Fluid Flow and Mass Transfer in Central San Joaquin Basin, California: Strontium Isotope Study of Carbonate Cements

J. L. Schultz, J. R. Boles, G. R. Tilton

Ratios of ⁸⁷Sr/⁸⁶Sr can be used as tracers in sedimentary basins for studying the evolution and migration of pore fluids. The central San Joaquin basin is well suited for such a study, because potential diagenetic sources and sinks for calcium (and thus strontium) are constrained in both space and time.

Analyses of carbonate cements from the marine Stevens sandstone (upper Miocene) at North Coles Levee reveal a trend of decreasing ⁸⁷Sr/⁸⁶Sr with precipitation temperature. Calculated isotopic temperatures for the calcite cements range from 50° to 80°C, and strontium ratios vary systematically from 0.7083 to 0.7074. Further evidence of progressive cementation events is the systematic decrease in interstitial cement volume with decreasing strontium ratio. The ⁸⁷Sr/⁸⁶Sr values of the cements at North Coles Levee lie between the Miocene seawater ratio (0.7090) and present-day pore waters (105°C, ⁸⁷Sr/⁸⁶Sr = 0.7072), and thus define a strontium evolutionary trend in the basin.

Potential sources of strontium in the central San Joaquin basin have been identified and analyzed for their strontium content and ⁸⁷Sr/⁸⁶Sr ratios. Preliminary analyses indicate that dissolution and/or albitization of plagioclase is the most likely source of lower ratio strontium in the carbonate cements. Because the timing of strontium availability from these reactions is known, potential fluid or oil-migration pathways and mass-transfer rates can be modeled. At North Coles Levee, mass-balance calculations indicate that a dissolution of approximately 10% of the detrital plagioclase would account for the strontium content and ratio of the present-day pore water.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.