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Geologic Controls on Formation Water Salinity Distribution, Southeastern Greater Natural Buttes Field, Uinta Basin, Utah


Tight-gas sandstone reservoirs of the Upper Cretaceous Mesaverde Group in the Greater Natural Buttes (GNB) Field have variable fluid saturations along with low matrix porosity and permeability. In order to build more reliable saturation models, it is significant to determine resistivity of formation water which is one of the input parameters in water saturation calculations. This study mainly investigates how formation water resistivity and salinity vary stratigraphically and spatially. For petrophysical analysis, the study interval was divided into seven stratigraphic zones based on net-to-gross ratio and variation in resistivity. Formation water resistivity derived from Pickett-plot analysis was used with formation temperature to determine formation water salinity distribution per zone. Temperature data from production logs show that the Wasatch Formation and Mesaverde Group have higher geothermal gradients than formations that are stratigraphically above. Therefore, formation temperature was estimated using these gradients which are consistent through the study interval. Petrophysical analysis indicates more fresh water is present in the western part of the study area coinciding with the trace of a basement fault. Salinity decreases stratigraphically downward while water saturation is variable within the study interval. Average formation water resistivity per zone ranges between 0.048 ohm-m to 0.064 ohm-m based on Pickett- plot analysis, while average formation water salinity per zone ranges between 55,000 ppm to 86,000 ppm. Furthermore, the average effective bulk-volume water is nearly constant around 3.5% suggesting that as being a basin-centered gas accumulation, most sandstones within the study interval are close to irreducible water saturation. A combination of different geological mechanisms might account for observed salinity variations. The increase in freshness stratigraphically downward may be due to basement faulting and associated natural fracture system enhancing upward movement of fresher formation water. In addition, coal and sediment dewatering in stratigraphic units below study interval might be the source of fresher formation water in this potentially closed hydrological system, whereas distinct horizontal layering and continuity of different petrophysical rock types might result in observed salinity trends in the area.