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

E. F. McBride, L. S. Land, T. N. Diggs, L. E. Mack

Sandstones from three conventional cores between depths of 11,140 and 14,910 ft (3,345 and 4,545 m) currently have an average composition of Q₈₆F₁₀R₄, but have lost approximately 5% feldspar to replacement by carbonates and to dissolution. Like other deeply buried Gulf Coast sandstones, plagioclase is being albitized and K-feldspar is dissolving. Patterns of feldspar distribution 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% and occludes all pores in samples containing more than 5% 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 up to 14% (mean = 5%) and is selective to coarser cleaner sandstones that had the highest permeability. ^dgr¹⁸O values of quartz overgrowths (34.2 ^pmil SMOW) suggest that quartz cement precipitated between temperatures of 35° to 50°C from water having a ^dgr¹⁸O of about +3 ± 1.5 ^pmil. Silica for quartz cement was largely imported because negligible pressure solution of silicate grains exists 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 up to 31% and averages 16%. Of total pores, 75% are intergranular, 17% are oversize pores inferred to have formed by feldspar dissolution, 6% are intragranular pores, 1% are oversize pores developed by fossil dissolution, and a trace are micropores within clay clasts, glauconite, rock fragments, and clay cements.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.