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Thermal maturity and organic composition of Pennsylvanian coals and carbonaceous shales, north-central Texas: Implications for coalbed gas potential

International Journal of Coal Geology

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DOI: 10.1016/j.coal.2008.05.006

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Abstract

Thermal maturity was determined for about 120 core, cuttings, and outcrop samples to investigate the potential for coalbed gas resources in Pennsylvanian strata of north-central Texas. Shallow (< 600??m; 2000??ft) coal and carbonaceous shale cuttings samples from the Middle-Upper Pennsylvanian Strawn, Canyon, and Cisco Groups in Archer and Young Counties on the Eastern Shelf of the Midland basin (northwest and downdip from the outcrop) yielded mean random vitrinite reflectance (Ro) values between about 0.4 and 0.8%. This range of Ro values indicates rank from subbituminous C to high volatile A bituminous in the shallow subsurface, which may be sufficient for early thermogenic gas generation. Near-surface (< 100??m; 300??ft) core and outcrop samples of coal from areas of historical underground coal mining in the region yielded similar Ro values of 0.5 to 0.8%. Carbonaceous shale core samples of Lower Pennsylvanian strata (lower Atoka Group) from two deeper wells (samples from ~ 1650??m; 5400??ft) in Jack and western Wise Counties in the western part of the Fort Worth basin yielded higher Ro values of about 1.0%. Pyrolysis and petrographic data for the lower Atoka samples indicate mixed Type II/Type III organic matter, suggesting generated hydrocarbons may be both gas- and oil-prone. In all other samples, organic material is dominated by Type III organic matter (vitrinite), indicating that generated hydrocarbons should be gas-prone. Individual coal beds are thin at outcrop (< 1??m; 3.3??ft), laterally discontinuous, and moderately high in ash yield and sulfur content. A possible analog for coalbed gas potential in the Pennsylvanian section of north-central Texas occurs on the northeast Oklahoma shelf and in the Cherokee basin of southeastern Kansas, where contemporaneous gas-producing coal beds are similar in thickness, quality, and rank.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Thermal maturity and organic composition of Pennsylvanian coals and carbonaceous shales, north-central Texas: Implications for coalbed gas potential
Series title:
International Journal of Coal Geology
DOI:
10.1016/j.coal.2008.05.006
Volume
77
Issue:
3-4
Year Published:
2009
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
International Journal of Coal Geology
First page:
294
Last page:
309
Number of Pages:
16