Abstract: Deposition and Diagenesis of Tensleep Sandstone, Eastern Big Horn Basin, Wyoming

David Mankiewicz, James R. Steidtmann

Variations in sandstone porosity due to precipitation or dissolution of cementing materials are primarily caused by differences in pore-water chemistry and/or alteration of hydrodynamic gradients. Folding and fracturing of permeable reservoirs alter fluid-pressure gradients causing pore-water movement and permitting invasion of foreign waters through cross-formational flow and subaerial exposure. Episodes of cementation or porosity development in sandstones are therefore likely in response to regional tectonic stresses. Petrographic investigation of thin sections from outcrops and subsurface cores of the Tensleep Sandstone in the eastern Big Horn basin suggests that the cement paragenesis is related to the tectonic history and can be divided broadly into pre-Laramide, Lar mide, and post-Laramide events. Pre-Laramide events are restricted to early diagenetic dolomitization of micritic carbonate rock, formation of quartz overgrowths, kaolinization of feldspar, and kaolinite infilling. Laramide cementing events consist of precipitation of rhombic dolomite followed by anhydrite and pyrite. Post-Laramide cementation is marked by silicification and precipitation of sparry calcite.

The type, variation, and regional distribution of cement in the Tensleep Sandstone from north to south along the eastern flank of the basin is reflected in the present pore-water chemistry, the composition of which is determined to a large extent by the aquifer's ability or inability to accept recharge from meteoric runoff. In the southern part of the basin where recent calcite spar dominates as a cement, the Tensleep section is thick and crops out over wide areas. Consequently, pore waters have a high bicarbonate to sulfate ratio. Northward, however, anhydrite is the dominant recent cement. Here, the Tensleep is thin and direct recharge is reduced because outcrops are narrow. Basinward, waters are more saline than those on the south and their bicarbonate to sulfate ratios are low.