Determining Water Saturation in Diatomite Using Wireline Logs, Lost Hills Field, California

Abstract

SPE Member Abstract There is a long-held paradigm in the California oil industry that wireline log evaluation does not "work" in Monterey Formation lithologies. This study demonstrates that it is possible to calculate accurate oil saturation from wireline log data in the diatomite reservoir at Lost Hills field, California. The ability to calculate simple but accurate oil saturation is important because it allows field management teams to map pay, plan development and waterflood programs, and make estimates of reserves more accurate than those based on core information alone. Core data from eight wells were correlated with modern resistivity and porosity logs, incorporating more than 2000 ft of reservoir section. Porosity was determined from bulk density and water saturation was determined using the Archie equation. Crossplots of corrected core oil saturation versus Archie oil saturation (1-Sw) confirm the accuracy of the algorithm. Improvements in the accuracy and precision of the calculated oil saturation will require more detailed reservoir characterization to take into account lithologic variation. Introduction Development of the Monterey Formation diatomite reservoir at Lost Hills field began with the discovery well in 1910 and continues today. Since logs were first run in the Monterey Formation, geologists and engineers trying to interpret them were puzzled because the usual oil indicators, high resistivity and a distinctive reservoir rock log signature, were either not present or inconsistent. Not only were oil productive zones difficult to identify, but porosity calculated from the bulk density log using measured grain and fluid densities was generally too low compared to core porosity. Consequently, the logs were used qualitatively and only for correlation and completion planning. Few successful attempts were made to relate log response to oil saturation. P. 369

Pressure and fluid saturation prediction in a multicomponent reservoir using combined seismic and electromagnetic imaging

“…Estimated shear modulus and grain density of the diatomite grains is very close to the values of 18 (GPa) and 2.3 (g/cc) estimated by Wang (2001). Bilodeau (1995) measured an average grain density of 2.37 g/cc from another location in the diatomite at Lost Hills; he also measured -1.84, -1.95, and 0.21 (S/m) for Archie's Law porosity exponent, saturation exponent and…”

Joint cross‐well and single‐well seismic studies of CO2injection in an oil reservoir

“…1987). The decrease in PR with depth is an indication of the compaction of the diatoms accompanied by an increase in density (Bilodeau 1995).…”

“…The reservoir at Lost Hills is comprised of diatomite, which consists of approximately equal parts of biogenic silica, clay and silty‐sands. As the diatomite is buried at higher pressures and temperatures, the silica, which is initially in a form called opal‐A, undergoes a phase transition to opal‐CT and subsequently to quartz (Isaacs 1982; Bilodeau 1995). Hydrocarbons are found in all three phases with enhanced production from fracturing in transition zones.…”

Heavy and Thermal Oil Recovery Production Mechanisms

“…9-10). The lower oil saturations of core-versus log-derived values may be due in part to flushing of the core by drilling, expulsion (drainage) of the oil during handling, shrinkage of the oil due to out gassing, and expansion of the core once confining stress was removed (Bilodeau, 1995). There was not a net change in saturation for the remaining pore types, PT1, PT2, and PT3.…”