ABSTRACT

The Pennsylvanian Tensleep Sandstone in the Lost Soldier field of Sweetwater County, Wyoming, is composed of texturally homogeneous, fine-grained sandstones and interbedded dolomite beds. Geologic setting and sedimentary features indicate that the Tensleep was deposited in supratidal-eolian environments. Detailed petrographic, cathodoluminescence, and SEM analyses of core samples from a well in the Lost Soldier field show authigenic features which resulted from nine diagenetic events in the Tensleep: 1) pervasive anhydrite cementation; 2) precipitation of feldspar overgrowths; 3) poikilotopic nodular calcite cementation; 4) nodular anhydrite cementation; 5) fabric-selective anhydrite cementation; 6) precipitation of silica overgrowths and silicification of carbonates; 7) alteration of feldspar; 8) dolomite cementation; and 9) hydrocarbon accumulation. Four diagenetic facies with their own specific patterns of alterations were identified: 1) early-carbonate-precipitation facies; 2) early-anhydrite-cementation facies: 3) primary-carbonate-grain facies; and 4) nodular-cementation facies. The carbonate-rich diagenetic facies are closely associated with the supratidal sebkha depositional environment. The anhydrite-rich facies is related to the interdune sebkha depositional environment, and the nodular-cementation facies is found within rocks deposited in the eolian dune environment.

Reservoir qualities (i.e., porosity and permeability) of the Tensleep Sandstone in the Lost Soldier field vary considerably, both vertically and laterally. Porosity and permeability are primarily controlled by compaction and cementation. Good correlation exists between reservoir quality and volume of cements for samples from nodular-cementation and primary-carbonate-grain diagenetic facies. Samples from other diagenetic facies can also be grouped into distinct areas on a cross-plot of porosity versus volume of cements. Therefore, the cross-plot can be used as a tool to differentiate different diagenetic facies at least for the evaporitic-eolian rock system.