A MODEL FOR THE FORMATION OF THE INVERSE GEOCHEMICAL ZONING OF GROUND WATERS IN DEEP HORIZONS OF OIL AND GAS STRUCTURES

Abstract

On the basis of the synthesis of natural data and thermodynamic computer modeling of geochemical processes in water-rock systems under various boundary conditions (S : L ratio, P-CO2, and temperature), the nature of inverse geochemical zoning in deep horizons of oil and gas bearing structures was evaluated. The inversion water was formed at the expense of low salinity (less than 10 g/l) bound and dehydration waters of Cl-Na and HCO3-Na compositions squeezed from the rocks of deep horizons under high geostatic pressures and temperatures. It was found that the most important factors in the formation of the geochemical patterns of inversion water are a change in the Eh-pH state of initial waters under the influence of organic substances contained in rocks and an increase in temperature at low S : L ratio and P-CO2 no higher than 10(-2) bar. At temperatures up to 50degreesC, the geochemical characteristics of inversion waters (high CO2 and low calcium and sulfate contents) result from their progressive redox transformation under the influence of organic matter from the rock. This transformation is driven by a decrease in Eh and a corresponding increase in pH up to values higher than 8.5, which results in calcium removal from initial waters and initiates the processes of sulfate reduction in them. Temperature is the most important factor of the formation of the geochemical characteristics of inversion waters. The most favorable temperature range is 75-100degreesC, when sulfate sulfur is transformed into sulfide forms. This displaces the hydrogeochemical systems of inversion waters into the region of low Eh and high pH, where high Ca2+ concentrations cannot be preserved. At higher temperatures, hydrocarbons are formed, and HCO3-Cl-Na and Cl-HCO3-Na waters are transformed into sulfate-free and carbonate-free Cl-Na waters. In addition to lowering Eh, an increase in temperature above 100degreesC results in a decrease in pH, the geochemical effect of which is manifested in elevated calcium concentration in the aqueous phase and corresponding transformation of Cl-Na waters into Cl-Na-Ca brines with the predominance of CH4 and H-2 in the gas composition. Consequently, two main directions of direct transformation are possible during the formation of the geochemical zoning of ground water in deep-seated hydrogeologic structures, calcic (at the initial relationships in the water 2m(Ca2+) > m(HCO3-) + 2m(CO32-)) and sodium carbonate (at the initial relationships in the water m(HCO3-) + 2m(CO32-) > 2m(Ca2+)).