Bulletin-23—Stratigraphy and Oil-Producing Zones of the Pre-San Andres Formations of Souteastern NM—Preliminary Report

By R. E. King, 1945, 31 pp., 1 fig., 3 plates, 1 index.

Since the beginning of 1944, exploration for oil in formations older than the San Andres has been greatly accelerated in southeastern NM, and has resulted in several important discoveries in Lea County. At the present early stage of development, there are considerable differences among geologists in the nomenclature of the formations. The purpose of the present paper is to attempt standardization of names and correlations of the older subsurface units in southeastern NM, and to compare the names used with those established for the surface outcrop sections in the state. It is recognized that southeastern NM is a part of a regional geological province, the South Permian Basin, more than half of which lies in Texas; but a full study of the pre-San Andres formations within the whole of that province is beyond the scope of this report. The area here considered is chiefly restricted to the NM counties of Chaves, Eddy, and Lea, although sections are described from Socorro, Torrance, Guadalupe, DeBaca, Lincoln, and Roosevelt Counties.

In order to demonstrate the lateral relations of the rock units, three stratigraphic cross sections are attached to the present paper. They are aligned on sea-level datum because it is believed that in spite of the small relative horizontal scale some conception of the regional structure may be gained from such an arrangement. Most of the logs on the cross sections have not appeared on any previously published section. The logs include representatives from each of the areas of deep production and all deep wildcat tests on which information was available before July 1, 1945. By deep is meant deep stratigraphic penetration rather than depth from the surface.

The logs used in the cross sections were obtained from a variety of sources. The fact that the logs are based on sample examination by a number of geologists results in some differences in lithologic interpretation. There is, in addition, considerable variation in the quality of the well samples: some logs were prepared from excellent sets of samples, whereas others, particularly for some of the rotary wells in Lea County, were prepared from poor sets, and allowance could not be made for all the effect of contamination by recirculated cuttings. Insofar as possible, an attempt was made to interpret the logs in order to eliminate recirculated material and to generalize the minor alterations of different types of lithology. It would have been preferable to show all the separate layers of different composition in the section by patterns extending horizontally the entire width of each column, but on the small vertical scale used, thin units could be shown only by means of percentage.

Most of the correlations shown are based on lithology, but some correlations in the Lower Permian and Pennsylvanian are based on the determination of fusulinids by various paleontologists. Fusulinids are known to be the most useful fossils in that part of the section, particularly in well cuttings where the larger fossils are generally broken into small unidentifiable fragments. Unfortunately, complete paleontological information on all wells containing fusulinids was not available for the present study, as much of this information is of confidential nature. Some of the paleontological data received were conflicting, because the faunas were composed of long-ranging types, the more distinctive short-ranging forms being absent.

Subdivisions of the Permian system that are used in classification of the subsurface section of the Permian Basin are, with few exceptions, taken from surface outcrops. Some difficulty is experienced in correlating these subdivisions from their distant type localities into the subsurface section, as there is a notable lack of lithologic and faunal uniformity between surface and subsurface, and breaks in deposition and local uplifts in the marginal areas took place at the same time with nearly continuous deposition within the Permian Basin. Different lithologic and faunal facies follow trends roughly parallel to the margins of the Delaware Basin, through which the Permian epicontinental sea connected with the open ocean, and to the margins of the lands bordering the epicontinental sea. After Permian time, the Permian rocks were exposed to erosion in the marginal facies of central Texas and central NM, and in the transition facies to the Delaware Basin in the mountain ranges of westernmost Texas and Eddy and Otero Counties, NM.