Hydrocarbon resource evaluation in the Woodford shale using well logs

Abstract

In the U.S. mid-Continent and west Texas areas, typically black, organic-rich shales include the Antrim, Chattanooga, and Woodford formations of Lower Mississippian and Upper Devonian ages. The Woodford shale in west Texas and southeast New Mexico is believed to be the source of at least 10% of the oil in the region. In southern and central Oklahoma, the Woodford shale is a carbonaceous shale, dark brown to black in color, with locally interbedded thin intervals of chert, siliceous shales, and siltstones. In the main oil-producing region of central and southern Oklahoma, 22 billin barrels of bitumen and 16 billion barrels of hydrocarbons have been expelled from the Woodford Formation. Locally, potential hydrocarbon production occurs directly from this Woodford section and is controlled by the presence of natural fracture systems, particularly in intervals of high carbonate, silt, and/or chert content. Well log responses in the Woodford shale include excessively high, natural radioactivity, a non-definite spontaneous potential, high formation resistivity, and low bulk density. The last two are caused by the presence of kerogen in the pore space. Natural gamma-ray spectral logging and associated interpretive techniques have proven useful particularly in locating the natural fracture systems from which the Woodford shale sequence often produces after initial completion or recompletion attempts. Field case examples illustrate these concepts.