Theory of Paleozoic Oil and Gas Accumulation In the Big Horn Basin, Wyoming

Abstract

This paper, to be presented at the 42nd Annual Fall Meeting of the Society of Petroleum Engineers of AIME, Houston, Tex., Oct. 1–4, 1967, is copyrighted by the American Association of Petroleum Geologists and will appear in the AAPG Bulletin. Permission to publish any portion of it must be obtained from the Publications Manager of that bulletin. SPE wishes to express appreciation for permission to preprint this paper, and to include it on the program. Abstract 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 related 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 multizoned fields with large oil columns, and some unusual reservoir pressure relationships 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 held in Paleozoic and Triassic reservoir rocks in the Big Horn basin were generated from the euxinic, dark-colored, organic-rich and phosphatic, finegrained sediments of the marine facies of the Permian Phosphoria formation. Primary migration was probably completed by Early Jurassic time when these hydrocarbons accumulated within regional stratigraphic traps created primarily by updip facies change, pinchout and truncation of the reservoir carbonates of the Phosphoria formation, and also by uneven Phosphoria-Goose Egg truncation of the underlying Tensleep sandstone both locally within the Big Horn basin and further east in the area beyond that was covered by marine Phosphoria rocks. The hydrocarbons held within these Phosphoria and Tensleep stratigraphic traps were later released as a consequence of fracturing and faulting associated with Laramide folding, and moved 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 of the basin either by selective hydrodynamic tilting within the Tensleep zone, by leakage or redistribution of fluids through fault zones, or by escape of hydrocarbons to the surface and inspissation resulting from breaching of the original Triassic cap rocks.