Evolution and Stress History of Low-Permeability Upper Cretaceous Gas Reservoirs, Rocky Mountains: ABSTRACT

The physical properties of the Mesaverde gas reservoirs in Wyoming, Utah, Colorado, and New Mexico are the result of stress-induced changes that modified the sediment's original properties. Consideration of the stress history aids in interpretation of well log data and in understanding the reservoir performance that is controlled by natural or artificial fractures. The poorly sorted and discontinuous reservoir sands were formed in a shifting nearshore environment in a region of nascent tectonic compression. Rapid sedimentation induced compaction and diagenesis, and later Tertiary burial continued compression and promoted some thermal stress. Laramide wrench-faulting could cause early shear fractures; a later uplift-reburial-uplift sequence, together with episodes of extensional tectonics, could promote tensional fracturing and a re-orientation of fractures and other structures in the well-indurated sediments. Reservoir style is a result of several aspects of basin history, e.g., the depth of Tertiary burial in the Green River versus the San Juan basin. The regions considered are now generally inactive seismically, and there has been very little igenous activity since Cretaceous time, as contrasted to most nearby areas in the Rocky Mountains. End_of_Article - Last_Page 794------------