--> ABSTRACT: Use of Rock-Eval Pyrolysis and Vitrinite Reflectance Data in Characterizing Type and Maturity of Organic Matter in Coal, Upper Cretaceous Fruitland Formation, San Juan Basin, New Mexico and Colorado, by B. E. Law, D. E. Anders, G. E. Michael; #91002 (1990).

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ABSTRACT: Use of Rock-Eval Pyrolysis and Vitrinite Reflectance Data in Characterizing Type and Maturity of Organic Matter in Coal, Upper Cretaceous Fruitland Formation, San Juan Basin, New Mexico and Colorado

B. E. Law, D. E. Anders, G. E. Michael

Rock-Eval pyrolysis data derived from coal samples in the Upper Cretaceous Fruitland Formation in the San Juan basin of New Mexico and Colorado indicate a moderate potential for the generation of liquid hydrocarbons. Hydrogen indices range from 75 to 400 and oxygen indices range from 10 to 44. Cross-plots of hydrogen and oxygen indices on a modified van Krevelen diagram indicate that the organic matter is either type I or II, capable of generating liquid hydrocarbons. However, a visual examination of these coal samples indicate that they are type III organic matter, predominantly composed of vitrinite (>75%) macerals with minor amounts of exinite (<10%) and inertinite (>15 %) macerals and should, therefore, be gas-prone. Although the coal samples are composed pre ominantly of type III organic matter and according to conventional wisdom should be gas-prone, we conclude that the samples contain sufficient hydrogen-rich organic matter to generate both oil and gas. Perhaps vitrinite macerals have higher liquid generating potential than commonly assumed. Mean random vitrinite reflectance (%Rm) values range from 0.42% in the southern part of the basin to 1.54% in the northern part. Pyrolytic temperatures (Tmax) derived from Rock-Eval pyrolysis range from 425 to 480°C and correlate well with Rm values. Cross-plots of the production index (PI) versus Rm show a well-defined hydrocarbon generation curve. Based on this set of data, the onset of thermogenic hydrocarbon generation occurs at a PI of about 0.04, Rm of about 0.60%, and a Tmax of about 430°C. Peak hydrocarbon generation occurs at a PI of about 0.12, a Rm of about 0.85%, and a Tmax of about 450°C. Hydrocarbon generation declines at higher levels of thermal maturity, and at a Rm of 1.5%, the PI is less than 0.04.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990