Strontium-isotope study of pore waters in San Joaquin basin, California

Nineteen pore-water samples from the late Miocene and younger marine formations were analyzed for strontium contents and isotopic compositions. All samples have /sup 87/Sr//sup 86/Sr ratios lower than the late Miocene seawater value (0.7090). The strontium values in many of these waters reflect silicate rather than carbonate reactions. In general, the strontium contents and isotopic ratios of the pore waters are related to the depth and location of the oil fields. The strontium contents as a whole show a tendency to increase with depth, although /sup 87/Sr//sup 86/Sr ratios have no simple depth relationship. The strontium contents and isotopic compositions of the pore waters fall into five groups related to geographic locations. This suggests localized evolution of pore waters in a given area and limited movement of these subsurface waters.