The Estancia Basin of central New Mexico is an asymmetric, north-south trending structural depression that originated during the Pennsylvanian. The present-day basin covers 1,600 mi2 (4,100 km2) and is defined by the Estancia valley. It is bounded on the east by the late Paleozoic Pedernal uplift, on the west by the Tertiary-age Manzano and Los Pinos Mountains, on the north by the Espanola Basin, and on the south by Chupadera Mesa. Depth to Precambrian basement ranges from more than 8500 ft in a narrow graben (Perro sub-basin) in the eastern part of the basin to less than 1000 ft on a shelf to the west.
Basin fill consists primarily of Pennsylvanian and Wolfcampian sandstones and shales in the graben and sandstones, shales, and marine limestones on the shelf. In the Perro sub-basin, reservoirs are fine- to coarse-grained sandstones with relatively low permeabilities and porosities. On the shelf, reservoirs are mostly fine- to coarse-grained sandstones with porosities that range from 0 to 16% and average approximately 10%. Most limestones on the shelf have less than 5% porosity and are poor reservoirs; however algal grainstones and recrystallized lime mudstones appear to locally form good reservoirs with porosities that can exceed 20%.
Mature to marginally mature dark-gray to black Pennsylvanian shales are probable source rocks. Thermal Alteration Index ranges from 2.0 to 3.2. Shales become thermally mature with depth in the eastern graben. On the western shelf, shales become mature to the west as a result of increased heating from the Rio Grande rift. Total organic carbon exceeds 0.5% in many shales, sufficient for hydrocarbon generation. Kerogen types are mixed algal, herbaceous, and woody, indicating that gas, or possibly gas mixed with oil, was generated. Kerogens in the shales of the eastern graben are entirely woody, gas-prone types. In limestones and shales of the western shelf, kerogens have a mixed marine and continental provenance, indicating that both oil and gas may have been generated in thermally mature parts of the shelf.
Forty-three exploratory wells have been drilled in the basin. Only 17 of those wells have been drilled to Precambrian. Density of wells that penetrate the lowermost Pennsylvanian is less than one well for every three townships. Most of the wells were drilled prior to 1950 and lacked modern logs and testing apparatus with which to fully evaluate the drilled section. In spite of this, numerous shows of oil and gas have been reported; many of these shows are well documented by modern logs and tests from wells drilled since 1970. During the 1930’s and 1940’s, carbon dioxide gas was produced commercially from two small fields on the western flank of the basin.