Stratigraphic Architecture of Lower to Middle Pennsylvanian Coal-beds in the Forest City Basin of Northeastern Kansas

W. Matthew Brown, Timothy R. Carr, and K. David Newell
Kansas Geological Survey and University of Kansas, Lawrence, KS

Abstract

The Forest City Basin of northeastern Kansas contains between 5 and 20 thin coal-beds (typically less than 1 m, but as thick as 1.2m) of variable depositional environments. These coals are interstratified within mixed siliclastic-carbonate cyclothem deposits of the Cherokee and Marmaton groups (Lower-Middle Pennsylvanian). These coals comprise a significant resource within the subsurface of Kansas and vary in rank from high volatile A to C bituminous. While not optimal for coal-bed methane production due to thinness and high volatile ranks, lateral persistence across a ten-county area in northeastern Kansas, coupled with sufficient overburden and competent seals make the Forest City basin an intriguing play for coal-bed methane exploration.

 

Continuous cores were described and linked to geophysical well-logs to construct structure and isopach maps and regional cross sections for a better understanding of depositional environments of major coals. Desorption tests of cores and cuttings of numerous coals show gas contents typically ranging from 10 to 60 scf/ton, although there have been measurements up to 160
scf/ton and methane and proximal coal analyses were performed to determine any variations in geochemical factors. A high-resolution sequence stratigraphic framework was developed for the Cherokee and Marmaton groups to develop depositional models and deduce potential relationships between depositional environment, coal quality, and gas content. A better understanding of spatial, geological and geochemical factors that influence the Cherokee and Marmaton coals is important to understanding coal-bed methane potential in northeastern Kansas.

 

Petroleum Significance - Active CBM Play

Conventional petroleum production in the Cherokee basin that began in the late 19th century continues to present, although in decline for the past fifty years. Since the mid-1980's and early 1990's unconventional shale and coal gas wells have reported cumulative production greater than 300 MMCF. Recent demands for natural gas, increasing prices, and new technologies have turned the Cherokee basin into an active energy play. Up to 14 relatively thin coals beds may be encountered in any one well.

 

The key to a successful coalbed play is to identify numerous coals with higher gas contents located near pipeline infrastructure.
Coals in the Cherokee basin are less than 2,500 feet deep, so drilling costs are relatively low. Top reported completions intervals are in the Mulky coal and overlying Excello Shale, Weir-Pittsburg coal, and Riverton coal. Many producers are completing in coals that are less than 2 feet thick with promising results. The Mulky coal on average is much thinner than other coals, but the combination of a thick black shale with moderate absorbed gas capping the coal, provides an economic target.