Abstracts

Dune field is located on the eastern margin of the Central Basin platform on the western side of the Midland basin. Major production is from Permian (lower Guadalupian) San Andres and Grayburg porous dolomites. The southern half of Dune field (University Land, block 30) was selected for study because of the large amount of data readily available at the University Lands Office and because a cooperative operator (Mobil) provided cores and core analyses. The study centered around the Mobil University Unit 15/16, where cores were obtained from nine wells, and logs from hundreds of wells.

Cores from all wells show a similar upward-shoaling sequence with fusulinid wackestone and crinoid wackestone in the lower part, pellet and ooid grainstone near the top, and pisolite grainstone at the top of the approximately 300-ft thick section. Very thin (5 to 10-ft thick) siltstone beds occur commonly in the upper part of the section studied. Core and log correlations of the siltstones have permitted delineation of the lateral distribution of carbonate facies and recognition of a logical facies tract comprising arid island, tidal-flat, and shallow-water subtidal depositional environments.

Production is highest in the eastern third of University Unit 15/16, where porosity is best developed in 20- and 30-ft-thick pellet grainstone reservoirs; porosity also occurs in the much thicker fusulinid wackestone facies. In contrast, production in the western part of the area is from a thick section of low porosity sponge and algal framestone, equivalent to the pellet grainstone and crinoid wackestone in the lower part of the eastern sequence. Consequently, the eastern and western areas essentially function as two distinct reservoirs that are located in different parts of the San Andres and Grayburg section and have considerably different performance characteristics. The nonporous siltstones probably form barriers to vertical migration of fluids throughout the field.

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