ABSTRACT: Isotopic Evidence for Km-Scale Vertical Fluid Flow, Elk Hills, CA

WOOD, JAMES R., Michigan Technological University, Houghton, MI

Isotopic measurements (C, O) on calcite cements and vein fills from samples collected from Well 934-29R at Elk Hills in the southern San Joaquin Valley of California suggest rapid, vertical fluid movements on the order of 2000-4000 ft originating at depths as great as 24,000 ft. Samples were taken from 24 sandstones ranging in depth from ~12,000 ft to >24,000 ft. Almost all samples are crosscut by a conjugate system of calcite-cemented veins oriented at ~45 degrees from the vertical.

Paired calcite samples were taken from interstitial cements and veins immediately adjacent to the cement samples. Oxygen isotopes from the vein calcites cluster in the range -13 to -15 (PDB) while the cements span the range -5 to -14 (PDB). The cement data appear to define two parallel, linear trends that approximately follow the geothermal gradient. The interpretation of these trends is that the cements constantly recrystallized along the geothermal gradient until conditions were met, perhaps complete porosity loss, that prevented further recrystallization and froze the isotopic signatures. It appears that some of the samples are still recrystallizing, but that others froze when the section was ~4000 ft higher than present.

It is clear that the vein calcites are not in isotopic equilibrium with the calcite cements (although one pair is within 2 o/oo). One interpretation of this disequilibrium is that the vein fill was rapidly injected and froze before thermal equilibrium could be established. If so, analysis of the O isotopes suggests that the likely source of the fluid was 3000 to 5000 ft deeper in the section.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)