Abstract

In the Delaware Basin the lower Guadalupian (middle Permian) Series includes the Brushy Canyon Formation, the Pipeline Shale and the upper part of the Cutoff Formation. The underlying upper Cisuralian Series (Leonardian) includes the lower part of the Cutoff Formation and the Bone Spring Formation. Our poster concerns three easily recognizable lower Guadalupian (Roadian Stage) subsurface markers which can be mapped regionally in the northern Delaware Basin. These important wireline log markers are: (1) Avalon (“Cutoff”) sand, (2) Pipeline Shale, and (3) top of the lower Brushy Canyon cyclic unit. Because of long time usage, the sharp break recorded on gamma ray and porosity logs at the Pipeline Shale - Cutoff Formation contact is generally called “top of the Bone Spring Formation” by the industry.

The long-recognized (i. e. WTGS, 1962, cross section) very fine-grained Avalon sand marker is locally productive, is a lowstand deposit within the basinal Cutoff Formation, and recently has been recognized in the upper part of the outcropping El Centro Member of the Cutoff Formation (Amerman, 2009). Because the world standard base of the Guadalupian Series (Guadalupian GSSP) is in the middle part of the El Centro Member, Stratotype Canyon, Guadalupe Mountains, the generally thin (<10 – 60 foot) and widespread Avalon (Cutoff) sand approximates the base of the Guadalupian Series in the subsurface. In addition to its importance as a time marker, it proves that the so called “upper Bone Spring” of subsurface usage is the Cutoff Formation. Thus the 1^st Bone Spring carbonate lies below the Cutoff Formation but the Cutoff – Bone Spring contact is difficult to pick with confidence in the subsurface.

On resistivity logs the Avalon sand is most easily recognized by its relatively low resistivity and moderate radioactivity compared to adjacent overlying and underlying El Centro siltstone/mudstone beds and has log characteristics similar to thicker sands in the older Bone Spring Formation and the younger Brushy Canyon Formation. In the subsurface it is present below the uppermost Cutoff carbonate (Williams Ranch Member) and in the upper part of the underlying shaly section thought to be equivalent to the outcropping El Centro Member. Upper Cisuralian (Leonardian) organic-rich siltstones/mudstones below the Avalon sand are locally productive in horizontally drilled wells. The Avalon sand is only patchily preserved on the slope (shelf to basin transition areas) probably due to bypass and/or erosion. The San Andres Formation shelf equivalent to the basinal Avalon sand may be the unconformity called “Brushy Canyon bypass surface” of Kerans and Fitchen (1995, 1996).

The Pipeline Shale marker overlies with sharp contact the uppermost Cutoff Formation carbonate (Williams Ranch Member). This contact is considered unconformable and the Pipeline is a thin black organic-rich siltstone/mudstone unit having high gamma ray kicks and high resistivity on wireline logs. Prior to widespread use of gamma ray logs, the Pipeline Shale was mapped as “top of Bone Spring Formation” because of its similar resistivity to the underlying limestone. Although commonly present, it is not recognizable throughout the basin probably due in part to erosion during basal Brushy Canyon deposition. It represents a basinal, highstand condensed unit and its shelf equivalent is within the post-Leonardian (post-Cisuralian) San Andres Formation lying below the Lovington sandstone.

Stratigraphic relationships of the Pipeline Shale remain controversial. In its type locality (Warren, 1955) along the El Paso pipeline road, southern Guadalupe Mountains and in the Delaware Mountains (King, 1965), and in the subsurface it appears to grade upward into typical siltstones and sandstones of the lower Brushy Canyon Formation. Therefore, most workers consider the Pipeline Shale to be the basal member of the Brushy Canyon Formation. However, its wireline log character more closely resembles shale beds in the underlying Cutoff Formation and some workers (i.e. Gardner and Sonnenfeld, 1996) consider it the upper part of the Cutoff Formation. An Exxon group (Beaubouef, et al., 1999) consider the Pipeline Shale to be a separate formation with sequence boundary unconformities at its top as well as its base.

The top lower Brushy Canyon marker (Broadhead, Luo and Speer, 1998) is also widespread in the northern Delaware Basin but cannot be confidently traced in areas near the basin margins nor to outcrops near the basin slope in the Guadalupe Mountains, Texas. It is a thin bed characterized by high to moderate radioactivity and high resistivity. Possibly it correlates with the outcropping sequence boundary siltstone at the top of the lower Brushy Canyon unit mapped by Gardner and Sonnenfeld (1995) and Beaubouef, et al. (1999). Where recognizable in the central part of the northern Delaware basin, this marker is an organic-rich siltstone/mudstone bed generally 200 to 400 feet above the Pipeline Shale – upper Cutoff carbonate contact. It marks the top of the locally productive and basinal lower Brushy Canyon cyclic sand - shale unit that onlaps the Cutoff Formation or older Victorio Peak –Bone Spring formations along the basin margin and slope.

These three lower Guadalupian wireline log markers (Avalon sand, Pipeline Shale, and top lower Brushy Canyon) allow regional mapping and help in the stratigraphic analysis of the lowstand lower Brushy Canyon unit and the lowstand Avalon (Cutoff) sand that are separated by the highstand upper Cutoff carbonate (Williams Ranch Member) and overlying Pipeline Shale. All three basinal markers are equivalent to parts of the post-Leonardian (post-Cisuralian) and pre-Lovington sandstone portion of the San Andres Formation on the Northwest Shelf but their precise equivalents are controversial.

Part 1 of our poster details the typical wireline log character of the section between the lower Brushy Canyon unit and the Avalon (Cutoff) sand in the north central Delaware Basin and in wells near the outcrop in the southern Guadalupe Mountains area. Part 2 consists of three regional cross sections and includes 3 outcrop sections and 58 wells showing log character and distribution of the three lower Guadalupian log markers. Work continues on regional mapping of the Cutoff Formation in the northern Delaware Basin.