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

R. A. Garber, P. M. Harris, J. M. Borer

Magnesite (MgCO₃) occurs pervasively in a 270-ft cored interval of Upper Permian (Guadalupian) 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 result in failure to recognize potential reservoir zones.

In our 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 magnesite consists of anhedral to euhedral crystals ranging in size from 1 to 4 µm. Within the dolomite intervals, magnesite occurs in three forms: (1) patches and discrete grains floating in dolomite matrix, (2) pervasive replacement of host dolomudstone and peloidal and pisolitic dolowackestone or packstone, and (3) discrete layers interbedded with anhydrite. Magnesite rarely fills pore spaces in dolomite, but within siliciclastic beds, magnesite occurs as a first generation cement and as a replacement of former carbonate grains. Magnesite and dolomite are mutually exclusive in siliciclastic beds, and magnesite occurs only when anhydrite is abundant. Preliminary measurements of stable carbon and oxygen isotopes yield normal Permian values for ^dgrC¹³, averaging + .84 ^pmil and slightly evaporitic values of ^dgrO¹⁸, averaging +1.04; corrected Sr⁸⁷/Sr⁸⁶ isotopic composition averages 0.70687.

Magnesite may have formed either (1) during the Guadalupian, syndepositionally or before significant burial, from dense surface brines or, more likely (2) during the Ochoan, after burial, from brines originating from the overlying Salado evaporites. The basin-wide extent of the magnesite occurrence is unknown. However, magnesite should be present elsewhere, based on the abundance of magnesite found in this core and similar gamma-ray response that is common in wells penetrating the same stratigraphic interval throughout the Permian basin.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.