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Facies and Hydrocarbon Reservoirs in Permian Cold-to-Warm Water Carbonate Cycles in the Big Horn Basin, Wyoming, U.S.A.


Outcrop and subsurface investigations indicate that deposition of Permian Phosphoria-age formations occurred under contrasting climatic conditions tied to sea-level cycles. Much deposition was under cool-humid conditions during sea level rises and early high-stands. Hot-dry conditions prevailed during late high-stands and sea-level lows. Climate/sea-level cycles may correlate to periodic melting of glaciers in the southern hemisphere.

Cool-humid conditions are reflected in westerly outcrops and wells by a) cherty phosphatic siltstones with brachiopods, crinoids, and bryozoans that grade upward into b) organic-rich shaley rocks and carbonates with calcite and high-Mg calcite faunas. These are interpreted as cold- to cool-water deposits. Overlying these are c) phosphatic mollusk-rich lime packstones, then d) pellet dolomite wackestone-packstones. These rocks have aragonitic faunas and are interpreted as restricted-marine, warm climate deposits.

Easterly outcrops and wells display fewer and thinner source beds, cherty units, and open-marine carbonates. Instead, the Park City Formation consists of cycles of restricted-marine dolomite capped by peritidal algal boundstone that commonly is teepee structured and micro-karsted late during low-stands. These successions indicate warming during late high-stands and sea-level falls.

Still farther east in the Bighorn Basin and western Powder River Basin, shales, microbial dolomites, and evaporites (Goose Egg Formation) formed in lagoons and salinas during sea-level rise-fall cycles. During maximim fall, stream incisions excavated shallow paleovalleys, assisted by paleotectonics. Subsequent cooling and rising sea levels resulted in filling of valleys with marine sediments.

This climate/sea-level model provides understanding of reservoir porosity development and preservation. Westerly, grain-rich, and less altered calcitic rocks deposited during sea-level rises produce primarily from intergranular and intercrystalline porosity (and tectonic fractures). Easterly reservoirs are mainly peritidal rocks, and secondarily restricted-marine rocks. Porosity in these reservoirs developed late during highstands and also during low-stands during extreme diagenesis of originally aragonite-rich sediments, by formation of mesovuggy-fenestral and fossiliferous-sucrosic dolomites.