Investigation of Aluminum Mass-Transfer in Sandstones, Southern San Joaquin Basin, California

Abstract

Dissolved plagioclase is the main aluminum source and authigenic caolinite is the principal aluminum sink in quartzofeldspathic San Joaquin basin sandstones. Samples include Oligocene to late Miocene age marine sandstones, which presently contain modified-marine and meteoric pore fluids. Plagioclase porosity and kaolinite are found from 600-1,500 m (30-50C) in the meteoric zone and from 2,100-3,300 m (80-105C) in the modified-marine zone. Leached plagioclase is absent in some meteoric-zone reservoirs but appears in up to 60% of uncemented sandstones in other reservoirs. In contrast, skeletal plagioclase is pervasive in Stevens sandstones, appearing in greater than 80% of all uncemented samples. Point-count data imply that aluminum is conserved in Stevens sandstones with modified-marine pore waters, despite enrichment in organic acids and anions capable of mobilizing aluminum in soluble complex ions. Some Stevens sandstones have excess kaolinite but no samples have a pronounced aluminum deficiency. The average Stevens sandstone has gained 0.06 {plus minus} 0.1 g of aluminum per 100-cm{sup 3} sample. Aluminum is conserved in Stevens reservoirs with high-temperature, slow-moving pore fluids. In contrast, aluminum may be exported from sandstones subject to meteoric flushing, although the aluminum deficiency is not extreme and complementary zones of aluminum import are not found. Average aluminummore » loss from Vedder sandstones in meteoric reservoirs is 0.3 {plus minus} 0.1 g per 100-cm{sup 3} sample. Aluminum apparently is mobilized in low-temperature, fast-moving meteoric waters, which may be focused along shallow faults and fractures, resulting in local zones of intense diagenesis.« less