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FELDMAN, MARK D., and J. R. BOLES, University of California, Santa Barbara, CA

Title: ABSTRACT: Regional Variations in 87Sr/86Sr and Sr Concentration of Formation Fluids, San Joaquin Basin, California

Formation fluids from the San Joaquin basin can be divided into three geographic provinces based on 87Sr/86Sr ratios and Sr concentrations. Connate marine fluids from the central basin have 87Sr/86Sr ratios between 0.7070 and 0.7080, and Sr concentrations between 15 and 200 mg/L. These results are interpreted in terms of 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 87Sr/86Sr ratios between 0.7061 and 0.7077, and Sr concentrations < 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 87Sr/86Sr 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 87Sr/86Sr ratios suggest that a 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 < 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 #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)