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

Tuba Evsan, Marc Connolly, and Matthew J. Pranter
University of Colorado at Boulder

Use of conventional petrophysical evaluation techniques at Greater Natural Buttes Field in the Uinta Basin is challenging. One reason for this is that sandstone gas reservoirs of the Upper Cretaceous Mesaverde Group have variable fluid saturation along with low matrix porosity and permeability. For petrophysical analysis, the study interval was divided into seven stratigraphic zones based on net-to-gross ratio and variation in resistivity. Pickett plot analysis was conducted for these zones in each well to determine formation water resistivity. Because many Mesaverde Group sandstones do not exhibit 100% water saturation, Pickett plot analysis often relied on interpreting irreducible water saturation. Moreover, water resistivity was used with formation temperature to determine formation water salinity. 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. Total, irreducible, and free-water saturation were determined in order to evaluate the relationship between movable water saturation and salinity variations. Water saturation calculations based on the Archie equation are higher than values based on the Waxman and Smits equation due to bound water associated with shale and clay. Petrophysical analysis indicates more fresh water is present in the eastern part of the study area, while salinity increases and water saturation decreases stratigraphically upward. This may be due to natural fractures enhancing imbibition of fresh water into deeper zones creating the observed variability in salinity and water saturation. Basement faults are believed to create these natural fractures impacting gas and water production across parts of the field.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013