Samples of 18 oil field brines were collected from producing wells of Gulf Oil Corp. Most of the wells were small producers not under flood. One was a water well in the Capitan reef. Two of the samples were condensates from deep gas wells. Producing formations range in age from the Permian Capitan to Ordovician Ellenburger Formations at depths of 1,900 to over 17,000 ft (570 to 5,100 m). Geologic environments represented by the host rocks include shelf and reef dolomites, backreef sandstones of the Central Basin platform, and sandstones from the Delaware basin. Older rocks represent shelf limestones and dolomites, and reef dolomites.

Certain chemical properties were determined in the field. Other major and minor constituents were determined in the laboratory. The chemical data are considered preliminary because analyses were not made for certain constituents.

Most of the waters have total dissolved solids of 100,000 ppm or over. The Permian brines are substantially more saline than waters from Gulf Coast Tertiary reservoirs of comparable depths. The brines are in general sodium-calcium chloride waters, with a substantial sulfate content in some samples. Basinal facies rocks produce brines enriched in chlorine, whereas concentrated brines from shelf-facies rocks are lower in chlorine than normal evaporite-depositing brines.

Diagrams show the degrees of variability among samples, and statistical correlations have been attempted. High iodine concentrations appear to be confined to the Delaware basin. All chlorine to bromine ratios are less than 300, which probably indicates brinewater solution of preexisting marine evaporites. Certain samples are anomalously enriched in calcium, sulfate, strontium, and bromine, and in some potassium is notably depleted.

Copper content of most samples was less than 1.0 ppm. Such low values are in agreement with brine compositions from other basins. Lead content ranges from 0 to 2.25 ppm, and zinc from 0.55 to 6.12 ppm. These numbers fall within observed values for other basins. The persistent occurrence of zinc in the brines suggests that favorable rocks in the Permian basin may have been mineralized. Slightly higher zinc-bearing waters appear to come from the Delaware basin or at the shelf edge.

The geochemistry of oil field brines may be used to: (1) identify overpressured zones, (2) correlate lithologic units, (3) locate depositional facies favorable for petroleum generation, (4) estimate subsurface temperatures from silicon analysis, and (5) locate geologic provinces favorable for concentration of lead and zinc as well as barite and associated nonmetallic minerals.

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