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Organic and Inorganic Diagenetic Responses to Meteoric Invasion of Marine Sandstones and Shales, Southeastern San Joaquin Basin, California

Karin F. Fischer, Ronald C. Surdam

Upper Miocene marine sandstones (Santa Margarita Formation) and shales (Edison and Fruitvale formations) of the southeastern San Joaquin basin have been invaded by meteoric waters to present-day depths of approximately 6,000 ft. By combining isotopic and microprobe data from carbonate cements with pyrolitic and elemental analysis data from organic shales, we determined the extent of freshwater incursion into basin-flank sediments. The shallow sands display diagenetic effects of meteoric invasion and are a major reservoir for hydrocarbons that migrated updip from the deeper, thermally mature basin axis. Mass transfer is documented in these reservoir sands by the spatial distribution of diagenetic smectite and feldspar dissolution. Subsurface waters from the area contain si nificant quantities of organic acids, which enhanced aluminum mobility and affected carbonate stability in the sandstones. These organic acids generated as a result of two events: (1) low-temperature alteration of organic compounds (migrated hydrocarbons and buried organic matter) and (2) thermal generation of acids along the basin axis, migrating updip in front of the hydrocarbons.

Time-temperature reconstructions of the southeastern San Joaquin basin place constraints on the timing and location of the thermal generation of organic acids; consequently, mass transfer within sandstones may be evaluated.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.