Abstract
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 2,000-6,000 and 1,000-4,000 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 Niobraramore » 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 proportionality larger thermal effects. More regional heating by larger volcanic bodies is an important factor in the oil generation history of the area. The authors 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.« less
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