New Mexico Geological Society Annual Spring Meeting — Abstracts

The City of Carlsbad, NM is on the northern edge of the Delaware Basin, a fore reef basin mostly encircled by the Capitan Reef and containing thick sequences of evaporites deposited in the Ochoan Age of the Permian Period.

Collection of subsurface rock core samples and downhole geophysical logging within Carlsbad are rare due to ordinances prohibiting oil and gas drilling within city limits. Thus, investigations and geotechnical remediation of the brine well cavity at the I&W Site provided opportunities for direct observation of local stratigraphic, structural and diagenetic changes, as well as comparison with stratigraphy of collapsed brine well sites on the north side (back reef basin) of the Capitan Reef. Several microseismic sensor array installation coreholes, ranging in depth from 400 to 700 feet, penetrated the Salado and the uppermost part of the Castile Formations.

Findings from the geotechnical investigation include:

Collapse breccias, comprising relatively insoluble clays and gypsum, and massive gypsum horizons in the uppermost Salado Formation that occurred as a result of dissolution of the youngest Permian halite layers from inundation of the area during development of the Ancestral Pecos River Valley since the Late Cretaceous or Early Paleogene. This contrasts with the profile in the Loco Hills area (site of a brine cavity collapse sinkhole), north of the Capitan Reef, where the upper Permian halite layers still exist with intermittent, interbedded claystone and sulfate (anhydrite) horizons.

Likely cyclical dehydration of gypsum into anhydrite during the Permian Period, followed by rehydration into gypsum with fresh groundwater inundation and during the Cenozoic formation of the Ancestral Pecos River Valley , á la R. C. Murray’s 1964 model, is evidenced by relict anhydrite horizons at the base of a thick, massive gypsum layer in the upper Salado above the shallowest Salado Fm halite.

Accumulation and consolidation of relatively insoluble remnants of dissolved halite layers and formation of massive gypsum layers above the halite at the I&W Site likely imparted additional strength to the roof of the brine well solution mining cavity, and delayed a full collapse that would have affected highway and city infrastructure in Carlsbad. This contrasts with the brine well operations at Loco Hills and Jim’s Water Service to the north, where the lack of halite dissolution by groundwater caused no such formation to occur; and the roofs of those cavities collapsed, leaving large sinkholes.

References:

1. Murray, R.C. (1964), Origin and Diagenesis of Gypsum and Anhydrite, Journal of Sedimentary Petrology, Vol. 34, No. 3, pp. 512-523
2. Rucker, Michael L. (2018), Technical Memorandum, Conceptual Design & Modeling, Carlsbad Brine Well Remediation, Carlsbad, New Mexico, Memorandum submitted to the Energy, Minerals, and Natural Resources Department by Wood Environment & Infrastructure Solutions, Inc, November 2018