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Dolomitization in the Uteland Butte Member of the Eocene Green River Formation, Uinta Basin, Utah
- Author / Creator
- Rueda Chaparro, Federico
The lacustrine Green River Formation (GRF) is an important oil-producing formation in the Uinta Basin, Utah (USA). In recent years, the unconventional carbonate reservoirs in the Uteland Butte member (UBM), base of the GRF, have been targeted because their estimated oil and gas resources. The stratigraphic interval of interest is limited by the D and C Shales, in which there are three dolomite layers named PZ2, PZ1â, and PZ1 that vary from 1.5 to 8 feet in thickness and have up to 30% porosity but only a maximum of 0.1 mD permeability. The objectives for this study are to characterize the depositional facies types and determine how they are related to dolomitization, to elucidate the dolomitization and how porosity and permeability are related this process, and to delineate the regional geometry of the dolomite layers. This study employed several methods: outcrop and core descriptions; petrographic analysis (transmitted light, cathodoluminescence, and scanning electron microscopy); mineralogical identification and ordering of dolomites (XRD); elemental composition of dolomite crystals (EDS and EMPA); major and trace element analysis (ICP-MS); and conventional and clumped oxygen and carbon stable isotope. Deposition of the Uteland Bute member took place during transgressive-regressive cycles that were driven by climate variations. The PZ layers were deposited in a shallow littoral environment as intraclastic, grainstones, and ooid ostracod grainstone-packstone, and in the sublittoral environments as mudstone, peloidal bioturbated mudstones, and ostracod wackestones. The carbonate facies are dolomitized to variable degrees and are characterized by (i) selective dolomitization of matrix, peloids, ooids, and intraclast, and (ii) variable preservation of the precursor textures. However, bioclasts are not dolomitized. The dolomitization process crosscut all facies boundaries from the distal sublittoral setting toward the littoral near-shore area. There is no discernible relationship between dolomitization and facies. Dolomites are characterized by %Ca that ranges from 49% to 59% with a strong mode in 52 to 55%, and low cation ordering that range from 0.2 to 0.5. Î´13C for dolomite range from 6â° to -5.4â° (VPDB) suggesting a high organic activity and/or organic matter decay, coupled with organic matter oxidation by sulfate reducing bacteria. The Î´18O for dolomite range from 0.9 to -7.3â° (VPDB) and it is interpreted as gradual increase in lake water temperature. The calculated dolomitizing fluid temperature is between 14 to 36 Â°C. Dolomitization was driven by climate changes. During the warmer climate periods, evaporation and reduced river inflow increased salinity sufficiently to create density-driven reflux of lake water in the subaqueous parts of the lake area, which promoted dolomitization. At the same time, the landward parts of the lake fell dry to form a playa, where dolomite formed from evaporating groundwater by evaporative pumping. This process overprinted the areas previously dolomitized by reflux. Dolomitization in the Uteland Butte member is thus interpreted as the result of two different and superimposed processes: density-driven reflux and evaporative pumping. Increased fresh water input during more humid climate periods stopped dolomitization and promoted deposition of lime mud. Î´13C for calcite range from 1â° to -1â° (VPDB), and the Î´18O range from -5.8 to -10.1â° (VPDB), with a calculated temperature for calcite precipitation of around 3 and 12 Â°C. The dolomite crystal sizes range from 0.25 to 6 Âµm and porosity is mainly intercrystal. Dolomite layers PZ1, PZ1â, and PZ2 display four dolomite textures: planar e, planar s, nonplanar-a, and planar-c. Dolomitization contributed to the lithification of the lime mud sediments preserving the primary porosity inherited from the parental lime mud sediments/limestone. Moreover, the mole-per-mole dolomite-calcite replacement likely enhanced the porosity of the PZ layers. Dolomite crystals planar-e, planar-s and nonplanar-a textures are correlated with porosity and permeability. Planar textures (planar-e and planar-s) are associated with the highest values of porosity and permeability. However, dolomite textures, crystal size, and shapes are highly variable throughout the PZ layers. In general, the variations in porosity and permeability within each PZ layer and across the entire study area are too small to influence exploration or development strategies. The best areas for exploration and development are the ones that are most highly overpressured, generally toward the basin center.
- Graduation date
- Type of Item
- Master of Science
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