SHALE-GAS POTENTIAL OF CRETACEOUS SHALES IN WYOMING LARAMIDE BASINS

Organic-rich Cretaceous shales in Wyoming are outstanding source rocks and should be considered potential shale-gas targets. Total organic carbon (TOC) from the Cretaceous shales in Wyoming basins typically range from 0.5% to 5.0%. These Cretaceous shales range in thickness from 250 to 5,000+ feet thick with especially organic-rich intervals within the shale units ranging up to hundreds of feet thick. T-max for samples ranges from 400o to greater than 455o. Vitrinite reflectance values range to 1.7 at depths greater than 10,000 feet. In the Mowry Shale, displacement pressures range from 300 psi at 5,500 feet depth to over 2,000 psi at 13,000 feet depth. The 13C nuclear resonance spectra indicate that the aliphatic carbon peak of the Mowry Shale is greatly diminished below 13,000 feet depth. Using the Mowry Shale as a guide suggests that at present-day depths greater than 10,000 feet, the remaining kerogen in the Cretaceous shales have very little capacity to generate liquid or gaseous hydrocarbons. Thermal maturation modeling suggests that at depths of 10,000 feet each gram of TOC in the shale generates about 80 milligrams (mg) of gas; but, only about 20 mg of gas is expelled leaving 75% of the gas stored in the shale. Sonic and seismic interval velocities show a pronounced slowdown where significant quantities of natural gas remain in the Cretaceous shales. Successful shale-gas plays in Wyoming will probably have the following attributes: 1) be below the regional velocity inversion surface; 2) have anomalously slow sonic and seismic interval velocities; and, 3) be in a geological setting conducive to fracturing. With new drilling and completion techniques, the recovery of stored natural gas in the Cretaceous shales of Wyoming could be substantial, particularly when the three key attributes converge.