Pennsylvanian and Permian Influence on Tensleep Oil Accumulation, Big Horn Basin, Wyoming

Abstract

The Big Horn basin is in northwestern Wyoming in the central Rocky Mountain province. Near the close of Desmoinesian time, regional uplift on the west and north elevated the Tensleep Sandstone of the Big Horn basin above sea-level. Broad, low-relief, northeast-trending folds developed during this orogenic uplift. Streams forming the drainage patterns superimposed on the exposed Tensleep surface furnished eroded Tensleep Sandstone sediment that subsequently was deposited in the younger upper Minnelusa Formation on the east and southeast. During middle Permian time, the Phosphoria sea transgressed the area and filled stream channels that had been incised in the Tensleep surface with impervious shale, anhydrite, and reworked Tensleep Sandstone. Subsequent Phosphoria depositi n onlapped post-Tensleep cuestas and monadnocks. Most of the oil discovered to date in the Tensleep has been in traps that are structurally controlled. The effects of hydrodynamics have been recognized by many as a factor in anomalous oil-water contact conditions. However, it is the writers' interpretation that accumulations in several of these traps are caused partly or wholly by three stratigraphic variables: (1) an intraformational change in permeability and (or) lithofacies, thereby providing a stratigraphic trap; (2) incised channels in the Tensleep surface that were later filled with basal Phosphoria impervious sediments, providing a truncation subcrop trap; and (3) a combination of (1) and (2) with later Laramide anticlinal folding superimposed on or near these primary traps, a situation which is in some places interpreted as a tilted oil-water contact. Meteoric waters percolating basinward from Tensleep outcrop areas also has had the effect of forming a tar seal in the oil-water transition zone; this tar seal caused a frozen oil-water contact, further preventing re-adjustment of paleo-accumulations into crestal positions in the Laramide closures. Structural accumulations in anticlines that have production established from the Phosphoria, Tensleep, and older Paleozoic reservoirs have a common oil-water contact datum for the producing formations. Many geologists believe that the Phosphoria Formation is the source for oil accumulations that occur in the underlying older Paleozoic reservoirs. Extensive vertical fracturing, making possible commingling of reservoir fluids, is a possible mechanism that allows Phosphoria-source oil to accumulate in underlying formations and accounts for the common oil-water contacts. Available subsurface data indicate that porosity decreases with increased depth in the Tensleep Sandstone. The writers suggest that the best possibilities for locating adequate porosity at greater depths involve exploration in those areas favorable for the accumulation of oil in primary traps which have not been modified greatly by Laramide folding and (or) in areas where younger Laramide structures are superimposed on earlier primary traps.