Permeability, Petrography and Small Scale Structural Element Analysis of Upper Cretaceous Channel Sandstones from the Rock Springs Uplift and Wind River Basin, Wyoming
Authors: Knutson Carroll F. , Hodges Lance T. , Righter Samuel B. 
Published: December 2007

Buried channel sandstones of the Green River and Wind River Basins in Wyoming now exist as low permeability gas reservoirs of limited extent. This has made gas production from these sandstone reservoirs difficult. This paper considers permeability, thin section petrography, sedimentary structural element geometry, and orientation of reservoir type sandstones in these two basins. Twenty sandstone outcrops were studied in the Lance, Ericson, and Rock Springs Formations on the northern end of the Rock Springs Uplift (RSU) and twelve sandstone outcrops were studied in the Mesaverde and Fort Union Formations on the rim of the Wind River Basin (WRB). At least 10 shallow rock cores, each two feet in length, were taken at various vertical levels from each sand unit. These included cores from closely spaced grids from each formation.

Cross bed elements in 12 sandstone units of the Wind River Basin were measured for height, width, and dip azimuths. Parallel inclined and trough cross bed heights were found to be larger in the Mesaverde Formation than in the Ft. Union Formation. Median heights of parallel inclined elements were .65 ft. and .42 ft., respectively. Directivity of the overall sand unit is proportional to element height and in the Ft. Union over ½ of the contribution is by elements less than ½ ft. thick. In the Mesaverde, elements over 1 ft. thick contribute to over half of the directivity. This means that use of a typical dipmeter log to calculate trends in a reservoir sand unit will be easier and more precise in Mesaverde units. Paleocurrent azimuth and directivity were calculated for 12 units. Directivities are higher and azimuths are more similar for units near Hudson than for those near Alkali Butte, where directivities range down to as low as .06. Highly variable directivity values indicate some sand units may have greater lateral extent than others. It is likely that more than one depositional environment is represented by the various units studied. Spot permeabilities were measured at 0.1 ft. intervals on each core. Permeabilities range from less than 1.5 md to 5300 md. Most cores were not cemented. Very low permeabilities were correlated with well-cemented sandstone. The Lance has the lowest permeabilities (median 460 md) and the Ericson the highest (median 2000 md). The Lance has some well-cemented zones, and the Ericson is the coarsest grained rock. The Rock Springs (RSU) and the Mesaverde (WRB) had similar permeability percentiles (medians 775 md and 820 md, respectively).

Limited petrographic analysis showed the sandstones to be detrital quartz arenites. Quartz grains were subangular, elongated, and made up 30.5% to 46.5% of the total rock. Chert grains were abundant in the Mesaverde (22%) and Ericson (16%). Claystone and shale made up most of the remaining detrital grains. Feldspars were rare and mostly altered to clay. Matrix ranged from 22% in the Mesaverde and Ft. Union (WRB) down to 3.5% in the Ericson. The upper parts of Lance units were well-cemented with calcite (47%). Compaction fabric (penetrated chert grains) was prominent, especially in the Mesaverde. This fabric reduces porosity and permeability.

Although this study was made on outcrops that have undergone leaching, it is believed that the relationships found for the sedimentary elements and for the orientation and shape of the larger sand units (composed of the elements) should apply directly to the buried reservoir sand units. This study indicates that overall effective permeability of channel sandstones is complicated by many factors. Correlation of permeabilities measured within sedimentary elements and overall effective permeability of the unit is not now possible. Structural element boundaries and geometries, grain sizes, and diagenesis all have an influence. Effective permeability can be increased by noting the nature of cements and diagenetic minerals present, and designing acid treatments accordingly. Fracture treatments which take into account the geometry and directivity of the channel sandstone reservoir will also increase effective permeability and increase gas production.