Late Pennsylvanian (Virgilian) to early Permian (Leonardian) conodont biostratigraphy of the “Wolfcamp Shale,” Northern Midland Basin, Texas

The “Wolfcamp Shale” of the Midland Basin in west Texas has been a prime source of oil reserves within the United States. However, correlation and definitive ages vary and have led to significant controversy. Fusulinid studies have given an approximate age of late Pennsylvanian (Virgilian) to early Permian (Leonardian) for the “Wolfcamp Shale,” though exact horizons for regional and international stage boundaries are not well defined. Analysis of conodont specimens from three cored basinal shale intervals (Core 1: 8836-9020 ft, Core 2: 9200-9502 ft, Core 3: 9660-9840 ft) of the Shell Stevens L1V well in the northern Midland Basin (Lynn County, Texas) allows resolution of both regional and international stage boundaries within the “Wolfcamp Shale,” and provides an accurate means of correlation to the Eastern Shelf and North American Midcontinent. Associated gamma-ray (GR) and resistivity (RES) logs show distinct log markers for the Pennsylvanian – Permian boundary, Leonardian Stage base, and Kungurian Stage base that are traceable to immediately adjacent wells and possibly to portions of the southern Midland Basin.

A total of 153 conodonts were collected from the cored intervals and comprise nineteen species over five genera. Specimens were collected from siliciclastic to mildly calcareous muds, indicating in situ deposition rather than deposition by debris flow. Conodont biostratigraphy confirmed that the “Wolfcamp Shale” ranges from Missourian to early Leonardian in age, and distinguished international and regional stage bases. The base of the international Late Pennsylvanian Gzhelian Stage was not recognized. The base of the international Asselian Stage (Pennsylvanian – Permian boundary) was recognized within the lowermost Wolfcamp C1, in the basal Core 2, by the co-occurrence of Streptognathodus invaginatus, S. wabaunsensis, and S. conjunctus at 9470.95’ – 9471.1’. The base of the international Sakmarian Stage was recognized higher in the Wolfcamp C1, in Core 2, by the first appearance datum of Streptognathodus barskovi at 9387’ – 9391’. The base of the international Artinskian Stage was not recognized by biostratigraphic means. The base of the Kungurian international stage was recognized in the uppermost Wolfcamp A, in Core 1, at 8841.95’ – 8842’ by the first appearance of Neostreptognathodus exsculptus. The base of the regional Late Pennsylvanian Virgilian Stage was not identified. The base of the regional Permian Wolfcampian Stage was taken at the same position as the base of the Asselian Stage. The base of the regional Leonardian Stage was recognized in the uppermost Wolfcamp B, in Core 1, at 8968.65’ – 8968.8’ by the first appearance of Neostreptognathodus ruzhencevi.

To better aid in correlation, 3rd and 4th order cycles were determined from GR log, core lithology, and conodont abundances. High GR values, calcareous mud, and high conodont abundances were associated with transgressive-highstand phases, while low GR values, siliciclastic mud, and low conodont abundances were associated with lowstand phases. Three 3rd order cycles (Cycle I, Cycle II, and Cycle III) and twenty-six 4th order cycles were identified from the Stephens L1V well and tied to well-established Midcontinent cycles using conodont biostratigraphy. Cycle I corresponds to the 3rd Order Gzhelian Sequence, Cycle II corresponds to the 3rd Order Council Grove Sequence, and Cycle III corresponds to the 3rd Order Chase Sequence. The base of the Asselian Stage lies within “cycle 10,” and corresponds to the 4th order “Red Eagle” sequence, while the base of the Sakmarian Stage lies within “cycle 13” and likely corresponds to the 4th order “Eiss” sequence. The base of the Artinskian Stage is taken to lie within “cycle 18” and corresponds to the “Barneston” sequence. These correlations were tied directly to the Eastern Shelf conodont succession. The base of the Asselian Stage in the lower Wolfcamp C1 correlates to the base of the Stockwether Limestone Member, while the base of the Sakmarian Stage in the Wolfcamp C1 was correlated to the base of the Santa Anna Branch Member. The base of the Leonardian in the uppermost Wolfcamp B correlates approximately to the base of the Talpa.