Diagenetic Carbonate in Miocene Sandstone Reservoir, San Joaquin Basin, California

Abstract

Stevens arkosic sandstones, deposited about 7 Ma in a deep marine environment, form the North Coles Levee reservoir. Dolomite and calcite cements occur as zones averaging 1 ft thick and having less than 300 ft of lateral continuity. Based on ^dgr18O measurements and estimated pore fluids between 0 and 2.8, the dolomite formed at 30°-40°C, an early calcite cement formed at 50°-60°C, and a late pore-filling and plagioclase-replacing calcite formed at 70°-80°C. Corresponding to these temperature estimates, dolomite formed in the zone of methanogenesis (^dgr13C = 4 to 8) and calcites in the zone of decarboxylation (^dgr13C = -3 to -12). Dolomite has between 4 and 12 mole % Fe, and calcites contain up to 8 mole % o Fe + Mg + Mn with a characteristic (Fe + Mn)/Mg ratio of about 3:1. The latest calcite is the most Ca rich, due in part to Ca sourced from plagioclase dissolution. Calcite cements in nonmarine sections have markedly high Mn contents (to 5 mole %) compared with the marine section at North Coles Levee. The volume of cement systematically decreases from about 40-20% over the cementation temperature interval, suggesting progressive compaction of the sandstones. Present-day mean porosity of the reservoir (15%) appears to be largely a result of mechanical compaction. A very late dolomite cement with high Fe content (13 mole %) and light carbon (^dgr13C = -14) is found in the cleavage flakes of crushed biotite. The dolomite is calculated to have formed from a fluid with a ^dgr18O of 0.9 at present reservoir temperatures of 105°C, suggesting precipitation in the past 20 years during release of the gas cap and injection of isotopically light water. Calcite also forms thick scales in production tubing primarily as a result of pressure drops from 4,900 to 400 psi in the tubing string. The calcites have high Mg contents (7 mole %) due to the composition of the injection water. Calculated distribution coefficients for Fe and Mn are similar to published values, but Mg is two times higher than published values at 90°C. Similar results are obtained for late calcite cements. A time-temperature plot for North Coles Levee constrains the timing of dolomite cementation between 5 and 6 Ma and the calcite cementation between 4 and 2.5 Ma. Plagioclase dissolution and hydrocarbon emplacement occurred within the past few million years, suggesting that the presence of the hydrocarbons and associated byproducts of kerogen maturation enhance porosity. Despite the narrow window for diagenesis, minimum fluid fluxes of only 0.02 cm3/yr per 100 cm3 rock are required to account for the volume of dissolved plagioclase.