The Permian Basin of the southwestern United States has produced approximately 35 billion bbl of conventional oil over the past century, and until recently, it was broadly viewed as an overdeveloped basin in the twilight of its effective life. Over the past decade, however, this perspective has changed because of the extraordinary success in oil recovery from unconventional upper Paleozoic shale reservoirs that are forecast to more than double the recovered oil from the Permian Basin. Among the assortment of shale reservoirs is the Upper Pennsylvanian Cline shale (also known as Wolfcamp D) of the southern Midland Basin that has oil-in-place estimates as high as 30–35 billion bbl and recoverable oil estimates for individual wells in the hundreds of thousands of barrels. The Cline shale extends across an approximately 10,000 mi2 (∼25,000 km2) fairway and is composed of eight basin-floor depositional facies that are variably enriched in organic matter, detrital clay, and calcite, all important attributes that influence reservoir quality and oil recovery. Gamma-ray and density logs are commonly available in wells throughout the Midland Basin and have a diagnostic response to sedimentary attributes that allows the grouping of eight lithofacies identified in core into six log-defined petrofacies. The correlation and mapping of petrofacies across the southern Midland Basin demonstrate a close correlation between the distribution of the most organically enriched petrofacies (organic-rich shale) and the burgeoning fairway of successful horizontal drilling and oil production.

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