Abstract

The Permian (Guadalupian) Grayburg and Queen Formations of Means field, Andrews County, Texas, consist of mixed siliciclastic, carbonate and evaporite sediments that were deposited in four general platform environments: (1) subtidal open shelf, (2) subtidal to intertidal shallow-shelf, (3) supratidal coastal flats and sabkhas, and (4) fluvial-dominated deltas.

The open shelf lithofacies consists of peloidal and fusulinid dolowackestones and rare dolopackstones that accumulated along the outer portion of the platform, generally below fair-weather wave base. The subtidal and intertidal shallow shelf lithofacies consists of four subfacies: a distal shallow shelf subfacies consisting of skeletal and peloidal dolowackestones and dolopackstones; a shoal subfacies of ooid dolopackstones and dolograinstones and oolitic quartzose sandstones; a restricted shallow shelf subfacies of peloidal dolowackestones, dolopackstones and dolograinstones; and a shoreface subfacies of mixed dolostones, silt-stones and sandstones.

The supratidal lithofacies consists of four subfacies: a carbonate supratidal flat subfacies consisting of coated grain dolograinstones and dolomudstones; a siliciclastic tidal flat subfacies of siltstones and very fine sandstones; a coastal sabkha subfacies of massive bedded and mosaic anhydrite, silty mudstone and dolomite; and an eolian subfacies of massive and ripple cross-laminated sandstone.

The fluvial-dominated delta lithofacies consists of three subfacies: a channel lag subfacies consisting of siltstones and very fine subarkosic sandstones; a river mouth bar subfacies of siltstones and fine sandstones; and an overbank subfacies of muddy ripple- and wavy-laminated sandstones and deformed convoluted wavy-laminated sandstones with small anhydrite nodules.

Means field produces from subtidal (shallow shelf and shoal) Grayburg dolostones and Queen river-mouth bar and siliciclastic tidal flat sandstones. Reservoirs appear to be both depositionally and diagenetically controlled, as lateral and vertical variations in porosity reflect variations in depositional and diagenetic texture. The most common pore types in subtidal Grayburg reservoirs are intercrystalline, moldic, and intergranular, while intergranular porosity is vital to Queen production.