbine Trend and its Application to Assessment of Gas Resources

Bulletin Vol. 88 (2004), No. 13 (Supplement)

AAPG Annual Meeting
Dallas, Texas
April 18-21, 2004

Pitman, Janet K.¹, Russell Dubiel², Phillip Nelson²
(1) U.S. Geological Survey, Denver, CO
(2) U.S. Geological Survey, Lakewood, CO

The overpressured Tuscaloosa-Woodbine trend downdip of the Lower Cretaceous shelf edge in Texas and Louisiana is a major gas-producing area in the on-shore Gulf Coast with cumulative production approaching 3 TCF. Current, U.S. Geological Survey assessment studies indicate that significant gas resources will be added to the hydrocarbon reserves over the next 30 years. Determining the number and size distribution of undiscovered fields in the trend involved the development of a comprehensive geologic, and (IES-PetroMod) multi-1D and 2.5D petroleum-system model of the Mesozoic-Cenozoic section that hosts the principal source rocks and reservoirs in the downdip trend. Sequence and seismic stratigraphy in major producing areas document a Cenomanian, shelf-margin to deep-water system overlain by Turonian marine shale. The Jurassic Louann Salt, which forms northwest-trending salt ridges and salt diapirs, influenced sediment accumulation in the deep-water system, and in turn, the formation and orientation of syndepositional growth faults within the basin. Seismic interpretations were used to numerically model the petroleum system (source rocks, reservoirs, traps, and seals) and processes (generation and migration) required for gas accumulation. Multiple source-rock units, high-stand and low-stand systems tracts, syndepositional growth faults, and rollover anticlines were incorporated into the numeric model to simulate gas generation and reservoir overpressuring. Model results indicate that the bulk of the gas in lower Tuscaloosa and Woodbine reservoirs was derived in situ from marine shales with minor input from source rocks deeper in the section. Migration simulations show that gas migrated locally over short distances and is compartmentalized by growth faults and overpressured shales.