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The Oligocene (Zemorrian) 64-Zone sandstone is an important oil and gas reservoir in North Belridge field, Kern County, California. The 64-Zone is a submarine-fan deposit that ranges from 83 to 137 m in thickness. Although diagenesis has played an important role in modifying porosity and permeability, the overall variations in reservoir quality reflect an upward increase in grain size associated with depositional processes. Three diagenetic events have had a significant impact on reservoir quality: (1) compaction, which has reduced intergranular volume to an average of 20%, (2) quartz cement, which has reduced porosity by an average of 6.2%, and (3) feldspar dissolution. Although an average of 2.7% porosity is directly associated with leached feldspar grains, mass balance calculations indicate that secondary porosity is roughly balanced by formation of authigenic kaolinite, resulting in little or no net gain in porosity.
Three episodes of calcite cementation reflect various stages of burial history. Petrographic and isotopic data demonstrate that calcite I formed at shallow depths in the zone of bacterial sulfate reduction. Fluid inclusion data indicate that calcite II precipitated at temperatures of 92 to 167°C from fluids less saline than seawater. Fracture-filling calcite III postdates calcite II, but formed at lower temperatures (85 to 125°C). The results of isotopic modeling indicate that calcite II precipitated in equilibrium with waters expelled by shales during I/S diagenesis (^dgr18OSMOW = +2 to +8^pmil) and that calcite III precipitated during tectonic uplift from a mixture of shale-derived and meteoric waters (^dgr18OSMOW = 0 to +4^pmil) Most fluid inclusions in calcite II yield temperatures greater than present bottom-hole values (^sim110°C). Assuming that fluid inclusions in calcite II record maximum burial and a geothermal gradient of 33°C/km, tectonic uplift of 0.6 to 1.7 km (^sim2000-5700 ft) would be required to explain the fluid inclusion data.
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