| The Cherokee Basin is an inter-cratonic depression that is part of the Pennsylvanian-age Western Interior Basin. The coals and carbonaceous shales of the Cherokee and Marmaton groups of Desmoinesian age have produced significant quantities of gas for nearly 100 years from depths of less than 2,000 feet. This study determined that the location of the gas in the Cherokee Basin is related to a pre-Pennsylvanian Paleozoic thin, underling the Chautauqua Arch that allowed migrating low temperature hydrothermal fluids (LTHF) to thermally mature Desmoinesian-age coals and carbonaceous shales. This event occurred in the Late Permian and is associated with the Ouachita-Marathon Orogeny to the south.;Two of the study areas presented in this study, Jefferson-Sycamore and Thayer, are located over the paleo-thin. In many parts of the basin, there are localized areas of production related to localize LTHF, structural and stratigraphic features. These are illustrated in the Chanute, Bourbon and Mound Valley study areas presented here. The seams in the basin are generally less than two feet thick, may be laminated with alternating bands of coal and shale, are High Volatile A Bituminous rank, high in sulfur, and contain low to moderate amounts of vitrinite as compared to equivalent coals in adjacent basins. The Tebo B, Tebo, V, Excello and Little Osage carbonaceous shales in ascending order, are deep water marine or lacustrine deposits, rich in organic matter and are capable of generating gas. The primary gas productive coals in ascending order across the basin are the Riverton, the Rowe, the Weir-Pittsburg and the Mulky coals. The Aw, Neutral, Drywood, Scammon, Mineral, Croweberg, Bevier and Mulberry coals are marginal or non-productive. The productive coals have sulfur contents over 5%, high fixed carbon or vitrinite and lower ash contents than the non-productive coals. The Excello Shale has over 2.5% sulfur, and high percentage of quartz and carbonates which results in excellent porosity and permeability.;The majority of hydraulic fracture stimulations used slick water, but cross-linked gel and nitrogen have proven to be as effective but were more expensive. The study found that a single stimulation of a single zone is more productive than multiple stimulations with multiple zones. Multiple stimulations generally are detrimental and result in fracture communication between several zones, thus losing their effectiveness. The decline curves of these unconventional reservoirs in the Cherokee Basin resemble conventional production decline curves rather than the typical dewatering curve for coalbed methane wells.;Throughout this study the integration of geology, production, reservoir characteristics and completion methods was used to understand why certain areas have significant gas in unconventional reservoirs. The overall conclusion of this study is that the gas being produced from the Desmoinesian unconventional reservoirs is a basin-centered gas play. |
