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Deposition and Diagenesis of an Upper Clear Fork Reef Trend, Palm Sunday Field, Hockley County, Texas

Brian R. Mosley

The Palm Sunday field produces from upper Clear Fork dolomite strata situated along a linear, shelf-edge reef trend just north of the Midland basin. Thickness of the reef core ranges from 65 to 140 ft. Oil production occurs from within sub-reef and reef-flank facies, but the reef itself is highly cemented. However, the influence of the reef buildup as an impermeable, localized structural high on the basinward-sloping shelf is an important factor in migration and trapping within adjacent beds.

The reef facies is comprised of a crinoid-sponge-bryozoan fauna that trapped and consolidated micritic mud as the reef developed. High initial porosity was lost to early emplacement of massive and nodular anhydrite. Other diagenetic characteristics include dolomite and late-stage anhydrite cementation, and the replacement of nodular anhydrite by chalcedony.

Productive facies below and adjacent to the reef trend are wackestones and packstones composed of crinoid and bryozoan skeletal debris. Reservoir porosity is predominately intergranular and vuggy. The lower initial porosity of these deposits enabled them to escape the massive anhydrite emplacement characteristic of the reef sequence. Late-stage anhydrite cements are present but do not critically occlude porosity.

The Palm Sunday field, currently defined by five producing wells, is still in an early stage of development. However, similar upper Clear Fork productive trends, though not possessing these reef-related characteristics, have shown production totals ranging from 1 to 2.5 million bbl of oil.

AAPG Search and Discovery Article #91034©1988 AAPG Southwest Section, El Paso, Texas, 21-23 February 1988.