About This Item
- Full text of this item is not available.
- Abstract PDFAbstract PDF(no subscription required)
Share This Item
The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
Volume:
Issue:
First Page:
Last Page:
Title:
Author(s):
Article Type:
Abstract:
The Bighorn basin is located 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 Bighorn basin above sea-level. Broad, low-relief, northeast-trending folds developed during this orogenic uplift. Drainage patterns superimposed on the exposed Tensleep surface provided stream courses which furnished eroded Tensleep Sandstone sediment for the younger, upper Minnelusa Formation deposited in the east and southeast. During Middle Permian time, the Phosphoria sea transgressed the area, and the stream channels which had been incised in the Tensleep surface were filled with impervious shale, anhydrite, and reworked Tensleep Sandstone. Subsequent Phosphoria d position onlapped post-Tensleep cuestas and monadnocks.
The majority of Tensleep accumulation discovered to date has been in traps which are structurally controlled. The effects of hydrodynamics have been recognized by many as factors in anomalous oil-water contact conditions. However, it is proposed here that accumulations in several of these traps are the result, partly or wholly, of three stratigraphic variables: (1) an intraformational change in permeability and (or) lithofacies, thereby providing a stratigraphic trap; (2) incised channels in the Tensleep surface which were later filled with impervious sediments, providing a truncational subcrop trap; and (3) a combination of (1) and (2) with later Laramide anticlinal folding superimposed on or near these primary traps, which commonly results in tilted oil-water contacts. Meteoric
End_Page 622------------------------------
waters percolating basinward from Tensleep outcrop areas also had the effect of forming a "tar seal" in the oil-water transition zone; this seal effected a "frozen" oil-water contact, further preventing re-adjustment of paleoaccumulations into crestal positions in the Laramide closures.
Anticlines on which production has been established in the Phosphoria, Tensleep, and older Paleozoic reservoirs, exhibit a common oil-water contact datum for each of the producing formations. Extensive vertical fracturing, allowing commingling of reservoir fluids, is a possible mechanism which would allow oil originating in the Phosphoria to accumulate in underlying formations and account for the common oil-water contact conditions.
Subsurface data presently available indicate a loss of porosity with increased depth in the Tensleep Sandstone. It is suggested that possibilities for locating adequate porosity at greater depths will be enhanced by exploration in those areas favorable to the accumulation of oil in primary traps which have not been modified greatly by Laramide folding.
End_of_Article - Last_Page 623------------