Petrology of Pennsylvanian Tensleep Sandstone, Lost Soldier Field, Wyoming

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 show nine diagenetic events in the Tensleep: (1) early calcium sulfate cementation; (2) precipitation of feldspar overgrowths; (3) poikilotopic nodular calcite cementation; (4) nodular anhydrite cementation; (5) late anhydrite cementation; (6) precipitation of silica overgrowths and silicification of carbonates; (7) alteration of feldspar; (8) dolomite cementation and dolomitization; and (9) hydrocarbon migration and accumulation. Four diagenetic facies with specific patterns of alterations have been identified: (1) early carbonate-precipitation facies; (2) early anhydrite-cementation facies; (3) primary carbonate-grain facies; and (4) nodular-cementation facies. Diagenetic alterations in each diagenetic facies were primarily controlled by lithologic characteristics within the original depositional environments. The carbonate-rich diagenetic facies are closely associated with the supratidal sabkha depositional facies. The anhydrite-rich facies is related to the interdune sabkha depositional facies, and the nodular-cementation facies is present within rocks deposited in the eolian-dune facies. Reservoir qualities (porosity and permeability) of the Tensleep Sandstone in the Lost Soldier field vary considerably and, with the mineral composition of the Tensleep, were modified significantly by diagenetic alterations. They can be estimated from knowledge about the distributions and associations of depositional and diagenetic facies.