Biogenic Coalbed Methane in Illinois Expands the Play Area

David G. Morse, Ilham Demir, Scott D. Elrick, and Keith C. Hackley
Illinois State Geological Survey, Champaign, IL

Shallow (300 to 1500') coals of the Illinois Basin have been attracting increased coalbed methane (CBM) drilling activity in the last few years. Net coal thicknesses compiled for the seven major seams, range from 10 to over 35 feet, with greater thicknesses in eastern and southern Illinois. Gas contents of coals on a dry mineral matter free basis in Illinois range from 50 to over 170 scf/ton, averaging about 100 scf/ton. Of exploration interest is the consistent biogenic isotopic signature of methane desorbed from basin coals. Stable carbon (δ¹³C_PDB) and deuterium (δD_SMOW) isotope values range from -57 to -74 ‰, and -197 to -232 ‰, respectively. Large CBM development projects are currently underway in Sullivan Co., Indiana, in coals at about 350' depth and in Saline County, IL at 500' to about 800' depth. Single seam well production ranges from 10mscf/day to over 50 mscf/day, but at today's prices and shallow depths, these appear to be economically viable. Removal expense of 2 to rarely more than 20% nitrogen and 1 to 3% CO₂ found in desorbed canister gas, must be considered. Multi-seam completions have improved yields. Horizontal wells have just been permitted in some areas and results are unknown. With 284 billion tons of coal resource, the Illinois Basin in-place CBM is estimated at 21 to 25 tcf. Because of the abundance of biogenic methane, CBM exploration is not limited to the heart of the basin, but can extend to ultra-shallow depths nearer the basin edge.