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Petroleum Inclusions and the Origin of Gas in the Upper Cretaceous Lewis Shale, San Juan Basin, New Mexico

Parris, T.M.1, N.S. Fishman2, Robert C. Burruss3, Mark J. Pawlewicz4 
1 Petro-Fluid Solutions, Loveland, CO 
2 U.S.Geological Survey, Lakewood, CO
3 U.S. Geological Survey, Reston, VA 
4 U.S. Geological Survey, Denver, CO

Gas in the Upper Cretaceous Lewis Shale, San Juan Basin, New Mexico, notwithstanding the unit's low total organic content (0.5-2.5%), has been interpreted to be internally generated during thermal maturation of gas-prone organic matter. Nevertheless, questions remain concerning the origin of the gas and timing of petroleum generation.

Petrographic and fluid inclusion analyses collected from samples with a range of present-day and deeper maximum burial depths reveal single- and two-phase condensate, and single-phase gas-rich inclusions within quartz overgrowths and fracture cements. The association implies that quartz cementation and petroleum emplacement were partly coeval. Condensate inclusions fluoresce light blue under ultra-violet light, implying thermally mature liquid hydrocarbons. Examination of condensate inclusions using a patented cryo-optical technique, confirmed they have API gravities >52º. 

Raman microprobe analysis, used to determine the composition of gas in the gas-rich inclusions, confirms the presence of methane, ethane, and propane in the inclusions. Remarkably, the composition of the gas-rich inclusions is similar to published data on the composition of wet gas produced from the Lewis in the same well. 

Aqueous fluid inclusion homogenization temperatures (113º-182ºC) from quartz containing petroleum inclusions, are close to maximum burial temperatures (110º-165ºC) determined from vitrinite reflectance data. Convergence of the vitrinite reflectance data and aqueous homogenization temperatures suggests that condensate and wet gas were generated in the Lewis in the late Tertiary, the time of maximum burial. Further, the presence of liquid hydrocarbons suggests organic matter in the Lewis was oil-prone and gas might have been derived from the cracking of oil.