Abstract

Black “shale” intervals within the Hermosa Group (Pennsylvanian), particularly those within the 7,000+ feet thick Paradox Formation, have a high potential for future shale gas production. The synorogenic Hermosa Group was deposited as a series of orbital-forced, transgressive-regressive cycles dominated by restricted-basin carbonates and anhydrite. Typically in the Paradox Formation, the base of each cycle is a 10 to 150 feet thick transgressive, dark gray, laminated, organic carbon-rich dolomitic mudstone or shaly dolomite. This interval grades upward into intercalated anhydrite, dolostone and limestone, with minor black shale, overlain by a thick halite interval deposited during the lowstand, arid-icehouse portion of the glacio-eustatic cycle. The TOC of the black “shale” averages 2% to 4%, and commonly exceeds 10%. Kerogen is dominantly amorphous (algal) with minor terrestrial organic matter. Limited Ro values show the Paradox Formation to be in the gas-generative window in the northwest, north and eastern parts of the basin, and in the oil-generative window in the south and southwest. Logs in the northeast half of the basin record mud gas in the black “shale” up to thousands of gas units. Currently, oil with associated gas is produced from the black “shale” intervals in the south-central part of the basin, and gas with minor condensate is produced from a few wells in the northeast part of the basin. Several factors favor the development of shale gas in the northeast half of the Paradox Basin: the high organic carbon content, the elevated level of thermal maturity, the known presence of gas in the “shale”, and a net “shale” thickness greater than 350 feet. The evaporite beds enclosing the “shales” are important both as a barrier to natural hydraulic fracturing during hydrocarbon generation and as an exceptionally effective seal to retain the hydrocarbons and maintain an overpressure to keep the fractures open. The black “shales” of the Paradox Basin constitute a potentially large natural gas resource waiting to be evaluated and exploited.