Preliminary Reservoir Characterization of Fluvial Tight Gas Sands of the Upper Mesaverde Group, Uinta Basin, Utah
The Greater Natural Buttes field of the northeastern Uinta basin produces gas from upper Cretaceous to Eocene “tight” fluvial sandstones of the Price River, Farrer, Tuscher and Wasatch formations. A key challenge to gas production is that low-heterogeneity reservoirs tend to be water saturated, while the high-heterogeneity reservoirs can be more productive but hard to recognize and correlate in the subsurface. This study identified and characterized 5 different channel types in the upper Mesaverde Group to elucidate the effect of heterogeneity on tight-gas sandstones. Channel types were defined based on their stacking pattern, internal architecture and facies: (1) single story meandering, (2) tidally-influenced meandering, (3) multilateral meandering, (4) multistory amalgamated meandering and (5) multistory meandering with preserved fines. Channel geometries were quantified for each channel type by mapping channels in outcrop, then projecting them in 3D Move with the appropriate paleoflow to get actual channel widths. 3-6 stratigraphic profiles with detailed facies descriptions, paleocurrent measurements and GR curves were collected for each channel type to help define flow units. Photo mosaics of the 5 channel types defined internal heterogeneity and architecture, geometry, and the distribution of 15 facies within them. Outcrop GR curves and porosity/permeability data from core plugs in Greater Natural Buttes field established the outcrop to subsurface connection. Using information about geometry, facies and facies connectivity the flow units were defined based on grain size, lithology, sedimentary structures, and porosity and permeability from core. Characterization and modeling of the 5 channel types showed that the main vertical and lateral barriers to gas flow occur at the top of fining up sequences, at floodplain deposits and mud plugs. Petrographic inspection showed that some channel types have early pore filling cement that may influence early gas trapping in tight-gas reservoirs. Collectively, abrupt vertical and lateral changes in grain size, changes in sedimentary structures, mudclast conglomerates, shale breaks and early cements increased reservoir heterogeneity may have helped trap gas early on, making them more productive even though the high heterogeneity makes them difficult to produce.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California