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Abstracts: Origin of Boron-Rich Pore Water in the Monterey Formation, San Joaquin Valley, CA



The high boron content of pore water in the Monterey Formation is due to the release of boron from diatom during the diagenetic alteration of opal-A to opal-CT.

The boron content of pore water in Monterey Formation generally ranges from 50 to 150 mg/L This is significantly higher then the mean boron content of oilfield water in Miocene reservoirs of the Gulf Coast (35 mg/L) and in pore water in pre Late Miocene rocks of the North Kettleman Dome (43 mg/L).

Purified samples of diatoms were prepared from three samples of diatomite from Lompoc. The average boron content of these samples was 110 ppm. The mean boron content of five nodules of glassy opal-CT in the diatomite was 5 ppm This suggests that boron released by the dissolution of diatomaceous silica is not coprecipitated with silica in opal-CT. Experimental alteration of diatoms at 150 to 250 degrees C over time periods of 14 to 20 days showed a decrease in the boron content of opal and an increase in the boron content of the pore water in all experiments.

The interpretation of pulsed neutron logs and carbon/oxygen logs is affected by the repartitioning of boron between opal and pore water during steam flooding of dolomite reservoirs.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California