Renee J. Perez¹, James R. Boles¹

(1) University of California, Santa Barbara, Santa Barbara, CA

ABSTRACT: Fracture diagenesis along the Wheeler Ridge thrust fault surface, Southern San Joaquin Basin, California

Cementation and porosity enhancement vary spatially along the Quaternary Wheeler Ridge thrust fault, southern San Joaquin Basin. The section of the fault under study extends from 2500 to 8500 feet below sea level and the fault zone is several hundred feet wide. More than seventy sandstones within the fault zone and up to 1000' distance from the zone were sampled for petrography, fluid inclusions, carbon and oxygen isotope and microprobe analysis.

Above 5000 feet depth, microfractures in porous sands, within the fault zone, promote quartz and plagioclase dissolution and kaolinite precipitation. Image analysis suggests an increase of IGV and total porosity by 40% with respect to its original values. In non-porous microfractured sandstones, calcite cement dissolution is pervasive increasing porosity by 50%.

Below 5000 feet depth, microfractures in porous and non-porous sandstones within the fault zone are partially cemented by calcite with oxygen isotopic signature consistent with present day formation waters. Image analysis suggests a 60% porosity decrease with respect to its value before microfracturing.

Evidence of fluid flow along the fault zone is inferred from carbonate precipitation at deeper levels in the footwall and by plagioclase and calcite dissolution in the shallower part of the fault.

At shallow depths (3000 to 5000) microfractures enhance porosity, but in deep intervals (5000 to 8000) they are cemented. As a result of the spatial variability of cementation and porosity enhancement along the Wheeler Ridge thrust fault, permeability along the fault zone is inferred to be highly variable as a function of depth.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado