ABSTRACT: Geologic Controls on the Occurrence of Methane in Coal Beds of the Pennsylvanian Hartshorne Formation, Arkoma Basin, Oklahoma
Hartshorne coals have long been known to be extremely gassy because early underground mines in the Arkoma basin were plagued by methane. Mine data, combined with recent advances in coal-bed methane technology, allow us to better quantify this gas resource.
The gas content of Hartshorne coals is controlled by a number of factors, including thickness, thermal maturity, ash content, and reservoir pressure. Hartshorne coal can be generally characterized as being thin (<6 ft), of relatively high thermal maturity (RO 0.7 to 1.7), and of relatively low ash content (<10%). Very little pressure data are available for the Hartshorne, but it appears that Hartshorne coals are underpressured to normally pressured with no indication of overpressuring. Estimates of gas in place for Hartshorne coal range from 2 to 6 bcf of gas per 640 ac.
Primary controls on the producibility of methane from the Hartshorne include the hydrology of the coal, and its permeability. Conflicting data exist as to the nature of the hydrologic regime of the coal. The existence of dry coal in a variety of structural settings is well established. Free-water production from the coal, where present, tends to be low volume (<10 bbl/day) and of a brackish chloride composition. Permeability is controlled by a number of factors, including cleat intensity and tectonic fracturing. Locally, diagenetic cements may significantly reduce permeabi1ity.
AAPG Search and Discovery Article #90991©1993 AAPG Mid-Continent Section Meeting, Amarillo, Texas, October 10-12, 1993.