Luncheon – May 1, 2009

Title: Influence of Compositional and Burial History on Reservoir and Petrophysical Properties of Mesaverde Tight Gas Sandstones in Western U.S. Basins

Speakers: John C. Webb, Alan P. Byrnes, Robert M. Cluff, Daniel A. Krygowski, and Stefani D. Whittaker

Date: May 1, 2009

Publication: The Outcrop, May 2009, p. 18-19

We have examined the relationship between lithofacies, detrital composition, grain size, core and log petrophysical properties in Mesaverde Group tight-gas sandstones from forty cored wells and shallow bore holes in the Uinta, Piceance, Sand Wash, Upper Greater Green River, Washakie, Wind River, and Powder River basins. The primary tool for lithofacies identification is a five digit rock typing system that has proven very useful in calibrating core data to log depths. Digital rock types are also an effective method for statistical sampling of lithofacies.

Typical non-reservoir, fine-grained intervals of the Mesaverde Group are dominated by mudstones and silty shales; burrowed, lenticular and wavy­bedded very shaly sandstones; and marginal reservoir and wavy-bedded to ripple-laminated shaly sandstones. Reservoir and non-reservoir sandstone intervals are dominated by ripple cross-laminated and cross-bedded, very fine to medium-grained sandstones, low-angle cross-laminated to planar laminated sandstones, and massive sandstones. Our data indicate that a given lithofacies can have a wide range of variability as far as environment of deposition, mineralogy, diagenesis, pore type and pore size distribution, porosity, permeability, electrical properties, and irreducible water saturation. Of all of the possible parameters available, grain size and shaliness are the most significant factors in determining porosity and permeability. Cementation by quartz, carbonates and clay minerals further subdivide lithofacies into productive, potentially productive, and nonproductive categories. Pore size distribution and the nature and distribution of clay mineral cements influence permeability, storage capacity, capillary pressure, relative permeability, and electrical properties. Integration of wireline log analysis and core petrophyscial relationships provides guidelines and equations for predicting reservoir properties. Variability in petrophysical characteristics is so complex and subtle that while we feel that we can reliably distinguish productive from nonproductive reservoirs, there is still some uncertainty when it comes to predicting just how “good” a particular reservoir might be. The Mesaverde Project website is (