Interpretation of chemical and isotopic compositions of brines based on mixing and dilution, "Clinton" sandstones, eastern Ohio, U.S.A.

Abstract

The chemical compositions of 24 selected brine samples from the gas and oil-bearing "Clinton" sandstones of eastern Ohio can be attributed to mixing of two components in the subsurface. The concentrations of major anions and cations of the mixed brines were subsequently altered by dilution with groundwater or surface water that entered the brines either during production or subsequent storage on the surface. The two brine types identified in this study are characterized by high concentrations of NaCl and CaCl2, respectively. After removing the effects of dilution, the abundance of the NaCl brine decreases with depth within the Clinton sandstones, whereas the abundance of the CaCl2 brine increases. These results support the conclusion that the Ca-rich brine component originates from sources at depth and is mixed with the Na-rich brine that may form by solution of rock salt in the overlying Salina Formation. The isotope compositions of oxygen and hydrogen of 12 brine samples from a linear array in coordinates of δ18O and δD. The array formed by the Clinton brines can be explained satisfactorily as a product of mixing of the two brine types characterized by differences in their chemical composition. The Ca-rich brine is enriched in 18O relative to the Na-rich brine. The 87Sr/86Sr ratios of Sr in 7 brine samples range from 0.70913 to 0.71364 and vary inversely with the Sr concentrations. This relationship is likewise attributable to mixing of the two brine types which have different isotopic compositions and concentrations of Sr. The Na-rich brine is enriched in radiogenic 87Sr compared to the Ca-rich brine. Therefore, both chemical and isotopic parameters support the conclusion that mixing and dilution account for the observed compositions of brine samples from the Clinton sandstones of eastern Ohio. © 1988.