Occurrence and Significance of Magnesite in Upper Guadalupian Shelf Strata, Delaware Basin, New Mexico: ABSTRACT

Abstract

Magnesite (MgCO/sub 3/) occurs pervasively in a 270-ft cored interval of Upper Permian (Guadalupina) shelf deposits from the northern rim of the Delaware basin, New Mexico. The texture of magnesite in core slab, thin section, and SEM is almost identical to that of dolomite. Because the density of magnesite is close to that of anhydrite and a high associated uranium content causes large gamma-ray deflections similar to that of shale, magnesite could lead to improper evaluation of lithology and porosity from logs and ultimately results in failure to recognize potential reservoir zones. In the authors' core example, magnesite is found in tidal flat and evaporative lagoon dolomites of the Tansill and uppermost Yates Formations. The interval is overlain by Tansill anhydrite and a thick halite section of the Ochoan Salado Formation. The magensite consists of anhedral to euhedral crystals ranging in size from 1 to 4 ..mu..m. Within the dolomite intervals, magnesite occurs in three forms: (1) patches and discrete grains floating in dolomite matrix, (2) pervasive replacement of host dolomudstoen and pelodial and pisolitic dolowackestone or packstone, and (3) discrete layers interbedded with anhydrite. Mangesite rarely fills pore spaces in dolomite, but within siliciclastic beds, magnesite occurs as amore » first generation cement and as a replacement of former carbonate grains. Magnesite and dolomite are mutually exclusive in siliciclasted beds, and magnesite occurs only when anhydrite is abundant. Preliminary measurements of stable carbon and oxygen isotopes yield normal Permian values for deltaC/sup 13/, averaging +6.84 o/00 and slightly evaporitic values of deltaO/sup 18/, averaging +1.04; corrected Sr/sup 87//Sr/sup 86/ isotopic composition averages 0.70687.« less