--> Abstract: Geochemical and Geological Controls on Generation and Accumulation of Oil and Gas from Coalbeds, Western United States, by J. L. Clayton, D. D. Rice, and R. W. Stanton; #91012 (1992).

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ABSTRACT: Geochemical and Geological Controls on Generation and Accumulation of Oil and Gas from Coalbeds, Western United States

CLAYTON, J. L., D. D. RICE, U.S. Geological Survey, Denver, CO, and R. W. STANTON, U.S. Geological Survey, Reston, VA

Coalbeds have long been recognized as sources of gas and more recently have been shown to be sources of liquid hydrocarbons. In-place gas reserves in the United States may total 800 TCF (trillion cubic feet). The recoverable resource may be as much as 145 TCF, according to recent estimates by the Potential Gas Committee. Only small amounts of oil in economic amounts have been produced from economically viable accumulations in coalbeds or other reservoirs associated with coalbeds in the United States; thus, no reserve estimates are currently available for coal-derived oil.

Coalbeds in western United States basins are estimated to contain large resources of gas, and major efforts have been made to produce this gas, especially in the San Juan Basin of New Mexico and Colorado. Small (subeconomic) amounts of oil have been produced in the San Juan Basin, whereas significant amounts of oil from either coal or coaly organic matter dispersed in shale have been produced in the Piceance basin. As described by previous workers, key factors that affect oil and gas generation and accumulation from coal include thermal maturity (rank), gas content, moisture content, permeability, and chemical or maceral composition.

Most western United States coal contains macerals of the vitrinite group and is classified as humic (type III) based on a general description of maceral composition. Such coal is considered to be capable of generating mainly dry gas. However, more detailed examination of macerals using etched polished surfaces under blu-light fluorescence and white light reveals that not all of the coal contains only structured humic (low-hydrogen) vitrinite. Some of the coal contains abundant matrix gel, a more hydrogen-rich type of vitrinite. Both types of coal produce methane-rich bacterial gas at low thermal maturity (Rm < 0.6%). At higher maturity (Rm > 0.6%), thermogenic hydrocarbons generated from these two types of coal vary. Structured humic, low-hydrogen coal generates methane-rich gas (>98% in the C1-C5 fraction), whereas coal containing the more hydrogen-rich vitrinite generates wetter gas (CH4 = 90% of the C1-C5 fraction) and liquid hydrocarbons. Coal containing significant amounts of matrix gel at relatively low thermal maturity (Rm 0.7-0.8%) generates liquid hydrocarbons in the San Juan and Piceance Basins. The amount of liptinitic material in the coal does not appear to be a major factor controlling the liquid generation potential. Oil generated from this coal is characterized by high pristane/phytane ratios (a common feature in coal-derived oil worldwide) and a slight predominance of odd-carbon-numbered n-alkanes in the C20+ fraction. However, the overall boiling-point distribution of saturated hydrocarbons is dominated by low-molecular-weight compounds (C1-C 0), which results in oil having high API gravities (about 50 degrees). Methycyclohexane is a predominant compound in the C1-C10 fraction and may indicate resin- or lignin-derived organic materials. The oil typically has very high saturated/aromatic hydrocarbon ratios (about 11), although the coal extracts have lower ratios. This difference is probably due to migration effects.

 

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