Reservoir Characteristics and Gas Production Potential of Woodford Shale in the Southern Midcontinent

Abstract

Woodford Shale is a prolific hydrocarbon source rock throughout the Southern Midcontinent of the United States, and in south-central Oklahoma it produces both oil and natural gas. The characteristic and dominant Woodford lithology is black shale, but chert, siltstone, sandstone, dolostone, and light-colored shale are common locally. In general, proximal lithofacies and basin depocenters contain more silt and sand and distal lithofacies more chert. The highest concentrations of organic carbon are found in intermediate settings remote from clastic source areas and bypassed by bottom flows. High concentrations of marine organic matter coexist with abundant biogenic silica, indicating that high biological productivity in surface waters was supported mainly by dynamic upwelling. The primary sites of hydrocarbon generation coincide mostly with the principal depocenters of the Delaware and Anadarko Basins; however, mature source beds are found in adjacent provinces. Mass balance calculations indicate that on the order of 830 x 1012 ft3 of natural gas and 250 x 109 bbl of oil reside in Woodford Shale in Oklahoma, northwestern Arkansas, West Texas, and southeastern New Mexico. Producing this resource is feasible where the subcrop contains competent lithofacies that are highly fractured (e.g., chert, sandstone, siltstone, dolostone). Areas having the greatest potential and most favorable lithologies include (1) northern flank of the Anadarko Basin and the Nemaha Uplift (chert, sandstone, dolostone), (2) Marietta-Ardmore Basin (chert), (3) southern flank of the Anadarko Basin along the Wichita Mountain Uplift (chert), (4) Arkoma Basin and frontal zone of the Ouachita Tectonic Belt in Oklahoma (chert), (5) the Central Basin Platform and Pecos Arch in West Texas and New Mexico (chert and siltstone), and (6) drillable flanks of the Delaware and Val Verde Basins in West Texas and New Mexico (siltstone and dolostone). In much of the Southern Midcontinent Woodford Shale is currently in the oil or gas generation window. Thus, fracture porosity would be continuously fed by hydrocarbons generated in the enclosing source rocks. Petroleum systems such as this typically produce at low to moderate flow rates for many decades.