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In southern Hidalgo and Grant Counties, New Mexico, the northwestern end of the Pedregosa basin has a high petroleum potential. Paleozoic rocks, dominantly shallow-marine carbonates, are over 11,000 ft (3,350 m) thick. Of the 11 formations, ranging in age from Cambrian to Permian, 7 contain favorable oil- or gas-source units. Mesozoic rocks, generally shallow-marine limestones and deltaic sandstones-mudstones, are nearly 10,000 ft (3,050 m) thick. Two of the three Lower Cretaceous formations contain favorable source units. Gas-prone kerogens are more abundant than the oil-prone types. At normal depths of burial, organic matter in the source units has reached thermal maturity, and some in the older formations is overmature. In the lower plate of a major Laramide thrust fau t, the Lower Cretaceous units are overmature and the older Paleozoic units may be thermally metamorphosed.
Best reservoir objectives are the porous dolostones, totaling 484 ft (148 m) in thickness, in the upper part of the Horquilla Formation (Pennsylvanian-Permian). They are located at the shelf margin of a deep-marine basin. Other favorable reservoir units are indicated in surface exposures.
In this frontier area, only 11 exploration wells have been drilled to Precambrian, Paleozoic, or Mesozoic rocks. Shows of oil or gas were reported in 6 of the wells. None of the wells have tested the better reservoir objectives in the deeper parts of the graben valleys, where commercial accumulations of petroleum are likely to be preserved. Tertiary-intrusive and volcanic-cauldron complexes have thermally metamorphosed older sedimentary units only locally. Basin and Range faulting has disrupted subsurface fluid systems in many parts of the area. Despite the challenging risks, the high potential encourages further exploratory drilling on selected petroleum prospects.
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