--> ABSTRACT: Rock Types and Lithostratigraphy of the Devonian Woodford Shale, Anadarko Basin, West-Central Oklahoma, USA, by Caldwell, Craig ; #90142 (2012)

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Rock Types and Lithostratigraphy of the Devonian Woodford Shale, Anadarko Basin, West-Central Oklahoma, USA

Caldwell, Craig *1
(1) Cimarex Energy Co., Tulsa, OK.

Since early 2008 over three hundred and fifty horizontal Woodford Shale wells have been completed in the Anadarko basin, west-central Oklahoma, along a northwest-southeast trend approximately 100 miles (161 km) long and 20 miles (32 km) wide. Shallowest production occurs at 10,000 ft (3,050 m) on the northeast side of the trend, and deepest production occurs at 16,100 ft (4,900 m) on the southwest side.

Seven mudrock lithologies, defined mainly on the basis of percent TOC and variations in mineral content (primarily quartz, clay, and dolomite), make-up the fifteen stratigraphic units that comprise the Lower, Middle, and Upper Woodford in the center (core) of the trend where the Woodford is 175 to 330 ft (53 to 100 m) thick. TOC-poor clayey mudrock (<2% TOC) makes-up the basal Woodford, recording the first transgression of the Woodford seas. The overlying middle and upper parts of the Lower Woodford and the Middle Woodford in the core area are composed of 10 to 30 ft- (3 to 9 m) thick intervals dominated by one of three lithologies: clayey mudrock (CM) (38% clay and 41% quartz), clayey siliceous mudrock (CSM) (27% clay and 55% quartz), and less common dolomitic clayey mudrock (DCM) (33% clay, 32% quartz, and 15% dolomite). The Upper Woodford in the core area is predominately CSM and siliceous mudrock. Siliceous mudrocks (SM) average 14.5% clay and 75% quartz. These mudrock lithologies, composing all but the basal most Woodford, are organic-rich with TOC values generally averaging 5 to 6.5%. Core analysis gas-filled porosities are generally 5 to 7%. Clay is predominantly illite, and dolomite is commonly ferroan. Sedimentary structures in these thin-bedded lithologies include parallel lamination, burrow structures, and rare soft-sediment deformation features.

CSM and SM units are characterized by density-neutron cross-over making these units readily distinguishable on wireline logs. In places thin, silica-rich layers in these “more brittle” lithologies display bed-limited, dolomite-healed fractures. Quartz in these rocks is predominantly biogenic. Rocks of the CSM and SM lithologies may record more distal deposition removed from the influx of detrital clay and silt.

Varying mechanical properties of the Woodford mudrock lithologies suggest an understanding of Woodford lithostratigraphy and rock types is potentially a critical component in predicting completion efficiencies and production.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California