On the origin of oil-field water in the Biyang Depression of China

Abstract

We have surveyed groundwater samples collected from oil and gas reservoirs in the Biyang Depression of China and quantitatively analyzed the chemical features of those samples using the proportional coefficients. Three different proportional coefficients, namely the de-calcium–magnesium, the boron–calcium–magnesium, and the chloration coefficients have been calculated. These three coefficients reflect the strength of de-calcium–magnesium reaction, the trace elements concentrations, and the degree of diagenesis of the oil-field water, respectively. The concentrations of calcium and magnesium ions are found to be very low in the groundwater of the Biyang Depression. The concentration of anion in the oil-field groundwater changes greatly with the salinity of groundwater in the Biyang Depression. In low salinity oil field, bicarbonate is generally the dominating anion; but when salinity increases, sulfate gradually replaces bicarbonate to become the dominating anion. However, in high salinity oil field, chloride is the dominating anion. Bromine, iodine, and boron are found to be relatively rich in oil-field water of Biyang Depression. The results show that extensive dolomite deposited near the center of the depression was resulted from de-calcium–magnesium reaction, and the degree of diagenesis of the oil-field water and concentrations of trace elements increase with buried depth in the reservoir.