Coal Bed Methane and Shale Gas in the Cherokee Basin - from Update
The Cherokee Basin is an intercratonic depression located in southeastern Kansas, southwestern Missouri and northeastern Oklahoma that has been producing gas for over 80 years from carbonaceous shales and coals. In the past fifteen years there has been a systematic exploitation of these unconventional reservoirs in the basin. New techniques are constantly being employed that work specifically in some parts of the basin but not in others. The coals are generally less then two feet thick and carbonaceous shales of the Cherokee Group generally average four feet thick. The productive coals and carbonaceous shales are found from 200 to 2,500 feet and the coals have ASTM rank of high volatile A. Using standard rock properties obtained from mechanical logs and cores the average well, using ten feet of net reservoir thickness can contain a total in place reserve of 50 to 94 MMCFG. However, in a number of areas many wells from a single coal or carbonaceous shale have produced 150 to 240 MMCFG. Original well spacing was at 160 acres, most operators have down spaced to 80 acres and in some cases 40 to 60 acres with no loss in total EUR.
The environment of deposition for these coals and carbonaceous shales represent a tidal dominated delta with limited accommodation, lack of sediment, depth of burial less then 8,000 feet and a tectonic history of limited impact. The cause of the high rank in coals when compared to the tectonic and thermal history is related to basin fluid expulsion and migration from the south through the basin to the north in late Paleozoic and Mesozoic times. Highly productive areas can also be related to basement fault blocks and intrusions defined by aeromagnetics. There are only certain coals that produce the majority of the gas are the Riverton, Rowe, Drywood, Weir-Pittsburg, Croweberg, Bevier and Iron Post. The carbonaceous shales are generally limited to the Excello (Mulky) and Little Osage intervals. The other coals and carbonaceous shales, which lie above and below many of the productive zones, seem to have a minimal amount of gas present. Analysis of palynological, lithologic and clay composition indicates that there are fundamental differences between the productive zones and the non-productive zones. The Cherokee Basin will continue to produce gas from unconventional reservoirs and provides a model for similar basins with thin reservoirs of Pennsylvanian or potentially any age.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009