A theory of hydrocarbon accumulation has been developed to account for the interrelations of stratigraphy, structure, and fluid distribution in the Paleozoic oil fields of the Big Horn basin, Wyoming. The proposed theory explains the common oil-water contacts observed in the majority of the multi-zoned Paleozoic anticlinal fields and the associated correlation between vertical oil column, formational thickness in the crestal area, and number of Paleozoic formations which are productive of hydrocarbons. Similar chemical composition of all Paleozoic crude oils (despite selective alteration effects) and of the associated formation waters, severe vertical density stratification of fluids in the multi-zoned fields with large oil columns, and some unusual reservoir-pressure rel tions are considered in support of the concept of a "common pool state." Several exceptional fields are explained as the result of post-accumulation modification of an original common pool.

The primary conclusion of this study is that essentially all the commercial hydrocarbons trapped in Paleozoic and Triassic reservoirs in the Big Horn basin (and probably all of western Wyoming) were generated in the euxinic, dark-colored, organic-rich and phosphatic, fine-grained rocks of the marine facies of the Permian Phosphoria (Park City) Formation. Similar dark-colored "source rocks" are essentially absent from the rest of the Paleozoic section in the Big Horn basin. Also, important intersystemic unconformities representing long periods of erosion appear to have eliminated effectively all sub-Phosphoria formations (except, locally, the underlying Tensleep Sandstone) as possible storage reservoirs for Paleozoic oil that might have migrated before the Laramide orogeny. Thus, consi erable importance is attributed to the fact that stratigraphic accumulations of commercial significance occur only in Phosphoria rocks, whereas all production from pre-Phosphoria formations is associated with structural closure.

The proposed theory states that, as a consequence of early generation and probable pre-Middle Jurassic flush migration within the Phosphoria marine facies, indigenous hydrocarbons became stored in the ideal primary stratigraphic trap of the western Wyoming shelf area. In the Big Horn basin area, this trap was created by facies change updip toward the east and south, and pinch-out and truncation of the widely distributed reservoir carbonates of the Phosphoria Formation toward the north. In addition, some hydrocarbons probably were expelled during early diagenesis into the underlying Tensleep Sandstone and, because of eastward lateral migration beyond the area covered by marine Phosphoria rocks, became caught mainly in several truncational subcrop traps at the Phosphoria-Tensleep unconf rmity.

As a result of the formation of many large anticlinal traps, and concomitant fracturing and faulting, during the Late Cretaceous-early Tertiary Laramide orogeny, the hydrocarbons held within the regional Phosphoria stratigraphic trap were released and spread into older Paleozoic reservoir rocks until they were adjusted fully to structure in common pools with common oil-water contacts. Segregation of an original common pool into several separate pools was accomplished in some exceptional fields of the basin 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 the breaching of the original Triassic cap rock.

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