Abstract

The widespread, basinal Manzanita Limestone Member crops out in the Guadalupe Mountains and Delaware Mountains where it has been studied for over 60 years. Nevertheless, there is no published regional subsurface study of this fascinating unit. This paper begins to fill that gap. Locally, oil production is found in very fine-grained sandstone reservoirs immediately overlying, within, and underlying the Manzanita Limestone Member. Unlike many basinal Guadalupian limestone tongues, it is fusulinid poor, but most workers consider it to be in the zone of Parafusulina (Guadalupian, Wordian) and by stratigraphic position to be younger than the Goat Seep Reef, older than the Capitan Reef, and probably equivalent to the Shattuck Member of the Queen Formation on the shelf. The Manzanita Limestone Member lies at the base of the outcrop-defined composite sequence 12 (CS 12) of Kerans and Tinker (1999) who place it in their high frequency sequences (HFS) G 15 and G 16.

Throughout the northern Delaware Basin, the Manzanita Limestone Member can be recognized by distinctive wireline log character. It underlies the locally productive uppermost siliciclastic unit in the Cherry Canyon Formation. We consider this upper sandstone to be the lowstand phase of the Lower Seven Rivers high frequency sequence of Tyrrell and Diemer (2003a, b) which they equate with G 17 HFS of Kerans and Tinker (1999). The widespread Hegler Limestone Member (also known as the “Two Finger Limestone” in the subsurface) of the Bell Canyon Formation makes up the highstand phase of that HFS, is in the zone of Polydiexodina, and is equivalent to the lower Capitan Reef on the shelf margin and the Lower Seven Rivers Formation on the shelf.

Over much of the eastern Delaware Basin our subsurface-defined Manzanita HFS consists of five siliciclastic to carbonate cycles which we map as cycles Mz-1 to Mz-5 from bottom to top. The basal part of cycle Mz-1 comprises the sandstone between the South Wells Limestone Member and the Manzanita Limestone Member. The carbonate phases of cycles Mz-2 through Mz-5 commonly can be recognized in outcrop. Regionally, the carbonate high-stand phases of the cycles change thickness relatively uniformly but locally some of the siliciclastic lowstand phases vary significantly in thickness. The Mz-3 cycle commonly has the thickest carbonate section and in its lower part generally contains two radioactive kicks on gamma ray logs considered to be bentonites (altered volcanic ash). The two bentonites are separated by a carbonate bed and together this triplet (herein called the BCB marker unit) can be recognized on most

wireline logs and used as a time marker throughout the northern Delaware Basin. The thin carbonate section at the top of the Mz-1 cycle also contains a medial radioactive kick on many gamma ray logs that is interpreted as a bentonite bed. The relatively thick siliciclastic unit in the lower portion of the Mz-1 cycle is interpreted as a lowstand deposit. It is locally an oil reservoir and probably is related to a bypass surface at or near the base of the Shattuck Member of the Queen Formation. We include this basal sandstone and overlying cycles of the Manzanita Limestone Member in one high frequency sequence, the Manzanita HFS. The Manzanita HFS thickens along the northern margin of the Delaware Basin where it locally exceeds 300 feet (100 m).

Our subsurface analysis using close well control from several oil fields along the northern Delaware Basin margin, suggests that the Manzanita HFS grades into massive carbonate which cannot be distinguished by wireline log character from the lowermost “Capitan Formation”. This observation concurs with the interpretation on numerous published regional subsurface cross-sections where the Manzanita Limestone Member is correlated with the Goat Seep or lower Capitan Formations. An alternative interpretation for the basin margin relationships is that the Manzanita HFS may be a prograding wedge which began on the lower Goat Seep Reef slope, or on lowstand siliciclastics that overlie this slope. This interpretation is supported by the observation that the lower two cycles of the Manzanita HFS extend the farthest shelfward. Rigby (1958) and Newell, et al. (1953), reported a debris flow deposit in the Manzanita Limestone Member exposed in Trew Canyon of the Apache Mountains. Together these relationships suggest the pinchout of the Manzanita Limestone Member along the west face of the Guadalupe Mountains (cf. King, 1942, 1948, and others) may be anomalous. Hampton (1989) reported that the mostly carbonate mudstones and wackestones of the outcropping Manzanita Limestone Member contain sparse elements of normal marine biota generally lacking fusulinids and suggested deep water deposition. In the Indian Draw Delaware Field, Cromwell (1979) showed the Manzanita Limestone Member draping the underlying bathymetry and slightly thickening in the area of a channel fill. This may be the result of a combination of deposition from suspended load and channel filling by turbidites. In the east-central basin (PDB-03 well, Loving County, Texas), Loftin (1996) interpreted the limestones of the Manzanita Limestone Member as fine-grained turbidites generated by carbonate debris flows. Some of the turbidity currents may have traveled across pycnoclines before the sediments settled out of suspension. The shelf equivalent Shattuck Member contains shelf margin carbonate beds which may have supplied carbonate debris to the basin. Ongoing mapping of the Manzanita HFS may shed more light on its basin margin relationships and depositional environment.