Geochemistry and origin of formation waters in the western Canada sedimentary basin--III. Factors controlling chemical composition

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Twenty major and minor chemical components are reported for 78 formation waters from oil fields and gas fields in Alberta, Canada. Using published pore volume and chlorinity distribution data, a volume-weighted mean composition of formation waters in the western Canada sedimentary basin is presented. The results of Q-mode, R-mode, and second-order R-mode factor analyses are tabulated and interpreted. The volume-weighted mean composition is similar to that of present day sea water and is compelling evidence for an ultimate origin from sea water of the major portion of the dissolved salts in the formation waters. Dilution by fresh water recharge and concentration by membrane filtration are the major factors controlling chemical composition. Together, they produce a chemical population ranging from freshwater to brines which is confirmed by the Q-mode analysis. Compared to sea water, the volume-weighted mean formation water has gained NaCl, which occurs as a separate factor in the R-mode analysis, and quantitative calculations demonstrate that sufficient halite has been dissolved from Middle Devonian evaporite beds to account for the observed gain in NaCl, and that the balance of halite dissolved from bedded evaporites in the basin has been, and is being, lost to the surface. The presence of Mg and Ca in separate factors, together with quantitative calculations, indicate that the ionically balanced loss of Mg and gain of Ca in the volume-weighted mean formation water cannot be attributed to dolomitization. Similarly, the total loss of SO 4 is not due to conversion of H 2 S. Chlorite formation could account for the loss of Mg. Other factors controlling chemical composition and suggested by the factor study include cation exchange on clays, a probable contribution of Br and I by desorption from the clay fraction of argillaceous rocks, as well as from organic matter, and solubility control of Ca-CO 3 and Sr-SO 4 concentrations.

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