Petrography Stable-Isotope Geochemistry and Diagenesis of Miocene Sandstones, Vermilion Block 31, Offshore Louisiana: ABSTRACT

ABSTRACT Sandstones from three conventional cores between depths of 11,140 and 14,910 feet currently have an average composition of Q86F10R4, but have lost approximately 5 percent feldspar to replacement by carbonates and to dissolution. Like other deeply buried Gulf Coast sandtones, plagioclase is being albitized and K-feldspar is dissolving. Patterns of feldspar distsribution in the deeper cores suggest that tight cementation by calcite retarded feldspar diagenesis and that waters of different composition or temperature must have invaded different sandstone beds in close proximity. Calcite, quartz and chlorite are most responsible for causing deterioration of reservoir quality. Calcite volume averages 4.8 percent and occludes all pores in samples containing more than 5 percent calcite. Textural relations and isotopic data indicate that calcite precipitated over a broad temperature range and from fluids of variable composition during burial. The 87Sr/86Sr ratio of cements is consistent with the derivation of the Sr from coeval, or only very slightly older skeletal material, prior to extensive silicate reactions. The volume of quartz cement ranges from 0 to 14 percent (mean - 5%) and is selective to coarser, cleaner sandstones that had the highest permeability. 18O values of quartz overgrowths (34.2 SMOW) suggest that quartz cement precipitated between temperatures of 35° to 50° C from water having a 18O of about +3+1.5 . Silica for quartz cement was largely imported because there is negligible pressure solution of silicate grains and the amount of silica released from feldspar alteration is insufficient to account for the present quartz volume. Chlorite is not abundant and its diagenesis is complex, including two stages of precipitation and a later stage of dissolution. The first episode of precipitation formed thin grain coats that rarely inhibited quartz cementation. A later episode partly filled both primary and secondary pores and strongly influenced permeability. Total thin section porosity ranges from 0 to 31 percent and averages 16 percent. Of total pores, 75 percent are intergranular, 17 percent are oversize pores inferred to have formed by feldspar dissolution, 6 percent are intragranular pores, 1 percent are oversize pores developed by fossil dissolution, and a trace are micropores within clay clasts, glauconite, rocks fragments and clay cements.