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Circular 176—Pennsylvanian stratigraphy, petrography, and petroleum geology of Big Hatchet Peak section, Hidalgo County, New Mexico

By S. Thompson III and A. D. Jacka, 1981, 125 pp., 7 tables, 42 figs., 3 appendices.

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The purpose of this report is to describe, analyze, and evaluate an exposure of the best petroleum objective found to date in Hidalgo County, New Mexico. One of the advantages of petroleum exploration in the Basin and Range Province is that source and reservoir units can be studied where they are exposed in the uplifts. This critical yet inexpensive surface control can be used in combination with available well data to extrapolate the objectives into the subsurface of the adjacent structural depressions, where the potential is greater for the preservation of oil and gas in commercial quantities.

Big Hatchet Peak section includes the uppermost 78 ft of Paradise Formation (Mississippian/Chesterian) and 3,230 ft of Horquilla Formation (Pennsylvanian-Permian/Morrowan to Wolfcampian). About 200 ft of uppermost Horquilla have been removed from the peak by erosion. Nearly all of the section consists of shallow-marine shelf carbonates. Paradise contains skeletal-oolitic limestone; cross-laminae indicate paleocurrents toward the southeast and southwest. In the Horquilla, the lower member (Morrowan to Desmoinesian) is 1,363 ft thick and consists of limestone with abundant chert in some beds. A disconformity may be present at the top. The upper member (Desmoinesian to Wolfcampain) is at least 1,867 ft thick and consists of alternating limestone and dolostone; chert is rare. Limestones contain biostromes of phylloid algae and a few ancient solution cavities. Dolostones range in thickness from 54 to 148 ft, are laterally extensive, and exhibit vuggy porosity. Petrographic evidence shows that limestones were stabilized within fresh-water diagenetic environments and that they contained much primary or secondary porosity; practically all of the porosity was occluded by marine- or fresh-water cements. Dolostones also contained much secondary intercrystalline and moldic porosity that was partially occluded by epitaxial cements and coarse recrystallization of initial neomorphic rhombs. Anhydrite porphyroblasts were emplaced by hypersaline waters and dissolved later by fresh ground waters to form molds with distinctive stairstep outlines; this tertiary (third step) porosity in dolostones was partially filled by gravitational cements, and some microstalactite tips were dolomitized paramorphically. Darker limestones in the Horquilla are rated as fair petroleum-source units. Land-derived kerogens indicate sources of gas, and alteration indices of 3 to 3+ indicate a moderate thermal history. Porous dolostones in the upper member are rated as fair to good reservoir units. Matrix porosity and permeability are poor, but they should improve farther south along the shelf edge and on the slope. The net thickness of 484 ft of dolostones with vuggy porosity supports the ranking of this facies as the best petroleum-reservoir objective in the Pedregosa Basin area.

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