The Arkoma Basin, an elongated, linear east-west-trending sedimentary basin, encompasses an area of approximately 13,488 sq mi in east-central Oklahoma and west-central Arkansas. Within this basin, extensive reserves of bituminous and some semianthracite coals are contained in Pennsylvanian age rocks. The Oklahoma Geological Survey and the U.S. Geological Survey have estimated that the total coal resource of this basin might be 7.89 billion tons.

More than ten individual coal seams have been identified in this area, eight of which have been mined or are being mined presently. The majority of the major coalbeds are continuous but do not maintain constant thickness. Individual beds range from a few inches to 7 ft in thickness. The coalbeds outcrop along the periphery of the synclines and dip steeply (up to 90°) toward the axis. Lower (Atokan) and Upper (Missourian) Pennsylvanian coals are thin and discontinuous, whereas the Middle Pennsylvanian (Desmoinesian) coals are thick, generally continuous, and provide the basin's major proven reserves. The thin Lower Pennsylvanian coals have not been studied in as much detail. The Hartshorne, the McAlester (Stigler), and the Secor coalbeds are the most extensive and uniformly thick c als in the Arkoma Basin; estimated coal reserves are over 7.4 billion tons, constituting 93% of the basin's total coal resource.

Methane Recovery from Coalbed Project (MRCP) data on the methane gas content of Arkoma Basin coals is presently limited to coring and well testing in Pittsburg, Haskell, and Le Flore counties, Oklahoma. In Pittsburg County, the Upper Hartshorne, Upper Booch, and McAlester coals were cored in two wells between the depths of 1,905 and 3,650 ft and samples were desorbed for approximately 5 months using the USBM's direct method. Total gas contents ranged between 73 and 211 cubic feet per ton (cf/ton). In Le Flore County, Oklahoma, the Upper Hartshorne was sampled at a depth of 192 ft and found to contain 310 cf/ton. Variability in gas content cannot always be directly related to coalbed depth, inasmuch as some of the gassier coals were from shallower horizons--the opposite of what might h ve been expected--but is related to other geologic controls on the gas content, analytical errors, or to the method used in determining the "remaining" gas content.

The Hartshorne coals in Oklahoma have been the most frequently sampled coalbeds. Estimated gas content of the Hartshorne ranges from 73 to 570 cf/ton. Other measured values are 200-211 cf/ton for the Booch coals and 131 cf/ton for the McAlester coal. The latter two estimates are from coals located in the subshelf part of the basin where the coals are thinner and not as well developed.

Methane liberated from mines during operation also provides an indication of the potential methane resource in an area. The correlation between the amount of gas emitted from coal mines and that predicted to be there by desorption testing is not obvious. Experience in the Arkoma Basin at the Howe mine in Le Flore County, Oklahoma, shows that about seven times more gas is liberated from a mature mining operation than what was measured by desorption testing.

Based on the limited data available, ranges for the maximum and minimum expected in-place gas resource have been made for all the major coalbeds. The Hartshorne coals are anticipated to have a minimum of about 5.4 billion short tons and a maximum of nearly 2.45 trillion cubic feet (Tcf) of in-place gas. The McAlester (Stigler) Coal, likewise, is estimated to contain a maximum of 0.69 (Tcf) of gas. The minimum in-place gas resource for the Hartshorne and McAlester coals totals about 1.4 Tcf.

It is reasonable to assume that the methane contained in the deeper parts of the various major synclines may add significantly to this figure. The gas content of the coals in the Arkoma Basin is generally higher towards the east. Several target areas totaling approximately 3,600 sq mi have been identified as having the greatest probability for early commercialization of gas. The main target area is located in the deepest portion of the Cavanal syncline in Le Flore County, Oklahoma. Another principal target is located in the Lehigh syncline in Coal and Atoka counties, Oklahoma. An alternate target would be the Cavanal syncline area in the western part of Sebastian County, Arkansas. These areas contained reasonably thick coal sections at considerable depths.

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