SECULAR VARIATION IN SEAWATER CHEMISTRY AND THE ORIGIN AND SIGNIFICANCE OF CALCIUM CHLORIDE BASINAL BRINES

The origin of hypersaline CaCl2-rich basinal brines (“oilfield brines”) stored in sedimentary basins has significance for understanding the origin and distribution in time and space of hydrothermal dolomite, metalliferous ore deposits and the world's major oil and gas resources. We propose that most of these CaCl2 basinal brines have inherited their chemistries and salinities from evaporated paleoseawaters formed during geologic periods when the oceans were Ca-rich and SO4-poor (“CaCl2 seas”/“--calcite seas”). These were times of fast seafloor spreading and high influxes of CaCl2-rich MOR brine into the ocean and high stands of sea level, conditions that favored the accumulation of primary CaCl2 marine evaporites in marginal and interior continental basins. Migration of these CaCl2-rich marine evaporite brines during burial was responsible for hydrothermal dolomitization of limestones and deposition of metalliferous ores such as hydrothermal Pb-Zn deposits. The connection between marine evaporite deposits and oil and gas resources lies in the fact that evaporite sedimentary environments are notable for their production and preservation of masses of phytoplankton and bacterioplankton from which type I and II kerogen, and ultimately oil and gas, are formed during burial diagenesis. In particular, CaCl2-rich marine evaporite brines have the highest potential of all such brines to produce and preserve kerogen because the paucity of sulfate ions in these brines discourages the destruction of sedimented organic matter by sulfate-reducing bacteria.