ABSTRACT

Knowledge of the evolution of carbonate cementation in hydrocarbon reservoirs is key to understanding the history of fluid flow during petroleum accumulation. The Stevens sands is a sequence of marine shales and deep-sea fan sands that was deposited within the Miocene Monterey Formation in the south-central part of the San Joaquin basin, California, during the upper Miocene (10-6 Ma). Rapid, high-precision in situ oxygen and carbon isotopic analyses of carbonate phases using the ion microprobe operated in multi-collection mode, in conjunction with electron microprobe analyses, indicate that carbonate cement zones within the Stevens sands at North Coles Levee (NCL) have had a complex and protracted fluid history. Three main generations of carbonate cement were identified.

The relative timing of carbonate cement precipitation within the Stevens sands at NCL was estimated using the thermal and burial history of the San Joaquin basin, in situ oxygen isotope data, and cementation temperatures derived from equilibrium oxygen isotope fractionation factors for calcite-water and dolomite-water. Precipitation of these cement zones began soon after sediment deposition ( 7 Ma) and is ongoing. Early dolomite was precipitated at a temperature of 10°C, near the sediment-water interface, and soon after sediment deposition. Calcite cements, which are the most abundant variety, precipitated semicontinuously between 4 Ma and 5 Ma, at temperatures between 50°C and 65°C, and depths of 800 m to 1300 m. Fe-dolomite, which is paragenetically late, appears to have precipitated at temperatures near 100°C in response to pore-pressure reduction, which accompanied exploitation of the gas cap within the last 35 years. Carbon in these cements was likely derived from several sources including marine, maturing hydrocarbons, and a zone of methanogenesis.