ABSTRACT: Diagenetic Patterns in the Forbes Sandstone, Great Valley Group, Central California
OSMOLOVSKY, PETER, PHILIPPE F. CLAEYS, and JEFFREY F. MOUNT, University of California at Davis, Davis, CA
Subsurface diagenetic studies of feldspathic sandstones of the Upper Cretaceous Forbes Sandstone, Great Valley Group, California, reveal extensive modification of porosity and systematic alteration of biotite and plagioclase with depth. Reservoir quality seems to be controlled by the competing effects of porosity enhancement involving dissolution of both carbonate cements and detrital plagioclase, and porosity reduction by precipitation of several phases of carbonate cement.
Diagenetic alteration of biotite is depth related and may record aspects of pore-fluid evolution during burial. Biotite alters to kaolinite at shallow depths and to chlorite at depths below 5000 ft. Authigenic carbonates, pyrite, and Ti oxides have been observed between expanded biotite cleavage planes.
The onset of widespread albitization of plagioclase appears to occur at 7000-8000 ft (140-160 degrees F estimated). Albitization and dissolution of plagioclase occur in close proximity, commonly at the thin section scale. This proximity implies that albitization and dissolution of plagioclase can occur under similar thermal and pore-fluid diagenetic regimes. Therefore, the compositional and structural states of plagioclase, inherited from source rocks, may be important in feldspar alteration. These source-controlled parameters include major and trace element composition (Ca, Na, K, Fe, Sr) and the degree of structural order (high-temperature vs. low-temperature feldspars). Preliminary analyses show that high-temperature volcanic plagioclase grains enriched in K, Fe, and Sr appear more susceptible to alteration and dissolution than do similar nonvolcanic low-temperature plagioclase grains depleted in those trace elements.