A thorough understanding of processes active during the evolution of sedimentary basins emerges when diagenetic studies of sedimentary rocks are integrated with geochemical trends in subsurface waters ( Moldovanyi and Walter, 1992 Land, 1995). Deep formation waters play an important role in many geological processes, such as metal mineralization, hydrocarbon migration and accumulation, and diagenesis of rocks ( Cathles and Smith, 1983 Land et al., 1988 Buzek and Michalićek, 1997). In terms of their origin, formation water may be connate – trapped in the pore space of sediment at the time of its deposition, or meteoric – derived from the atmosphere and percolated down through the vadose zone. Strictly speaking, the term “brine” refers to water with more than 35,000 mg/L (about 3.5 percent) total dissolved solids (TDS) ( Hem, 1985). Most oil and gas wells produce a saline solution, or brine, in addition to the desired oil and gas ( Collins, 1975). Oilfield brines or saline formation water, generally referred to as deep formation water in this paper, accompany the production of crude oil and/gas from reservoirs in sedimentary environments. Multivariate statistical analyses (correlation and factor analysis) indicate multiple sources of enrichment of ions in the formation waters. Chemical data suggest that the formation waters were derived from seawater, dominantly altered by reverse ion-exchange processes and subsequently by water-rock interactions.
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. The preponderance of alkali elements suggests the presence of feldspars, which could have resulted from sediments through which the water flows. 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.
Relative abundance of major ions is in the following order: Na > Ca > Mg > K and Cl > HCO 3 > SO 4. On the basis of bicarbonate, chloride, and sulphate ions, they are shown to be of connate origin. Chemical analysis revealed that the waters are slightly alkaline and can be generally classified as saline water of the Na–Cl type based on their total dissolved solids (TDS). Oilfield brines from nine producer wells in the Miocene reservoirs (2,472.25–3,532.48 m.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.
The Niger Delta is one of the most prolific hydrocarbon provinces in the world, but information on basinal hydrogeology is scanty.
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