Abstract

A core from the EVGSAU 0524-007 well recovered a thick, dolomitized calcisponge-microbial reef facies from the upper San Andres Formation (Guad 9 HFS) that has exceptional preservation of reef fabrics, diagenetic cements, and karst features. The reef occurs within the oil column at the shelf margin of the productive reservoir at EVGSAU and has unique reservoir characteristics compared to other producing facies in the field.

The reef facies comprises a relatively small portion of the reservoir volume and is therefore a minor contributor to the unit’s production. The well has produced 59,000 barrels of oil and 39,500 barrels of water from 1994 to present through perforations in doloboundstone and fusulinid dolograinstone of the San Andres and sandstone and dolostone of the lower Grayburg. No tests are available to determine the exact contribution of the doloboundstone facies to that production. There is no evidence that the reef forms a separate trap.

The 240-foot core comprises, from top to base, 46 feet of Grayburg Formation; the Grayburg-San Andres contact; 76 feet of inter-bedded doloboundstone, reef-talus dolorudstone, fusulinid dolopackstone-grainstone, and skeletal dolowackestone-packstone; and 108 feet of massive doloboundstone. The core did not reach the base of the reef. Doloboundstone occurs at three levels in the interbedded interval. A karstic fracture extends 30 feet downward from the top of the middle of those doloboundstone-bearing cycles and is filled by geopetal internal sediments, fusulinids and other bioclastic debris, and massive anhydrite.

Porosity in the doloboundstone intervals is patchy and partially occluded by anhydrite. Large (up to at least 1.0 mm in diameter) intraskeletal pores in the calcisponges and other macrofossils may have either isolated and touching vug characteristics. Smaller inter-crystalline to small vuggy pores within matrix, cements, microbial fabrics, and partially open microfractures are less than 20-micron diameter. Vuggy and intercrystalline pores locally form connected channels. Average core porosity is 6%, and average core permeability is 26 md. There are 120 net feet of permeability above 1.0 md, and 70 net feet between 10 md and 100 md. The corresponding porosity is only four net feet above 10% and 70 net feet above 5%. Proximal backreef dolograinstones-dolopackstones consistently have porosity-permeability relationships corresponding to rock fabric numbers between 0.5 and 2.5 in the Lucia classification. In contrast, the porosity-permeability crossplot of the doloboundstone reef facies forms a tightly organized cluster outside of and inconsistent with those Lucia petrophysical classes, probably because of the more patchy porosity system, including separate and touching vugs, and the partial occlusion of pores by anhydrite cement.

The reef occurs at the upper San Andres shelf margin, as evidenced by its basinward position relative to backreef subtidal dolograinstones-packstones; the large accommodation space implied by the thickness of the reef; the presence of an onlapping siliciclastic-carbonate cycle in the basal Grayburg Formation; and the updip edge of basinal sands of the Delaware Mountain Group within less than 500 feet of the 0524-007 well. The siliciclastic-dolomitic Lovington member of the San Andres (base of the Guad 9 HFS) pinches out updip of the reef facies, and its equivalent basinward by-pass surface appears (by a tenuous gamma ray correlation) to be 30 feet below the base of the core. If that correlation is correct, the reef may be rooted on the Lovington bypass surface, in which case the massive reef facies could be up to 138 feet thick.

The presence of boundstone facies within the higher order cycles of the uppermost Guad 9 HFS suggests analogy to the offlapping reef-bearing clinoforms in Last Chance Canyon of the Guadalupe Mts. (Sonnenfeld and Cross, 1993, AAPG Memoir 57). In Last Chance Canyon, San Andres shelf-margin carbonates prograde via several clinoforms over basinal siliciclastics of the Cherry Canyon Tongue. At Vacuum, the presence of an uppermost San Andres carbonate section above Delaware Mountain siliciclastics basinward of the 0524-007, and carbonate intervals within those siliciclastics, supports this possibility. Reef talus in the uppermost bed of the core indicates that boundstones were subaerially exposed at the end of San Andres deposition.

Orchard et al, 2011 (abstract, AAPG Annual Convention and Exposition) and Wahlman et al, 2011 (abstract, this symposium) report additional details of the fabrics, fossils, and diagenesis of this reef.