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Petroleum Source Potential and Thermal Maturity of Cantwell Formation (Paleocene), Central Alaska Range: A Reconnaissance Study

Richard G. Stanley

The Paleocene Cantwell Formation is a sequence of nonmarine sedimentary and volcanic rocks that is widely distributed in the central Alaska Range and locally is more than 3,000 m thick. Mudstones and coals in the Cantwell were deposited in a variety of lacustrine and fluvial overbank environments and are of interest as potential source rocks of petroleum. Thirty-eight samples of mudstone and coal were collected from ten outcrops and analyzed using Rock-Eval pyrolysis, vitrinite reflectance (Ro), and thermal alteration index (TAI).

The results suggest that these rocks may be potential sources of gas and possibly oil. Total organic carbon (TOC) in the samples is relatively high, averaging about 2.8% (range 0.5-14.2%) in the mudstones and about 28.8% (range 5.1-50.2%) in the coals. Plots of hydrogen and oxygen indices (HI and OI) on modified van Krevelen diagrams indicate kerogens of types III and IV. In addition, HI values are generally less than 150 and S2/S3 values are generally less than 3, indicating that these kerogens are gas prone. A few samples exhibit somewhat higher values of HI (up to 170) and S2/S3 (up to 18.6) and therefore may be capable of generating small amounts of oil. Values of Tmax (range 437-537°C), median Ro (range 0.63-4.28), and TAI (range 2.3?-3.8) show that the thermal maturity of the samples varies from mature to postmature with respect to the oil and gas windows.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.