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Dolostone reservoirs with net thickness of 484 ft (148 m) in upper Horquilla, are exposed in the Big Hatchet Peak section of southwestern New Mexico and constitute the best petroleum objective demonstrated to date in the Pedregosa basin.
This shallow-marine carbonate section includes uppermost Paradise Formation (Chesterian) and 3,230 ft (985 m) of Horquilla (Morrowan-Wolfcampian). The lower member of the Horquilla consists of limestone and chert; a disconformity at the top formed during the Desmoinesian. The upper member is at least 1,867 ft (569 m) thick; limestones contain phylloid-algal biostromes and ancient solution cavities; alternating laterally extensive dolostones are 54 to 148 ft (16.5 to 45 m) thick and exhibit much vuggy porosity.
Petrographic evidence demonstrates that the limestones were stabilized within freshwater diagenetic environments, that they contained much primary or secondary porosity, and that practically all of it was occluded by marine or freshwater cements. Dolostones also contained much secondary intercrystalline and moldic porosity that was partly occluded by epitaxial cements and coarse recrystallization of initial neomorphic rhombs. Anhydrite porphyroblasts were emplaced by hypersaline waters and were dissolved later by fresh ground waters to form molds with distinctive stairstep outlines; this tertiary (third stage) porosity in dolostones was partly filled by gravitational cements, and some microstalactite tips were dolomitized paramorphically.
Dark Horquilla limestones are rated as fair petroleum-source units. Land-derived kerogens indicate sources of gas; alteration indices of 3 to 3+ indicate moderate thermal history. Dolostones in the upper member are rated as fair to good reservoir units. Matrix porosity and permeability are poor but should improve farther south along the shelf edge and on the slope into Alamo Hueco basin.
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