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Formation fluids from the San Joaquin basin can be divided into three geographic provinces based on (87)Sr/(86)Sr ratios and Sr concentrations. Connate marine fluids from the central basin have (87)Sr/(86)Sr ratios between 0.7070 and 0.7080, and Sr concentrations between 15 and 200 mg/l. These results are interpreted in terms of the mixing of Tertiary seawater Sr with Sr derived from Sierra Nevada plagioclase by dissolution and albitization reactions. Fluids from albitized reservoirs can be further distinguished by both elevated Sr concentrations and strongly modified Ca/Na ratios.
Meteoric fluids from the eastern flank of the basin have (87)Sr/(86)Sr rations between 0.7061 and 0.7077, and Sr concentrations less than 2 mg/l. The isotopic ratios suggest that plagioclase dissolution is the dominant source of Sr in these fluids.
Fluids from the southern basin have anomalously high (87)Sr/(86)Sr ratios (>0.7080), although Sr concentrations are comparable to fluids from the central basin. Stable isotopic and chemical data show that the southern basin fluids are of marine or mixed marine-meteoric origin. The elevated (87)Sr/(86)Sr ratios suggest that an Sr source more radiogenic than Sierran plagioclase is involved in the evolution of these waters to a much larger extent than elsewhere in the basin. Possible Sr sources include Tertiary marine carbonate, detrital smectite, and K-feldspar.
The fluid provinces outlined here limit the scale of lateral fluid communication to distances less than 10 km. Isotopic variations also occur within individual fields; in these cases, the scale of fluid communication may be much smaller.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)