Mechanisms of sulfate removal from subsurface calcium chloride brines: Heletz-Kokhav oilfields, Israel

Abstract

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 composition. Particular emphasis was given to the brine-oil interaction in the oilfields and to the sulfate depletion and lower SO4/Cl ratio in brines in contact with hydrocarbons (oil brines) relative to “oil-free” from dry wells in the same oilfields.A method is presented for a calculation 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 different stages of its evolution, from early diagenetic processes to the contact with crude oil. In the present study, based on the δ34SSO4 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 their interaction with the hydrocarbons and only a negligible fraction of few percent of the sulfate was removed during the crude oil-water contact.The Ca-chloride brines evolved from Messinian (Upper Miocene) seawater that underwent evaporation during the desiccation of the Mediterranean. Sulfate was removed from Messinian lagoon (s) during gypsum precipitation due to evaporation and dolomitization. Bacterial sulfate reduction further depleted the brine in sulfate and changed its isotopic composition, from its original Miocene seawater composition of δ34SSO4 ∼ 20%o, 26%o. Overall, some 50% of the original sulfate, as normalized to chloride, was removed from the original lagoon through the above processes, mostly by gypsum precipitation. Eastward migration of the Messinian Ca-Chloride brine into the Heletz Formation was accompanied by dolomitization of the country rock.Final depletion of sulfate from the brines took 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 δ34SSO4, which reaches a maximum value of 59%o. The above correlation is explained by bacterial sulfate reduction facilitated by the contact with the crude. Samples collected from the same boreholes at time intervals of several months show two opposing trends: sulfate concentration decrease accompanied by increase in δ34SSO4, and vice versa. While the first can be explained as in situ bacterial sulfate reduction, the latter attest to subsurface brine migration, as would be expected in oil-producing fields.