ABSTRACT: Hydrocarbon Source-Rock Evaluation of Desmoinesian (Middle Pennsylvanian) Coals from Part of the Western Region of the Interior Coal Province, U.S.A.

HATCH, JOSEPH R., U. S. Geological Survey, Denver, CO

Coals in the Desmoinesian (Middle Pennsylvanian) Cherokee Group and Marmaton Group in the Western Region of the Interior Coal Province, U.S.A., are potential source-rocks for methane and liquid hydrocarbons. To help evaluate this potential, 85 samples were collected from cores and surface-mines at 21 locations in southeastern Iowa, Missouri, southeastern Kansas, and northeastern Oklahoma. Analyses of these coals are summarized from three areas defined by increasing thermal maturity. Organic matter in coals from area 1 (southeastern Iowa and northern Missouri) is marginally mature with respect to petroleum generation (Rock-Eval Tmax = 430 +/- 4 degrees C, n = 48; Ro less than or equal to 0.55%, n = 9); in area 2 (southwestern Missouri, southeastern Kansas and northeastern Oklahoma near the outcrop) organic matter is mature (Tmax = 446 +/- 5 degrees C, n = 25; 0.57% less than or equal to Ro less than or equal to 0.80%, n = 7); and in area 3 (Osage County, northeastern Oklahoma) organic matter is near peak oil generation (Tmax = 460 +/- 5 degrees C, n = 12; 0.8% less than or equal to Ro less than or equal to 1.0%, n = 3).

For coals from area 1, mean genetic potential (Rock-Eval S1 + S2 mg HC/g sample) is 126 +/- 41 mg/g; from area 2, 139 +/- 46 mg/g; and from area 3, 115 +/- 35 mg/g, which shows that coals from the three areas all have excellent potential to generate hydrocarbons. For coals from area 1, mean carbon-normalized volatile hydrocarbon content (S1/TOC, mg HC/g TOC) is 4.4 +/- 3.1 mg/g; from area 2, 6.8 +/- 4.6; and from area 3, 12 +/- 5 mg/g, which indicates that some hydrocarbon generation has taken place.

For coals from area 1, mean content of CHCl3 extractable organic matter is 27 mg/g sample (geometric deviation [GD] = 1.7; n = 14); from area 2, 15 mg/g (GD = 1.6; n = 9); and from area 3, 9 mg/g (GD = 1.1; n = 5). This decrease is thought to be caused by a decrease in micropore interconnections with increased thermal maturity, which results in lower extraction efficiencies. For coals from areas 1 and 2, saturated hydrocarbon distributions are dominated by pristane with low relative amounts of n-C25 to n-C31; in area 3, relative amounts of n-alkanes and isoprenoids are similar. Extracts from Desmoinesian coals immediately overlain by marine black shales are intermediate in composition between those of black shales and other coals. This may result from the addition or preservation of b cterial biomass in the upper layers of the peat during early diagenesis or from migration of hydrocarbons from the overlying shale down into the coal.

All of the coal extract compositions are dissimilar to oils produced from the Cherokee Group, suggesting minimal generation or migration of liquid hydrocarbons from these coals. There is, however, active exploration for coalbed methane in southeastern Kansas, and at least 74 gas wells have been completed.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)