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Conventional and Unconventional Petroleum Systems of the Delaware Basin, USA


Although one of the major producing basins in the U.S.A., details of Permian Basin petroleum systems have not been published except for the Ordovician (Katz, 1994). The Permian Basin is comprised of the Delaware and Midland basins with the Central Basin Platform juxtaposed between them. A classical petroleum systems study of the entire Permian Basin was originally initiated in 2000 that included thousands of source rock and hundreds of oil samples. This effort was an update on the work of Smith and Jones (1965) and Williams (1977) that Hill et al. (2003) presented. However, this work is further updated focusing on the unconventional systems in the basin. Early efforts to establish unconventional production to produce gas from the Barnett Shale occurred in Reeves County, Texas in the Delaware Basin. However, the shale was less brittle than in the Ft. Worth Basin and was oil window mature in southcentral or very deep (ca. 12-15,000 ft) in northern Reeves County. In the southwestern portion of Reeves Co., the Fasken State 36 #1 penetrated and tested the Barnett Shale at just over 7,000 ft. This well did produce minor amounts of gas (ca. 50 mcf/day) and small amounts of condensate (8 bbls/day) for a limited time. Thermal maturity is post-peak to volatile oil window in agreement with canister gas data from this well and carbon isotopic analysis of the Fasken State 34 #1 gas. The 34 #1 well shows oil crossover as indicated by S1/TOC (x100) ratios of over 100 mg oil/g TOC in a 200 ft interval often indicative of potential production, all other factors being suitable for hydraulic stimulation. Petroleum source rocks in the Permian Basin include Ordovician, Devonian, Mississippian, Pennsylvanian, and Permian Wolfcampian, Leonardian and Guadalupian series. Most of these systems are oil prone kerogens. While these systems are primarily siliciclastic shales, the Bone Springs is a carbonate to marly shale source rock. Maturation-wise there are distinct thermal histories that show different relationships to depth particularly in the Delaware Basin. This paper focuses on Delaware Basin petroleum systems and on characterization and quantification of resources for unconventional production. The amounts of petroleum generated, expelled, and retained in situ for each source rock are calculated. The goals are volumetric calculations and prediction of reservoir properties from these analyses including the impact of alteration processes such as gas exsolution.