AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
USGS Definition of a Greater Gulf Basin Lower Cretaceous and Upper Cretaceous Lower Cenomanian Shale Gas Assessment Unit, Gulf Coastal Plain and State Waters, USA
(1) Eastern Energy Resources, U.S. Geological Survey, Reston, VA.
An assessment unit (AU) for undiscovered continuous shale gas in Lower Cretaceous Aptian and Albian, and Upper Cretaceous lower Cenomanian fine-grained clastic rocks in the Gulf of Mexico coastal plain was recently defined by the U.S. Geological Survey (USGS). The AU is part of the Upper Jurassic-Cretaceous-Tertiary Composite Total Petroleum System (TPS) of the Gulf of Mexico Basin. This work was conducted as part of the USGS assessment of Jurassic and Cretaceous stratigraphic intervals in 2010.
The stratigraphic upper limit of the AU is the mid-Cenomanian unconformity at the top of the Washita Group. Included in the AU are the Maness Shale, the Del Rio Formation and equivalent Grayson Formation, the Pawpaw Formation, the Weno Limestone, the Denton Clay, the Duck Creek Limestone, the Kiamichi Shale, the Walnut Formation, the McKnight Formation, the Bexar Shale and the Pine Island Shale exclusive of the Maverick Basin, and unnamed shales in the Glen Rose and Sligo Formations. The Pearsall Formation in the Maverick Basin was assessed separately.
The updip boundary of the AU follows the TPS boundary northeastward from the USA-Mexico border in Texas to Arkansas. The updip boundary across the Mississippi Embayment in Louisiana, Mississippi and Alabama is the erosional subcrop of the Lower Cretaceous. The eastern boundary in Florida is defined by the limit of clastic deposition according to well data. In Texas and Louisiana, a line extending 10 miles downdip of the Lower Cretaceous shelf margin defines the downdip limit of the AU. Where the Lower Cretaceous shelf margin extends offshore from Mississippi, Alabama and the panhandle of Florida, the AU boundary is the State water boundary.
Lithofacies include thin calcareous shales, marls, sandy clays and sand lenses. Total organic carbon(TOC)values for the Del Rio Formation in the Maverick Basin and Bastrop County, Texas range from 0.54 to 4.63 % and hydrogen indices (HI) range from 67-504.TOC values for Washita and Fredericksburg Group formations in Mississippi range from 0.39 to 0.83 % and HI values range from 38 to 109. There currently is no indication of biogenic gas. This AU was not quantitatively assessed by the USGS at the present time primarily because there is too little production data available.