AAPG ACE 2018

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Assessment of the Woodford Shale Petroleum System Within a Deep Subbasin on the Central Basin Platform, Permian Basin

Abstract

The thickest and most distal section of the Upper Devonian Woodford Shale in the Permian Basin was deposited near the axial depocenter of the early to mid-Paleozoic Tobosa Basin. Today, a >560 foot thick section of the Woodford is preserved in a subbasin on the Permian Basin’s Central Basin Platform (CBP). Conventional core was cut from 11,468 to 11,515 feet TVD in northwestern Winkler County, Texas, to define the geologic properties of the Woodford at this location and assess its viability as an unconventional resource play. A full suite of core, log, and cuttings data is used to characterize rock compositions, facies, reservoir properties, and thermal maturity. The Woodford at this location is an organic-rich, slightly dolomitic, silty, pyritic, highly-siliceous mudstone, averaging 55-75% silica, 10-30% detrital clay, and 5-9 weight% TOC. Thin-section analysis of the core reveals three main facies: pyritic radiolarian chert, radiolarian-poor siliceous mudstone, and mixed chert-mudstone.

Trace element data from across the full Woodford section suggest dramatic changes in basin restriction through time, possibly recording to a second-order sea level highstand during the early Frasnian followed by regression and overprinting by higher-order cycles through the latest Famennian. Porosity ranges from 6-10%, and permeability was measured in the nanodarcy range, with oil and water saturations averaging >47% and <25%, respectively. Rock pyrolysis, oil extract, oil biomarkers, and gas isotopes indicate that thermal maturity ranges from 0.91-0.94 %Ro equivalent, indicating late peak-oil generation with little migration. We use a strong depth-maturity relationship to demonstrate that the entire subbasin area is within the oil generation window.

High inorganic porosity filled with organic matter and common organic-hosted nanoporosity is visible in FE-SEM images. High magnification images reveal that most of the Woodford’s silica-rich matrix consists of silica nanospheres 200-500 nm in diameter interpreted to have formed penecontemporaneously with deposition and preventing formation compaction through time. The sum of our findings indicate that the thick, distal Woodford Shale section in the Winkler County subbasin has key attributes required to be a viable source-rock reservoir, whereas the Woodford elsewhere on the CBP typically lacks thickness and/or adequate thermal maturity.