Stratigraphy, Depositional Environments and Coalbed Methane Resources of Cherokee Group Coals (Middle Pennsylvanian)--Southeastern Kansas

Jonathan P. Lange
Kansas Geological Survey
University of Kansas

Abstract

The Cherokee basin of southeast Kansas, part of the western region of the Interior Coal Province, is a hydrocarbon-bearing foreland province. Middle Pennsylvanian Cherokee Group coals form a large portion of an estimated 48 billion metric tons of deep coal resources (greater than 30 meters) in eastern Kansas. Cherokee coals are of high-volatile bituminous A and B rank. With sufficient overburden and thick seals they have high potential for coalbed gas production. In Kansas, economic coalbed gas production requires coals of higher quality (low ash, high Btu, and high gas content), seams generally thicker than 0.3 meters, and multiple coals within close proximity to pipeline infrastructure.

Structure and isopach maps, along with cross sections constructed from cores, outcrops and well logs, provide a better understanding of the lateral variability and extent of the major coal bearing sequences. Integration of core descriptions with well logs was used to correlate depositional environments across the Cherokee basin. Cherokee Group coals accumulated in a variety of depositional settings such as, marshes, non-barred and back barrier coastlines, estuaries, and fluvial flood basins. Variations in coal quality, thickness, and lateral distribution can be understood by placing Cherokee Group coals within a sequence stratigraphic framework. Thicker and laterally extensive coals developed toward the end of the transgressive systems tract, and beginning of the highstand systems tract. Pre-existing topography played a major role in the growth, distribution and quality of peatlands that developed into coal.

Desorption of coal and shale samples from core holes within the study area determined total gas content. Gas contents varied from 3 to more than 300 standard cubic feet/ton (scf/ton). Black shale gas contents range from 3 scf/ton to 35 scf/ton. Within the study area, an estimated 6.6 trillion cubic feet of original gas in place is estimated from twelve coals and two black shales. An improved geologic understanding of the Cherokee Group coals can aid in coalbed gas exploration and development in southeastern Kansas.