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Circular 203—Petrographic analysis of Cenozoic-Mesozoic-Permian well cuttings from two exploration wells in south-central New Mexico

By R. E. Clemons, 1993, 28 pp., 18 figs., 2 appendices.

Two deep oil and gas exploration wells were drilled in southern Doña Ana County in 1972–1973 and 1985, the Grimm, Hunt, Brown, and Am. Arctic Ltd. No. 1 Mobil 32 (TD 21,759 ft) and the Phillips Petroleum Corp. Sunland Park Unit No. 1 (TD 18,232 ft), respectively. This study involved scanning cuttings from the two wells and petrographic analysis of 265 thin sections made from the cuttings in order (1) to provide more detailed data on subsurface lithologies in southern Doña Ana County, (2) to provide information to support or negate the presence of Jurassic and early Tertiary marine rocks, (3) to compare lithology of the well cuttings with correlative strata exposed in the nearby mountains, (4) to determine if subsurface structures in the Potrillo Basin are necessary to explain different depths of reported stratigraphic tops in the two wells, and (5) to provide additional information on lithology and thickness of Cretaceous rocks along the northern edge of the Chihuahua trough.

The study provides data essential for interpretation of stratigraphy and structure between fault-block ranges in south-central New Mexico. The Grimm well penetrated 1,920 ft of basin-fill sediments; 3,880 ft of andesitic and volcaniclastic rocks, probably correlative with the upper Eocene Palm Park and Rubio Peak Formations; 7,000 ft of muddy volcanic arenites with minor conglomerates, correlative with lower Eocene-Paleocene Love Ranch and Eocene Lobo Formations; 2,400 ft (including a 640 ft monzonite sill near the base) of fossiliferous shales, siltstones, sandstones, lime, mudstones, and wackestones, believed correlative with the Lower Cretaceous U-Bar and Hell-to-Finish Formations. About 5,360 ft of Permian-Pennsylvanian rocks and 1,200 ft of Mississippian-Lower Ordovician rocks underlie the Cretaceous.

The upper 200 ft of Permian rocks are shales, mudstones, sandstones, wackestones, and packstones, probably correlative with the Lower Permian Hueco Formation. The Sunland Park well penetrated 1,530 ft of basin-fill sediments overlying Tertiary volcanic and volcanistic rocks. Distinctive characteristics of stratigraphic units in this well are mostly obscured by alteration and contact metamorphism adjacent to felsic intrusions encountered frequently between 1,530 and 17,150 ft depths. Eighteen hundred ft of Cretaceous section, composed of fossiliferous shales, lime mudstones, wackestones, siltstones, and sandstones, was penetrated at 15,100 ft. Actual top of the Cretaceous is probably within the immediately overlying 4,000 ft of muddy siltstones, sandstones, skarn, hornfels, and felsites. The Cretaceous section is underlain by 1,332 ft of fossiliferous shales, silty-lime mudstones, wackestones, packstones, and minor siltstones and sandstones correlative with the Lower Permian Hueco Formation. Oligocene ash-flow tuffs and rhyolitic volcaniclastics, abundant north of the well sites, are not in the subsurface where these two wells were drilled. No evidence of earlier reported early Tertiary marine rocks or Jurassic rocks was found in either well during this study.

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