Abstract Middle Pennsylvanian coal units in eastern Kansas produce commercial quantities of coal-bed natural gas. Annual coal-bed natural gas production in 2008 was 49.1 billion cubic feet (Bcf) (13% of state output); cumulative production since 2000 is 165 Bcf. Coal beds are commonly less than two feet thick and are mostly produced by vertical wells at 80- to 160-acre spacing. Wells usually have comingled gas production from several coal beds. The main producing region is a fourcounty area (Labette, Montgomery, Neosho, and Wilson counties) in southeastern Kansas immediately north of the Oklahoma state line. Most wells are not prolific; their average maximum production rate is approximately 67 mcf/day, peaking about 14 months after initial production. Decline rates are low, as some coalbed natural gas wells have produced 15 years and beyond. North-northwest–south-southeast trending production fairways can be defined by mapping maximum production rates. These fairways generally correlate to where coal beds are individually and compositely thick. The most prolific wells in the thickest coal units record maximum production rates as great as 615 mcf/day. The median as-received gas content for coals in southeastern Kansas is 139 scf/ton, with maximum gas content of approaching 400 scf/ton. Gas content in eastcentral and northeastern Kansas coal beds generally runs half that of southeastern Kansas, indicating economics of coal-bed natural gas production are harsher northward. Coals increase in depth westward at a rate of approximately 20 feet per mile. Their gas content commensurately increases by 10 to 20 scf/ton for each 100 feet of burial. Thin (<4 foot) black shale beds interbedded with the coal units may have commercial potential, for their as-received gas content can be great as 65 scf/ton, but 20 scf/ton is the median of all shale samples assayed.

Abstract The Middle Pennsylvanian (Desmoinesian) Strawn Formation in the Trans-Pecos area of Texas was deposited during relative tectonic quiescence that prevailed before rapid infilling of the Val Verde Basin. It represents one of a series of backstepping carbonate ramps formed on the craton side of this foreland basin. Strawn Formation carbonate rocks in three cores—Conoco Anna McClung #3-1, Alex Mitchell #2-1R, and Creek Ranch #10-1—show several shallowing-upward sequences, each a few meters thick. The Creek Ranch core displays the deepest-water characteristics of the three cores; the lower part of this core is dominated by graded bedding. The Mitchell and McClung cores contain skeletal-rich carbonates. Both of these cores display characteristics of shallow-water bank or lagoonal environments. All three cores have approximately the same diagenetic history. Primary fluid inclusions indicate early porosity-occluding interparticle and mold-filling calcite precipitated from water with a narrow range of salinities. Modal salinities are that of seawater, but slightly lesser salinities (indicating mixing of seawater and meteoric water) and slightly greater salinities (indicating evaporative concentration of seawater) are also indicated. The influence of meteoric groundwater can be detected by stable-isotope analyses of the early cements at stratigraphic levels that correlate to the tops of the major shallowing-upward depositional sequences. However, subaerial exposure surfaces are not demonstrated in these cores but were likely to be present updip. Most porosity is cement-reduced vugs, dissolution-enlarged (and cement-reduced) molds (> 1/16 mm, < 4 mm), and fractures. Minor intraparticle, intercrystalline, and shelter porosity is also present. Reservoir porosity is caused by fracturing and a late-stage dissolution event. Dissolution in the Creek Ranch core is not as pronounced as in the other cores because of a dearth of skeletal material. Porous zones in the McClung and Mitchell cores are associated with open fractures spatially, which commonly interconnect with nearby molds and vugs. This complex porosity system occurred after stylolitization, as evidenced by “cuticles” of insoluble stylolitic residue that bridge across small dissolution-enlarged fractures. Porosity detected by wireline logs therefore is mostly effective porosity. The open-fracture network may have been caused by thrusting of the Strawn Formation, most likely in Permian time. Late-stage cement reduction of porosity occurs in two stages—first by calcite spar, then saddle dolomite. These cements are unevenly distributed. Both of these cements contain primary oil-filled fluid inclusions. Homogenization temperatures of primary aqueous fluid inclusions in saddle dolomites indicate that the Strawn Formation has been subjected to a temperature of at least 136°C (roughly 45°C over present formation temperature), which correlates to a vitrinite reflectance equivalent of 1.22%. Homogenization temperatures, in conjunction with oxygen isotope compositions, indicate that fracture-filling calcite spars and the later saddle dolomites precipitated from isotopically positive fluids, which were probably connate waters that had undergone extensive rock-water interaction. These observations suggest that thrusting of carbonate shelf strata, in a proximal foreland setting, was responsible for creation of latestage fracture porosity. In turn, tectonic expulsion of undersaturated, heated, connate water into the Strawn Formation enhanced the porosity. As this expulsed water cooled, it reached saturation with respect to calcite and dolomite, and these cements partly filled the available porosity. These processes of reservoir creation might be expected in other proximal foreland settings.