Circular 93—Structure, stratigraphy, and hydrogeology of the northern Roswell artesian basin, Chaves County, New Mexico

By K. C. Havenor, 1968, 30 pp., 6 tables, 2 figs., 2 appendices, 6 sheets, 1 bibliography.

Structural and stratigraphic controls of the quantity and quality of waters within the northernmost part of the Roswell artesian basin were studied using outcrops, cores, samples, and electric logs. Three structural zones are recognized: the Border Hill, Six Mile, and YO. A fourth near the town of Dexter is suggested, and a fifth in the vicinity of T14S R27E is considered because of the close dating of faulting (early Desmoines). Three structural blocks, Six Mile, Roswell, and Orchard Park, between the first four of these zones are delineated.

The strata in the Orchard Park block are most uniform and compare generally to the complete section preserved in the subsurface east of the artesian basin. The Glorieta, lower and upper units of the San Andres, and most of the Artesia Group are present. The artesian aquifer is developed in the top one third of the San Andres (all in the upper unit) and in the overlying Grayburg. The Queen forms the aquitard between the artesian and shallow aquifers. The latter comprises part of the Seven Rivers Formation and the overlying Tertiary-Quaternary alluvium. Structural uplift and consequent deep, irregular erosion have stripped the Artesia Group and the upper unit of the San Andres from the Roswell block. Secondary porosity enlargement has been developed in the Slaughter zone of the lower San Andres and constitutes the principle porosity zone of the artesian aquifer. The Slaughter zone, a regionally porous and permeable unit, is the principal hydrocarbon-producing zone of the San Andres of the Northwest Shelf of southeast New Mexico and west Texas.

Reduced head, because of regional over-appropriation, and reduced pressure, caused by locally intensive pumping on the Roswell block, relieve the retarding hydrostatic pressure on the saline waters in the regionally porous Slaughter zone and permit saline water encroachment into the Roswell block. The positioning of the Queen aquitard on the downthrown side of the YO structural zone probably inhibits the movement of the saline water from the Roswell to the Orchard Park. Erosion of the Roswell block has developed a collapse breccia and overlying terra rossalike residual clay, heretofore referred to the Artesia Group. This younger deposit forms the aquitard on the Roswell block. Physical differences of the Queen aquitard indicate that its leakage factor will be much higher than the Queen.

The Six Mile block is a grabenlike tilted fault block. A more complete San Andres comprises part of this block than the Roswell block. Remnants of the Artesia, previously reported, are not present in part of the block. Considerations for control of saline encroachment include additional stratigraphic work to determine the areal extent of the base of the aquifer porosity and Slaughter porosity interface. Retardation might result from the employment of injection wells. Further investigations should include acquiring more electric logs, sample examination, continued study of the Glorieta, and adaptation of hydrologic methods to the geology.