Abstract Yowlumne is a giant oil field in the San Joaquin Basin, California, that has produced over 16.7 million m 3 (105 million bbl) of oil from the Stevens Sandstone, a clastic facies of the Miocene Monterey Shale. Most Yowlumne production is from the Yowlumne Sandstone, a layered, fan-shaped, prograding Stevens turbidite complex deposited in a slope-basin setting. Well log, seismic, and pressure data indicate seven depositional lobes with left-stepping and basinward-stepping geometries. Log-derived petrophysical data, constrained by core analyses, indicate trends in reservoir quality. Concentration of channel and lobe facies along the axis and western (left) margin of the Yowlumne fan results in average net/gross sandstone ratios of 80%, porosity (cj>) of 16%, and liquid permeability (K U quid) °f 10-20 md. By contrast, more abundant levee and distal margin facies along the eastern margin result in shale-bounded reservoir layers with higher clay contents and lower net/gross sandstone ratio (65%), porosity (12%), and permeability (2 md). Although a waterflood will enable recovery of 45% of original oil in place along the fan axis, reservoir simulation indicates 480,000 m 3 (3 million bbl) of oil trapped at the thinning fan margins will be abandoned with the current well distribution. Economic recovery of this bypassed oil will require high-angle wells with multiple hydraulic fracture stimulations to provide connectivity between the reservoir layers.

Abstract Carbonate cements in the early Miocene Temblor Formation at Kettleman North Dome oil field, on the western flank of the San Joaquin basin in California, formed in marine and mixed meteoric-marine pore waters. Arkosic sands were deposited in deltaic to shallow marine and deep marine environments. Carbonate cements preserve the degree of compaction at the time of cementation. Micritic calcite cements are interpreted to have formed at the sediment-sea water interface when intergranular pore space was about 40%. Later, dolomite cements formed during shallow burial, when intergranular porosities were about 30%. Coarse crystalline calcite cement and grain-replacement cement precipitated during deep burial, when intergranular porosity was less than 25%. These carbonate cements originated from three types of formation waters based on oxygen isotopic data. The micrite formed in nearly pure meteoric water at the sediment-water interface. The dolomite precipitated from mixed marine-meteoric water during shallow burial. Late calcite that formed during deep burial (70–120°C) precipitated from diagenetically modified marine waters. Studies of six fields in the central and eastern San Joaquin basin indicate that carbonate cements originate from both marine pore waters and from meteoric incursion during deposition and uplift of the basin perimeter. Sediments deposited in non-marine to shallow marine environments in the basin flanks were subjected to meteoric water infiltration during shallow burial. Early carbonate cements with meteoric isotopic signatures are found at distances of up to 5 km (Round Mountain Field, eastern flank) to 15 km (North Kettleman Dome Field, western flank) from potential recharge areas. Meteoric recharge is also recorded late in the cement history, due to locai uplift on the west flank. The uplift focused meteoric water into deep marine sands up to 15 km from the basin edge (North Belridge Field). In the central basin, sands deposited in deep marine environments were isolated from meteoric influence due to their distance from meteoric recharge areas and lack of hydraulic continuity with the basin flanks. Thus, these sands only contain carbonate cements with marine or evolved marine geochemical signatures (e.g., North Coles Levee Field).

Abstract: Mass transfer of aluminum is investigated on a thin-section scale by comparing volumes of dissolved plagioclase and authigenic kaolinite in quartzofeldspathic sandstones from the San Joaquin Basin. Samples include Oligocene marine-shelf sandstones, which have been infiltrated by meteoric water, and Late Miocene turbidite sandstones, which contain diluted sea water. Other aluminum sources and sinks are volumetrically minor in these sandstones. Dissolved plagioclase and kaolinite presently appear from 600 m to depth of sample control (30–70°C present temperature) in the meteoric zone and from 2,100 m to depth of sample control (75–130°C present temperature) in the marine zone. Leached plagioclase and kaolinite are rare in the matrix-rich or carbonate-cemented sandstones, but appear in more than 80% of the uncemented turbidite sandstones and in up to 60% of the uncemented sandstones in some meteoric-zone reservoirs. Point-counted volumes of plagioclase porosity and kaolinite in all sandstones are compared with relative volumes calculated from a mass-balance reaction in which aluminum is conserved between An 30 plagioclase and kaolinite (25 to 50% microporosity). Aluminum is conserved on a centimeter scale in shale-encased turbidite sandstones exposed to limited fluxes of marine pore water, despite enrichment in organic-acid anions, which potentially may mobilize aluminum in soluble complexes. The average marine-zone sandstone has a volume of kaolinite approximately equal to that calculated for plagioclase porosity, based on relative volumes of the mass-balance reaction. In contrast, the average sandstone with meteoric pore water has a kaolinite shortfall of 0.4 ± 0.3 volume percent of total rock relative to plagioclase porosity. This average aluminum loss is 0.2 ± 0.1 gm/100 cm 3 rock volume. Complementary zones of aluminum import are not found in the meteoric zone. A small amount of aluminum is mobilized beyond a centimeter scale in shelf sandstones flushed by low-temperature, dilute waters. Plagioclase dissolution and kaolinite precipitation in sandstones from both porewater settings result in compositional shifts of less than 0.4 weight percent Al 2 O 3 , too small to discriminate from the natural bulk chemical variation of 1 to 2 weight percent Al 2 O 3 . Data from this study do not support models proposed for transfer of large masses of aluminum over significant distances.

Abstract Secondary porosity in Miocene Stevens sandstones of the North Coles Levee Field results from dissolution of calcite, ferroan dolomite, and calcic plagioclase (An30). Kaolinite is a leach product of plagioclase, and mass balance calculations indicate that alumina is conserved on a thin-section scale. Iron released from dissolution of Fe-carbonate is possibly conserved in late-stage pyrite. Detrital K-feldspar and early formed albite fracture filling in plagioclase are unaffected by the leach fluids, suggesting that these components are stable with respect to the leach fluids, whereas the anor-thite component is unstable. Thermodynamic considerations indicate present-day pore waters at 100°C, 260 bars fluid pressures, are nearly at equilibrium for the reaction: CaCO 3 + CaAI 2 Si 2 O 8 + H 2 O + 3H + = 2Ca +2 + HCO 3 − + AI 2 Si 2 O 5 (OH) 4 and for a similar reaction involving dolomite. Compaction following or perhaps contemporaneous with leaching has resulted in at least two diagenetic events. One involves albitization of plagioclase at stressed grain contacts with quartz, possibly as a result of higher silica activities stabilizing albite at these contacts. The other involves crushing of detrital biotite resulting in crystallization of carbonate adjacent to it. This phenomenon is due to depletion of H + in the pore water adjacent to newly formed mica surfaces as H + exchange occurs between pore water and mica.