September Luncheon

Please RSVP by 3:00pm on Friday, September 22, 2023.

Reservations made after the deadline will incur an additional fee of $10.

Thanks to DIVERSIFIED WELL LOGGING for Sponsoring our September Luncheon!

LOCATION: In-Person luncheon meeting will be held at the Bush Convention Center. Virtual meeting on Zoom. 

TOPIC: "Old Rocks, New Ways: The Lower Paleozoic of Far West Texas

and Southeast New Mexico," - Andy Roark

ABSTRACT: Despite the importance of the Fusselman Formation and Wristen Group as oil, gas, and saltwater disposal reservoirs in the Permian Basin, and the potential association of these zones with seismic activity, few modern studies have analyzed well-exposed, age- and facies-equivalent rocks in far western Texas and southeastern New Mexico. This study presents newly-collected logs of outcrop “pseudowells” through these strata and the immediately underlying Montoya Group, all deposited in ramp carbonate settings, at four strategic locations along a thinning, loosely distal-to-proximal transect through the outcrop belt in the El Paso-Las Cruces-Alamogordo corridor.  These data replicate previously ambiguous features in subsurface data, enabling unprecedented recognition of stratigraphic and diagenetic features.

This study subjected the outcropped Upper Ordovician to Lower Devonian Fusselman Formation (age-equivalent to both the Fusselman Formation and the Fasken Member of the Wristen Group in the subsurface) and the Upper Ordovician Montoya Group to an array of field methods that replicate responses to common wireline and mudlog datasets. Measurements from a handheld gamma ray scintillometer produced curves of total gamma ray and potassium, uranium, and thorium spectra. Response to a Leeb rebound hammer resulted in a log of hardness, similar to sonic log-based unconfined compressive strength data. Reporting the reactivity with dilute HCl flagged limestone intervals, analogous to a photoelectric log. Reflected light photomicrographs from a handheld USB device allowed generation of grain/crystal size and grayscale color curves, as one could also derive from cuttings data. All data were collected at 0.3’ to 0.7’ sampling intervals, reproducing the resolution of wireline logs.

Measured sections range in thickness from ~1000’ to ~350’ and sample an array of depositional settings. Facies descriptions reveal an overall shallowing upward trend within the Fusselman. From base to top, the formation shows increasing abundance of above-ramp-crest facies (laminated strata often with restricted fauna and evidence of exposure) while ramp crest and below facies (wackestones/packstones with open marine fauna to distal mudstones), which dominate the base of the section, disappear. The Fusselman shows more-dramatic lateral facies changes, generally becoming more proximal to the north and west, than the Montoya. Additionally, there is no evidence of hardgrounds or exposure surfaces at the contacts between the lithostratigraphic members, a significant distinction from the Montoya whose lithostratigraphic subdivisions also correspond to third order sequence boundaries (M. Pope, 2014, AAPG Bull.). These differences explain why the Fusselman’s 3 members are only well-defined in the immediate vicinity of the type section.

Without exception, sequence boundaries within the Montoya correspond to sharp inflections in multiple log datasets, particularly Leeb hardness and total gamma ray. These observations validate the ability of the outcrop logging data suite to identify stratigraphically significant features. Within the Fusselman, the data replicated a common subsurface feature in the western Delaware Basin: lenses of limestone surrounded by uranium-enriched dolomite. Three-dimensional outcrop exposures show that the features are clearly diagenetic, representing the edges of fronts of burial dolomite crystallization that frequently cross bed boundaries. These observations rule out alternative hypotheses (e.g. karstic cavern fill or biohermal buildups). At the ~100’ scale, coarsely crystalline intervals appear to correlate laterally, potentially representing exposure surfaces, and correspond to lower (softer) Leeb rebound data. Combined, these datasets establish a basis for extending outcrop-based high-resolution stratigraphy to the subsurface, improving predictivity of mapping products and potentially helping assess reservoir potential and seismicity hazards.

BIO: Andy Roark is a development geologist at Chevron with more than 8 years of experience working exploration and development in shale and tight plays and associated saltwater disposal assets across the Rockies, Anadarko-Ardmore, Permian, and Gulf Coast basins. He currently supports fields in Lea and Eddy Counties of southeastern New Mexico. He received a bachelor's degree in geology from the University of Alabama and a Master of Science in Geology at Texas A&M University. For the past three years, he has led multiple recurring geologic field trips to the Permian Basin and surrounding areas for Chevron and regional professional societies.