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ABSTRACT: Source Rock Evaluation, Oil-Source Rock Correlation, and Kinetic Modeling--San Juan Sag, CO

J. L. Clayton, Robbie Rice Gries

Recently, oil and gas shows have been reported in Cretaceous and Tertiary rocks of the San Juan sag, and minor oil production was established from volcanic rocks (Kirby Petroleum 1 Jynnifer well, Sec. 9, T40N, R5E.). Potential source rocks present in the San Juan sag are the upper and lower (including the Niobrara Member) Mancos Shale (Upper Cretaceous). The combined upper and lower Mancos Shale is about 666 m thick and contains between about 0.5 and 5.5% organic carbon, although most values are between about 1.5 and 2.0%. The Niobrara Member of the lower Mancos Shale has the highest overall organic matter content in the section (organic carbon averages <2.0%). Pyrolysis and solvent extraction data (typically 2000-6000 and 1000-4000 ppm, respectively) indicate that the upper and lower Mancos Shale and the Niobrara Member are all good potential source rocks for oil and gas. Oil-source rock correlations using gas chromatography, mass spectrometry, and stable carbon isotope ratios indicate that the upper Mancos Shale is the most likely source for the oil produced from the 1 Jynnifer discovery well. The source of the oil produced from the nearby Gramps field is less certain, but may be the lower Mancos Shale or Niobrara Member. The hydrocarbon generation history of the San Juan sag is complex because of highly variable heat flow in the area caused by Oligocene volcanism. Sills have caused thermal alteration of organic matter in shales on a local scale, and larger volcanic bodies may have produced proportionally larger thermal effects. More regional heating by larger volcanic bodies is an important factor in the oil generation history of the area. We have constructed kinetic models at several locations in the area to estimate the timing and amount of hydrocarbon products generated from the source rocks. The main phase of oil and gas generation and expulsion occurred during the Oligocene.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990