New Mexico Geological Society Annual Spring Meeting — Abstracts

The City of Jal (COJ or the City), located in the Permian Basin in southeastern New Mexico, obtains its entire drinking water supply from the Tertiary - Quaternary-age Pecos Valley Alluvial (PVA) aquifer and obtains non-potable (higher total dissolved solids [TDS]) water for Jal Lake and municipal parks from the Triassic Santa Rosa Sandstone aquifer. The City is therefore entirely reliant on groundwater for its water supply and produces water from wells drilled to depths ranging from 400 to 800 ft. The same aquifers that COJ relies on are also the target for use in non-conventional oil and gas (O&G) development (hydraulic fracturing, AKA fracking), and wells that produce water for the O&G industry are located in close proximity to COJ wells.

In an effort to better understand this aquifer system, we have conducted a basin analysis of the northern Monument Draw Trough, a collapse structure within the Delaware Basin (one of the subbasins within the Permian Basin), utilizing natural gamma logs from the New Mexico Oil Conservation Division (OCD) on-line database, lithologic and borehole geophysical data from the COJ exploratory drilling program, New Mexico Office of the State Engineer (NMOSE) well records, geologic mapping, and an aquifer testing program using new and existing COJ production wells. Our analysis focused on the upper 2500 ft of Permian and Triassic evaporite and clastic sedimentary rocks overlain by eolian, fluvial, and lacustrine sediments of the PVA that fill the Monument Draw Trough, which was formed as a result of dissolution of Permian evaporite deposits resulting in the collapse of overlying younger strata. Our analysis demonstrates that collapse of the Permian/Triassic sediments was accommodated, at least in part, by brittle deformation resulting in juxtaposition of PVA and Santa Rosa aquifers against lower permeability lithologies. Basin-bounding faults that delineate a large graben structure can be traced across a series of six east-west cross sections drawn on one-mile spacing, and unit contacts in two north-south cross sections match those determined from the east-west cross sections. Structure contour mapping of the top of the Permian Rustler Formation provides additional evidence for the location of these structures. Several other faults within the basin delineate additional horst and graben structures. GGI’s documentation of brittle deformation in a salt-collapse structure provides important new information for understanding these features.

PVA isopach and saturated thickness maps developed from our basin analysis are used in conjunction with the geometric mean of hydraulic conductivity determined from aquifer tests conducted on COJ wells and other wells in the basin to develop a transmissivity array for the PVA in the vicinity of Jal administrative basin for on-going development of a regional groundwater model. Mapped Triassic outcrops and well data are utilized to delineate the northern extent of the saturated PVA in hydrologic communication with the portion of the aquifer tapped by COJ wells. The Santa Rosa Sandstone is absent or unsaturated on the upthrown side of the western basin-bounding fault. Delineation of the northern extent of the PVA and the western extent of the Santa Rosa Sandstone identified through this basin analysis suggests that the water resources available to the City of Jal and other stakeholders in the region are more limited than previously estimated.