Woodford Shale: From Hydrocarbon Source Rock to Reservoir

Brian Cardott
Oklahoma Geological Survey

The Woodford Shale (a.k.a. Woodford Chert or Woodford Formation) is mostly Late Devonian in age with the uppermost part as Early Mis-sissippian. Three informal members, related to environments of ma-rine deposition, are recognized based on differences in palynology, organic geochemistry, and electric log response. As a hydrocarbon source rock, the bulk organic composition is Type II Kerogen (oil gen-erative organic matter), the organic quantity is up to 28% total or-ganic carbon content (highest in black shale facies), and the thermal maturity is up to meta-anthracite equivalent (5-6.5% Ro vitrinite re-flectance). The Woodford Shale is the oldest rock in Oklahoma that contains vitrinite (derived from wood) and is a good indicator of the thermal influence of Pennsylvanian-age orogenies and maximum burial depth.

As a reservoir, the Woodford Shale has a brittle, biogenic-silica-rich lithology important for the generation of natural and induced frac-tures. Post-oil solid bitumen forms a network that has nanoporosity which is important for gas storage and migration.

A total of 2,461 Woodford Shale well completions from 2004-2012 delineate three primary Woodford Shale gas and oil/condensate plays in the Arkoma, Anadarko, and Ardmore basins. Since 2009 there has been an emphasis on liquid hydrocarbon plays. Gas, con-densate, and oil production types are related to thermal maturity based on vitrinite reflectance. Much is still unknown about the im-portance of hydrocarbon window boundaries on commercial oil/condensate/gas production. For example, what is the lowest thermal maturity needed for commercial oil production and what is the high-est thermal maturity possible for commercial condensate produc-tion? Lower and upper limits demonstrated so far for proven Wood-ford Shale oil and condensate production, respectively, are 0.56% Ro and 1.67% Ro.

AAPG Search and Discovery Article #90184 © AAPG Woodford Shale Forum, Oklahoma City, Oklahoma, April 11, 2013