Lithofacies, porosity and log Rresponse of the lower San Andres formation in the Palo Duro Basin

Abstract

The lower San Andres Formation of Curry, DeBaca, Quay and Roosevelt counties, New Mexico, was deposited in the Tucumacari - Palo Duro Basin which was a swale on the Northwestern Shelf of the Permian Basin. Unlike other areas of the Permian Basin where San Andres forms prolific hydrocarbon reservoirs, in the study area it is devoid of hydrocarbons. The study of lithofacies distribution and associated depositional environments, porosity and permeability trends and geophysical log response could serve as a useful model in analyzing and interpreting other similar lithofacies.

The lower San Andres consists of dolostones, limestones, dedolostones, anhydrite, halites and thin shales. Four major progradational depositional cycles are recorded. Upward from the base the cycles generally consist of subtidal and intertidal carbonates, supratidal carbonates, sulfates and halite. The vast bulk of the sulfate was emplaced as replacement of dolostones; thus, the presence of anhydrite within a cycle may have little genetic significance.

As compared to carbonates, percentages of anhydrite and halite increase both towards the NW (proximally) and within successively younger cycles of deposition. Dolostones to limestone ratio also increases towards the NW and in younger cycles. Porosity and permeability generally increase towards the SE (distally) and in older cycles of deposition. This reflects’ a general progradation of supratidal environments towards SE (distally).

Primary porosities are very rarely preserved. Most of the preserved porosities are secondary, tertiary and quaternary in nature. Secondary intercrystalline porosity is more abundant in distal (SE) areas, while moldic porosity is predominent in proximal (NW) areas. Dolostones generally have much greater porosity than limestones.

Most lower San Andres intervals have been subjected to two or more cyclic fluid invasions sequences wherein dolomitizing, anhydritizing and low salinity fluids have successively registered their overprints. Dolomite, anhydrite and calcite cements formed during multiple cyclic diagenesis are the prominant porosity occluders.

Generally most of the lithofacies can be differentiated on the basis of the log response; however, cross plots of various log values aid in differentiating the lithofacies more explicitly.