Abstract: Rank of Coal as Index of Organic Metamorphism for Oil and Gas in Alberta, Canada

Peter A. Hacquebard

The degree of organic metamorphism (DOM) of producing oil and gas pools in Alberta has been obtained from rank variations of subsurface and near-surface coals. Of the former, which are coals of the Lower Cretaceous Mannville Formation, vitrinite-reflectance measurements were used as rank parameter, and of the latter the moisture content was employed.

From the two sets of data the rank changes of the Mannville coals have been plotted in relation to maximum depth of burial, which in turn permitted the construction of subsurface isoreflectance contours. These contours not only give the degree of organic metamorphism, but also the corresponding paleotemperatures, which were obtained from Karweil's correlation diagram of coalification time, rank, and temperature.

Results show that in Alberta 88% of initial oil reserves have a DOM comparable with a coal rank of 0.5 to 0.9% R_o (reflectance), or with high-volatile C to middle of high-volatile A bituminous coal; the corresponding paleotemperatures range from 68 to 116°C. No oil was found above 1.3% R_o (medium-volatile bituminous coal), or above 143°C.

With regard to the DOM of initial gas reserves, biogenic methane is present at a coal rank of 0.4 to 0.5% R_o (subbituminous coal), or between 52 and 68°C; thermal gas ranges from 0.8 to 2.0% R_o (middle of high-volatile A bituminous coal to semianthracite), or between temperatures of 106 and 177°C.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC