Abstract

During ongoing work on an atlas of well log cross sections in southeastern New Mexico and adjacent Texas, we use two subsurface marker units to help refine some correlations and relate these markers to well known outcrops in the Guadalupe Mountains. The Bowers Sand marker unit is a shallow water, late Wordian, cyclic siliciclastic – carbonate (or evaporite) unit on the Northwest Shelf and the Two Finger Limestone marker unit is a slightly older late Wordian deep water deposit in the northern Delaware Basin.

The widespread Bowers Sand in the Seven Rivers Formation is an oil and gas reservoir in the Hobbs Field area, Lea County, New Mexico where it was first described in cores by Deford and Wahlstrom (1932). Although not named, it is present in the Artesia Group type subsurface reference section located 36 miles north-northeast of Carlsbad where it has typical wireline log character for the Northwest Shelf. The Bowers Sand marks the base of the middle part of the Seven Rivers Formation and commonly consists of three sandstone/siltstone beds separated by carbonate beds in the middle to outer shelf facies tracts or by anhydrite beds over much of the inner shelf facies tract. It can be traced in the subsurface by well-to-well correlation to outcrops in the Guadalupe Mountains where it defines the base of High Frequency Sequence (HFS) SR-2 of Tinker (1998) and equates to siltstone beds X, Y and Z of Hurley (1978, 1989) in North McKittrick Canyon. It also can be recognized in measured sections in the Bear Canyon to Rocky Arroyo area. The sandstone/siltstone – carbonate/evaporite cycles in the Bowers Sand are interpreted as 5^th order cycles related to sea level variation.

The distribution of the Bowers Sand is similar to the slightly older, better-known, and commonly productive Shattuck Member sandstone (Artesia Red Sand or Queen Sand) of the uppermost Queen Formation. Like the Shattuck Member, the Bowers Sand can be recognized by radioactive “kicks” on gamma-ray logs. Near the down dip pinchout of the Shattuck Member, the Bowers Sand locally has been mistaken for the “Queen Sand”. The basinal equivalent of the Bowers Sand unit may be the sandy interval separating the Hegler Limestone Member and the overlying Pinery Limestone Member of the Bell Canyon Formation.

The Two Finger Limestone is a widespread marker unit in the northern Delaware Basin. “Two Finger Limestone” refers to its wireline log character where two limestone beds are overlain, underlain, and separated by more radioactive very fine-grained feldspathic sandstone. Except near its gradation into the lower Capitan Reef it is commonly less than 30 feet thick. It is not an oil or gas exploration objective. It lies above a variable thickness of locally productive sandstone that overlies the Manzanita Limestone Member in the upper Cherry Canyon Formation. It was cored in the Gulf PDB-04 research well located 16 miles northeast of Carlsbad where it was not named but placed in the upper Cherry Canyon Formation by Garber, Grover and Harris (1989). We have traced the Two Finger Limestone in the subsurface by well-to-well correlation to the Guadalupe Mountains where it equates to typical outcrops of the Hegler Limestone Member of the Bell Canyon Formation. Thus the Two Finger Limestone in the Gulf PDB-04 core and in the northern Delaware Basin could be called the Hegler Limestone Member and the top of the Cherry Canyon Formation should be placed at its base. The shelf equivalent of the Hegler Limestone Member is probably the lower Seven Rivers Formation below the Bowers Sand (HFS G 17 of Kerans and Tinker, 1999).