A theory of oil and gas accumulation has been developed to account for the relations among stratigraphy, structure, and fluid distribution in the oil fields that produce from Paleozoic reservoirs in the Big Horn basin of Wyoming. This proposed theory relates the common oil-water contacts observed in the normal multi-zoned Paleozoic anticlinal fields to height of the oil column, formational thicknesses in the crestal area, and number of Paleozoic formations productive of hydrocarbons in each field. The similar chemical composition of the Paleozoic crude oils and of the associated formation waters, the vertical density stratification of fluids in the multi-zoned fields with large oil columns, and some unusual reservoir-pressure relations are cited in support of the concept of a “common-pool state.”

The major conclusion of this study is that essentially all of the hydrocarbons in Paleozoic and Triassic reservoir rocks in the Big Horn basin were generated from the euxinic, dark-colored, organic-rich and phosphatic, fine-grained sediments of the marine facies of the Permian Phosphoria Formation. Primary migration probably was completed by Early Jurassic time when these hydrocarbons accumulated within regional stratigraphic traps created primarily by (1) updip facies change, pinchout, and truncation of the reservoir carbonates of the Phosphoria Formation, but also by (2) uneven Phosphoria-Goose Egg truncation of the underlying Tensleep Sandstone, both locally within the Big Horn basin and farther east beyond the area covered by marine Phosphoria rocks.

The hydrocarbons in these Phosphoria and Tensleep stratigraphic traps were later released as a consequence of fracturing and faulting associated with Laramide folding, and migrated into older Paleozoic reservoir rocks until fully adjusted to anticlinal structure in common pools. Vertical segregation of an original common pool into several separate pools was accomplished in some exceptional fields by (1) selective hydrodynamic tilting within the Tensleep zone, (2) leakage or redistribution of fluids through fault zones, or (3) escape of hydrocarbons to the surface and inspissation resulting from breaching of the original Triassic cap rocks.

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