Preliminary geochemical characterization of saline formation water from Miocene reservoirs, offshore Niger Delta

The Niger Delta is one of the most prolific hydrocarbon provinces in the world, but information on basinal hydrogeologyis scanty. Oilfield brines from nine producer wells in the Miocene reservoirs (2,472.25-3,532.48m.b.s.l.), offshore Niger Delta, have been investigated along with two seawater samples to understand their hydrogeochemical characteristics in relation to the host rock mineralogy. Chemical analysis revealed that thewaters are slightly alkaline and can be generally classified as saline water of the Na-Cl type based on their total dissolved solids (TDS). On the basis of bicarbonate, chloride, and sulphate ions, they are shown to be of connate origin. Relative abundance of major ions is in the following order: Na > Ca > Mg > K and Cl > HCO3 > SO4.Saturation indices (SI) of selected mineral phases calculated using PHREEQC indicate that the dissolution of iron oxide and carbonate minerals may contribute major ions in the formation water. The preponderance of alkali elements suggests the presence of feldspars, which could have resulted from sediments through which the water flows. An inverse relationship is observed between the resistivity of formation water and its TDS, which could be used to calculate resistivity values of formation waters in the area if the TDS contributions are known. Chemical data suggest that the formation waters were derived from seawater, dominantly altered by reverse ion-exchange processes and subsequently by water-rock interactions. Multivariate statistical analyses (correlation and factor analysis) indicate multiple sources of enrichment of ions in the formation waters.

Automated summary

The Niger Delta is rich in hydrocarbons, but data on basinal hydrogeology is limited. Chemical analysis of oilfield brines and seawater samples in the area revealed that the formation waters are slightly alkaline and saline, originating from connate sources with a dominance of alkali elements. The presence of feldspars and the contribution of major ions from iron oxide and carbonate minerals were also identified through saturation indices.