Generation Storage and Expulsion of Oil and Gas From Mesaverde Group Coals, Washakie Basin, Wyoming

Mario Garcia-Gonzalez, Ronald C. Surdam, Milton L. Lee, and Charles R. Nelson

An integrated study of hydrocarbon maturation reactions of coals from the Mesaverde Group (Washakie Basin of Wyoming) demonstrates that these coals are a significant source of both liquid and gaseous hydrocarbons. Petrographic studies of subsurface coal samples from a variety of depths (e.g. thermal exposure) show that with increasing temperatures, oil is generated during alteration of desmocollinite and liptinite macerals into exsudatinite (waxy oil) and inertinite solid residue; the waxy oil is stored in porous structures and vesicles in the coal; significant oil is expelled if the coals are tectonically fractured; and in the absence of tectonic fractures, with increased thermal exposure, the oil is thermally cracked to gas and as a result the coal is fractured and gas is expelled and migrates. Similar hydrocarbon reaction, storage and expulsion scenarios are observed in hydrous pyrolysis experiments designed to simulate the thermal maturation of coal.

Compositional studies utilizing supercritical CO₂ extractions and gas chromatography/mass spectrometry show that liquids from natural coal samples, hydrous pyrolysis experiments and oil fields in the Washakie Basin are similar. The data suggests that coal is the main source of the oil and gas associated with the Mesaverde Group in the Washakie Basin.

This research was supported by the Gas Research Institute contract #5091- 221-2146.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California