AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
USGS Assessment of Undiscovered Oil and Gas Resources of the Cretaceous Fredericksburg-Buda Carbonate Platform-Reef Gas and Oil Assessment Unit, Gulf Coastal Plain and State Waters, USA
(1) U.S. Geological Survey, Reston, VA.
Lower Cretaceous Albian to Upper Cretaceous lower Cenomanian carbonate rocks that comprise the Fredericksburg and Washita Groups (including the Edwards Limestone, Georgetown Limestone, Buda Limestone, and equivalent formations) in the Gulf Coastal Plain and State waters were recently assessed for undiscovered oil and gas resources by the U.S. Geological Survey (USGS). This work was conducted as part of the 2010 USGS assessment of Jurassic and Cretaceous stratigraphic intervals in the Gulf Coastal Plain area. The assessed intervals are part of the Upper Jurassic-Cretaceous-Tertiary Composite Total Petroleum System of the Gulf of Mexico Basin, defined by the USGS in 2007.
The Fredericksburg-Buda Carbonate Platform-Reef Gas and Oil Assessment Unit (AU) was assigned to carbonate rocks in the Albian to lower Cenomanian interval. This conventional AU was defined using a geologic model based on depositional environment (back-reef lagoon, reef, and fore-reef environments), source rock potential, structural features, traps, seals, migration pathways, porosity, and permeability. The AU extends from south Texas to the East Texas Basin, central Louisiana, southern Mississippi and Alabama, and the panhandle area of Florida. In Texas and Louisiana, the downdip boundary of the AU is defined by a line that extends 10 miles downdip of the Lower Cretaceous shelf margin, to include potential reef-talus reservoirs. In Mississippi, Alabama, and the panhandle area of Florida, where the Lower Cretaceous shelf margin extends offshore, the downdip boundary of the AU is defined by the State water boundary. Updip limits are defined primarily by basin margin fault zones and the updip extent of carbonate rocks in the stratigraphic interval assessed.
The assessment required an in-depth analysis of hydrocarbon production reservoir and well data. Also, facies changes, diagenetic modifications, faults, dolomitization, secondary dissolution, and fracture porosity were considered in estimates of undiscovered oil and gas reservoirs. Other factors were examined, including the mid-Cenomanian sea-level fall and erosion that removed large portions of platform and platform margin carbonates in the Washita Group of central Louisiana. Results for the USGS assessment of Jurassic and Cretaceous stratigraphic intervals in the Gulf Coastal Plain area will be available in 2011.