Bulletin 157—Subsurface geology and oil and gas potential of Estancia Basin, New Mexico
By R. F. Broadhead, 1997, 54 pp., 7 tables, 20 figures, appendix, index.
The Estancia Basin, a structural depression that initially developed during Early Pennsylvanian (Morrowan) time, is a frontier exploration area. Although no oil or hydrocarbon gases have been produced from the basin, CO2 gas was commercially extracted from two small fields west of the town of Estancia during the 1930s and 1940s.
Forty-three exploration wells have been drilled in the Estancia Basin; only 17 of those wells were drilled to the depth of the Precambrian basement, which ranges from less than 1,000 ft on the western flank of the basin to more than 8,500 ft in a narrow graben near the town of Willard in the eastern part of the basin. Density of wells that penetrate the lowermost Pennsylvanian is less than one well for every three townships. 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, especially from wells drilled since 1970. During the 1930s and 1940s, carbon dioxide gas was produced commercially from two small fields on the western flank of the basin.
The purpose of this report is to describe and analyze the subsurface structure and stratigraphy of the Estancia Basin in order to characterize reservoirs, source rocks, and hydrocarbon occurrences. Most data used in the study are from the petroleum exploration wells. Structural analyses were synthesized from drill-hole data, reflection seismic profiles, gravity and aeromagnetic maps, and published surface geologic maps. Data relating to stratigraphy, source rock, and reservoirs were obtained from geophysical logs, drill cuttings, scout reports, and source-rock analyses on file at the NM Bureau of Mines and Mineral Resources.
The Estancia Basin of central NM is an asymmetric, north-south-trending structural depression that originated during the Pennsylvanian. The present-day basin covers 1,500 mi2 and is defined approximately by the Estancia Valley. It is bounded on the east by the late Paleozoic Pedernal uplift, on the west by the Tertiary-age Sandia, Manzano, and Los Pinos Mountains, on the north by the Española Basin, and on the south by Chupadera Mesa. Depth to Precambrian basement ranges from more than 8,500 ft in a narrow graben in the eastern part of the basin to less than 1,000 ft on a shelf to the west.
Basin fill consists of Pennsylvanian and Wolfcampian sandstones and shales in the Perro sub-basin 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 016% and average approximately 10%. Most limestones on the shelf have less than 5% porosity and are poor 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 2.03.2. Shales become thermally mature with depth in the Perro sub-basin. 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 Perro sub-basin are entirely woody, gas-prone types. In limestones and shales of the western shelf, kerogens have mixed marine and continental provenance, indicating that both oil and gas may have been generated on thermally mature parts of the shelf.