Abstract

A project to create an atlas of well log cross sections for the Northwest Shelf and the northern Delaware Basin of southeast New Mexico and adjacent west Texas has been underway intermittently for 3 years. Emphasis is on Permian lithostratigraphy and log correlation but an attempt is made to relate these to sequence stratigraphy of the Guadalupian and Cisuralian (Leonardian - Wolfcampian) Series. As of mid 2002, sixteen Paleozoic cross sections (many with multiple panels) have been constructed, mostly in Eddy County, NM and northern Culberson County, TX and include over 200 wells. The structurally simple, grid of sections is tied to published Permian composite sequence (CS) and high frequency sequence (HFS) studies in the Sierra Diablo, Delaware, and Guadalupe Mountains, as well as, to surface geology, several surface type sections, Artesia Group type well, the Gulf PDB - 04 cored well, regional EPR seismic line northeast of Carlsbad, and to 9 published regional cross sections. Although the scales (1” = 400’ vertical and mostly 1” = 4000’ horizontal) are somewhat compressed, and a variety of log types used, careful hand drafting of selected curves preserves much of basic log character.

The well log cross sections (many to be published by the NMBG&MR) and yet to be prepared maps are designed to provide:

1. Subsurface data for first-time field trip visitors or students who may be unfamiliar with the entire Paleozoic section of the greater Guadalupe Mountains area.

2. Background data and my interpretation of regional subsurface relationships of possible use in complementing published sequence analyses by other workers in nearby classic Permian outcrops

3. Regional subsurface data useful in evaluating stratigraphic changes along strike, as well as dip, and away from published outcrop studies.

4. An available grid of uninterpreted well log sections useful for making independent interpretations of lithostratigraphy and sequence stratigraphy in Permian, Pennsylvanian, and older Paleozoic units.

“Old” west Texas logs (combined GR, SP and Resistivity curves) tend to filter out some of the higher order sequence cycles and commonly are helpful in visually subdividing the Guadalupian, Leonardian, and Wolfcampian on the cross sections. These log curves plus more modern geophysical logs help establish potentially useful “E-Log defined units” that may approximate outcrop- and core-defined CS and HFS in some system tracts. Recognition of sequences within the various Permian shelf margins remains a problem, as does log picks of the base Lopingian (Ochoa Group), Guadalupian, Leonardian, and Wolfcampian without paleontological control and agreement among fusulinid and conodont workers. Nevertheless, using the lowstand - highstand sea level reciprocal sedimentation model, most Permian CS and many HFS defined by others in Guadalupian and upper - middle Leonardian outcrops commonly can be recognized (or approximated) from geophysical log plus sample data in the subsurface of the northern Delaware Basin and to a lessor extent on the Northwest Shelf. In contrast, pre-3^rd Bone Spring Sandstone lower Leonardian and Wolfcampian CS and HFS in slope and condensed basinal facies are difficult to impossible (?) to define by log criteria alone.