MECHANISMS OF SULFATE REMOVAL FROM SUBSURFACE CALCIUM-CHLORIDE BRINES- HELETZ-KOKHAV OILFIELDS, ISRAEL

The evolution of the Ca-chloride brines in the Heletz Formation, Lower Cretaceous, in the southern coastal plain of Israel was reconstructed through the study of its sulfate concentration and isotopic compositi on. Particular emphasis was given to the brine-oil interaction in the oilfields and to the sulfate depletion and lower SO4/Cl ratio in brine s in contact with hydrocarbons (oil brines) relative to ''oil-free'' f rom dry wells in the same oilfields. A method is presented for a calcu lation of the amount of sulfate removed from the original seawater in the various stages of its evolution to Ca-chloride brine. These stages include evaporation, dolomitization, and sulfate reduction in differe nt stages of its evolution, from early diagenetic processes to the con tact with crude oil. In the present study, based on the delta(34)S(SO4 ) and SO4/Cl ratio, it was found that in the Heletz brines most of the sulfate (80-94%) was removed from the original seawater prior to thei r interaction with the hydrocarbons and only a negligible fraction of few percent of the sulfate was removed during the crude oil-water cont act. The Ca-chloride brines evolved from Messinian (Upper Miocene) sea water that underwent evaporation during the desiccation of the Mediter ranean. Sulfate was removed from Messinian lagoon(s) during gypsum pre cipitation due to evaporation and dolomitization. Bacterial sulfate re duction further depleted the brine in sulfate and changed its isotopic composition, from its original Miocene seawater composition of delta( 34)S(SO4)approximate to 20 parts per thousand, to 26 parts per thousan d. Overall, some 50% of the original sulfate, as normalized to chlorid e, was removed from the original lagoon through the above processes, m ostly by gypsum precipitation. Eastward migration of the Messinian Ca- Chloride brine into the Heletz Formation was accompanied by dolomitiza tion of the country rock. Final depletion of sulfate from the brines t ook place, and possibly still occurs, in the presence of crude oil in the oilfields. The two oil-producing fields, Heletz and Kokhav, occupy different areas on a Rayleigh distillation diagram. Sulfate depletion in both fields is accompanied by an increase in delta(34)S(SO4), whic h reaches a maximum value of 59 parts per thousand. The above correlat ion is explained by bacterial sulfate reduction facilitated by the con tact with the crude. Samples collected from the same boreholes at time intervals of several months show two opposing trends: sulfate concent ration decrease accompanied by increase in delta(34)S(SO4), and vice v ersa. While the first can be explained as in situ bacterial sulfate re duction, the latter attest to subsurface brine migration, as would be expected in oil-producing fields.