ABSTRACT

The clean, orthoquartzitic, Pennsylvanian Tensleep Sandstone of the Big Horn basin, Wyoming, contains three prominent mineral cements: quartz, dolomite, and anhydrite. Present areal and stratigraphic distribution of these cements is the product of two major factors: environment control at the time of original precipitation, and subsequent redistribution by a hydrodynamic system that was created during the Laramide orogeny. Quartz cement shows a marked increase toward a northern and northeastern land source. Massive silica cementation of tightly folded, crest-faulted anticlines, however, is the result of solution and reprecipitation during cross-formational artesian flow. Original accumulation of microcrystalline carbonate “mud” matrix was strongly affected by the winnowing ability of currents moving over a shallow-water platform in north-central Wyoming. Some recrystallization and replacement of microcrystalline carbonate by coarser dolomite occurs throughout the Tensleep as a consequence of downdip flow of ground water. Secondary dolomite cementation is especially prominent below oil-water contacts in Big Horn basin fields where reprecipitation was not inhibited by petroleum. Anhydrite is present in various basin areas where circulation over the former cratonic shelf was restricted. Subsequent anhydrite replacement by dolomite in some of these areas may be related to oxidation-reduction reactions with petroleum.

Characteristic differences between Tensleep and Mesozoic oils in the basin are believed to have been caused by degradation of mature Tensleep petroleum as a result of contact with artesian ground water. Degradation probably occurred through oxidation of petroleum by gases and mineral salts dissolved in the downdip flowing ground water, by bacterial activity, by solution of petroleum in the water, or by a combination of these actions. Oxidation-reduction reactions between artesian water and petroleum can also explain the fact that API gravity, per cent light gasoline fraction, and sulphur content of Tensleep oil display regional variation patterns which closely conform to the potentiometric surface in the Big Horn basin. Oxidation could have taken place during migration of petroleum to structural traps created by the Laramide orogeny. Sufistantial oxidation also occurred as a result of post-accumulation contact with artesian ground water in the oil-water transition zones of oil fields, and throughout faulted traps during cross-formational migration of water.

Big Horn basin fields may be divided into three classes on the basis of prevalence and location of faulting, potentiometric position, and API gravity of produced oil. The classes are: (1) relatively unfaulted, “normal” or main trend fields displaying inclined oil-water contacts and having no active water drive; (2) fields which possess higher than average API gravity oil for their potentiometric position, display flat oil-water contacts with active edge-water drive, and which are associated with updip “ramp” faults; (3) highly faulted fields with lower than average API gravity oil for their potentiometric position, and multiple oil-water contacts, either horizontal or tilted.

Solid hydrocarbon developed on detrital grain surfaces appears to be the product of in situ oxidation of petroleum by artesian water and anhydrite cement. The observed hydrodynamically tilted oil-water contact in some Big Horn basin fields is actually a “fossil” or paleohydrodynamic tilt maintained by secondary dolomite and solid hydrocarbon cementation that prevents or strongly inhibits an edge-water drive. Where solid hydrocarbon is found coating grains in producing fields, the reservoir rock behaves as if partially oil-wet. Unrecognized oil-wet reservoirs can lead to incorrect reserve estimates, and possibly to completion of wells in water-producing zones.

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