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Formation Water Composition as a Guide to Fluid Flow and Diagenetic Processes in Three Southern California Basins


Fluid composition can provide important evidence of active basin processes. In the offshore Santa Barbara basin, changes in fluid composition in wells close to the fault demonstrate the influx of sea water into the South Ellwood field along the Ellwood fault. The reservoir is about one km below the sea bed in siliceous, Monterey shale and is currently underpressured (about 50% of hydrostatic). The opal to quartz transition has resulted in the shift of δ18O from near zero to about +4 in the present day reservoir fluid. Wells close to the South Ellwood fault have δ18O values approaching zero due to the influx of sea water, in addition to having elevated SO4= and Mg++ content consistent with sea water influx. Formation waters in the San Joaquin basin (SJB) and Los Angeles basin (LAB) are classic examples of arkosic sediment-sea water interaction in young, largely late Tertiary, first-cycle sedimentary basins. Maximum depth of sample control is about 4.1 km in the SJB to about 3.5 km in the LAB, where temperatures are 130–160°C to 150°C, respectively. Both basin waters are generally more dilute than sea water (60 to 80%), which mostly reflects dilution by the I-S clay reaction and the lack of salt or other evaporite underpinnings to these basins. Abundant SO4= and Mg++ in sea water is sequestered by early pyrite, siderite and dolomite cements. One similarity in the deep waters from the two basin, and offshore waters is abundant organic acids in the waters (up to 90 % of the total alkalinity) from Monterey type kerogen. Water from both basins show trends of positive oxygen and negative hydrogen relative to sea water and the meteoric water line. There are some significant differences in the water composition in the deep part of both basins. In the SJB, the albitization of plagioclase results in high Ca/Na ratios and low Sr isotopic ratios relative to initial Tertiary marine values. In contrast, the LAB has a relatively radiogenic Sr isotopic values and the δ18O is not as positive as in the SJB. High 87/86Sr values are attributed to clay diagenesis in the LAB but this conclusion is not well documented. Overall the formation waters are consistent with petrographic observations that the basin systems as a whole have not been subjected to large scale vertical fluid flow. Formation waters from the deep basins can be distinguished from those at shallow levels, allowing detection of upward fluid movement along pathways.