Lithofacies, Depositional Environment, Burial History and Calculation of Organic Richness from Wireline Logs: A Study of the Barnett Shale in the Delaware Basin, Pecos Co., West Texas, and Comparison with the Barnett Shale in the Fort worth Basin

Ali, Walaa ¹; Gale, Julia ²; Ruppel, Stephen C.²; Carr, David L.²; Loucks, Robert ^2
1 Jackson School of Geological Sciences, The University of Texas at Austin, Austin, TX.
² Bureau of Economic Geology, Jackson School of Geological Sciences, The University of Texas at Austin, Austin, TX.

Studies of core and logs through the Barnett Shale in Pecos County in the southern part of the Delaware Basin have allowed us to compare the lithofacies and depositional environment of the Mississippian in this area with that in the Fort Worth Basin. Overall, mudrock facies are similar although, in contrast, the studied core contains no skeletal debris layers. Limestone concretions were not seen, but there is substantial dolomite in many horizons. Total clay contents are broadly similar. There is more bioturbation than in the Fort Worth Basin. Agglutinated foraminifera, Tasmanites, radiolarians, conodonts and echinoid spines are present. The transition zone between the Barnett and the Woodford Formation is a chert rather than a carbonate, as reported in the northern part of the Delaware Basin. This transition zone is regionally referred to as the "Mississippian Limestone". Recognizing this, we made cross-sections, isopach maps, and structure maps based on well-log correlations penetrating the Barnett Shale in Pecos, Reeves, Culberson, and Hudspeth counties. We then use published methods to calculate organic richness from wireline logs. We built a 1-D burial history of the cored well using 'Genesis' software, making use of vitrinite reflectance data. Constructing the burial history is crucial for being able to track likely diagenetic changes in the shale with time. These diagenetic changes in turn control hydrocarbon generation, overpressuring, natural fracturing, petrology and petrophysics, and present-day mechanical rock properties, all of which are important factors in determining whether Barnett Shale-gas in the Delaware Basin will be economic.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009