Regional Geologic Characteristics Relevant to Horizontal Drilling, Woodford Shale, Anadarko Basin, Oklahoma
HESTER, TIMOTHY C., and JAMES W. SCHMOKER, U.S. Geological Survey, Denver, CO
Horizontal drilling in the Late Devonian-Early Mississippian Bakken Formation of the Williston basin has spurred new interest in other black shales as primary hydrocarbon reservoirs. The Late Devonian-Early Mississippian Woodford Shale, which is similar in some respects to the Bakken Formation, is a major source of oil and gas in the Anadarko basin of Oklahoma and could prove to be a significant reservoir rock as well. The three regional geologic characteristics of the Woodford discussed here are of likely importance to horizontal drilling programs, although direct relations to drilling strategy cannot be developed until empirical data from horizontal tests become available.
First, the Woodford Shale is composed of three distinct depositional units (the upper, middle, and lower informal members) with different physical and geochemical properties. The middle member has higher kerogen content [average total organic carbon (TOC) = 5.5 wt.%] than the upper and lower members (average TOC = 2.7 and 3.2 wt.%, respectively).
Second, a paleotopographic high that was rising before and during Woodford deposition divided the Woodford Shale into northeast and southwest depocenters. This high is interpreted as a basin-margin forebulge that developed as the direct result of loading and subsidence along the central trough of the southern Oklahoma aulacogen.
Third, Woodford depositional patterns are overprinted by thermal-maturity trends shaped primarily by differential burial of the Woodford during Pennsylvanian and Permian time. The Woodford Shale northeast of the forebulge is generally immature to marginally mature, whereas its thermal maturity southwest of the forebulge ranges from mature to postmature with respect to oil generation. A formation resistivity of about 35 ohm-m approximates the updip limit of oil-saturated Woodford Shale from which free oil might be produced from fracture systems.
AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)