Lithofacies, depositional environment, burial history and calculation of organic richness from the wireline logs : a study of Barnett Shale in the Delaware Basin, Pecos Co., West Texas, and comparison with the Barnett Shale in the Fort Worth Basin
Studies of core and logs from the Pioneer Reliance Triple Crown #1 (RTC#1) through the Barnett Shale in Pecos County in the southern part of the Delaware Basin, west Texas allow comparison of the lithofacies and depositional environment of the Mississippian section in this area with that in the Fort Worth Basin (FWB). Overall, mudrock facies are similar although, in contrast, the studied core contains no skeletal debris layers. Limestone concretions are absent, but there is substantial dolomite in many horizons. Total clay contents are broadly similar. There is more bioturbation than in the FWB. Agglutinated forams, Tasmanites, radiolarians, conodonts and echinoid spines are present. The depositional environment is a euxinic basin with a more distal setting than has been reported for the Barnett Shale in the FWB producing area. The transition zone between the Barnett Shale and the Woodford Formation is a chert rather than a carbonate, as reported in the northern part of the Delaware Basin in some locations and a mix of chert and lime in the rest of the basin. This transition zone is regionally referred to as the "Mississippian Limestone". Recognizing this, cross-sections and isopach maps for the transition zone between Barnett Shale and Woodford Formation have been constructed based on well-log correlations penetrating the Mississippian section in Pecos, Reeves, and Culberson counties. Additionally, core from the Hamon Regan #1 well has been examined to investigate the lithology and mineralogy for the transition zone and to interpret the origin of chert in the transition zone, between the Barnett Shale and Woodford Formation, and has been compared to the RTC#1 well in Pecos County. Moreover, gas content for the Barnett Shale interval has been estimated based on calculating the original TOC and original hydrocarbon potential. A published method to calculate organic richness from wireline logs has been tested for the Barnett Shale in the RTC#1 well and it gives fair correlation with measurements of TOC in cored samples. A 1-D burial history model was constructed for the Delaware Basin at the location of the RTC#1 well making use of vitrinite reflectance data from the Barnett Shale and Woodford Formation. Constructing the burial history is crucial for tracking likely diagenetic changes in the shale over 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 production of gas from the Barnett Shale in the Delaware Basin will be economic.