Geologic Observations Concerning Reservoir Development in the Glorieta
Formation (Permian, Leonardian) in the Robertson, Wharton and South Harris
Units; Robertson and Harris Fields, Gaines County Texas
John K. Southwell/Tome Brown Inc., Midland TX and Emily L. Stoudt/Texaco North American
Producing, Midland TX
Abstract
The Glorieta formation (Permian, Leonardian) is a shallow water carbonate unit that occurs
between the prolific Clear Fork (Leonardian) and San Andres (Leonardian-Guadalupian) formations on the
Central Basin Platform and northwestern and eastern shelves of the Permian Basin. Along the eastern side
of the Central Basic Platform, the Glorieta interval is often a productive reservoir in its own right, as is the
case in the Texaco-operated Robertson, Wharton and South Harris units in Gaines County, Texas.
In Robertson, Wharton and South Harris units, the Glorieta formation occurs a an average depth of
5800 feet and is roughly 500 feet thick. Examination of cores from numberous wells in the Wharton and
South Harris units reveals that the Glorieta consists of several successions of shallowing upwards cycles
culminating in tidal flat capped facies. These peritidal deposits are characterized by laminated,
stromatolitic dolowachestones and dolopackstones containing pisolites, intraclasts, pellets, and fenestral
textures. Tidal flat bases are generally argillaceous and sandy and pores in the dolopackstones are plugged
with anhydrite. Thus, thse tidal flat intervals are tight and are typically barriers to fluid flow.
Subtidal cycles beneath the tidal flat capped intervals consist of skeletal, pellatal mud-rich to
grain-rich dolopackstones. Skeletal grains in the deeper parts of the cored Glorieta interval include
numerous crinoids, bryozoa, and scattered brachiopod and sponge (?) fragments, suggestive of depostition
in an open marine environment. Cycles are typically extensively burrowed. The subtidal cycles
immediately beneath the tidal flat caps are characterized by abundant pellets, peloids and sparse bivalve
fragments, suggesting depostition in more restricted environment.
Individual cycles in the cored wells have been grouped into 4 high frequency sequences that can
be traced across the Robertson-Wharton-South Harris area. The younger two sequences (HFS 1 and HFS
2) display tidal flat caps in both dip and strike directions across most of the study area. The older (deeper)
sequences (HFS 3 and HFS 4) are capped with tidal flats on the western side of the study area. However,
the eastern side of the area displays subtidal capped cycles in these deeper HFS’s. The best reservoir
intervals occur in these subtidal capped cycles.
The Glorieta formation produces from various subtidal facies exhibiting a wide range of reservoir
characteristics; however, the best reservoir facies consists of grain-rich dolopackstones. Porosities in
subtidal much-rich dolopackstones average 7.7% and permeabilities average .76 md. In grain-rich
dolopackstones, porosities average 10.5% and permeabilities average 3.0 md. Pore types of include
interparticle, moldic, fracture and some intercrystalline. Variations in reservoir quality are mappable and
may follow trends of depositional facies patterns inherited from paleotopography.
The Robertson-Wharton-South Harris Glorieta trend was discovered in 1942. Through primary
and waterflooding activities, the units have produced over 115 million barrels of oil to date. Significant
amounts of movable oil remain to be produced, but enhanced production requires detailed geologic
modeling and understanding of the controls on the reservoir heterogeneity resulting from depositional and
diagenetic variation.