Origin And Chemical Evolution Of Brines In Sedimentary Basins

Abstract

Abstract A brine may be defined as a solution containing more than 100,000 mg/l dissolved solids. Brines in sedimentary basins are genetically related to evaporites in one of three ways:dissolution of evaporite minerals generally halite;expulsion of interstitial fluids from evaporites; andalteration of hydrous evaporite minerals such as carnallite. Most Na-Ca-Cl brines originate as interstitial fluids in halite-rich rocks. These fluids initially are rich in magnesium, sulfate, and potassium but evolve to a Na-Ca-Cl brine during migration to their present host rock. The principal reactions involved in present host rock. The principal reactions involved in the chemical evolution of these brines are dolomitization, sulfate reduction, albitization of plagioclase, and the formation of authigenic potassium aluminosilicates. Introduction Occurrences of brines in sedimentary basins containing intervals of bedded halite are known in a number of areas around the world. In many of these locations, certain stratigraphic intervals are important producers of hydrocarbons and, in some cases, contain producers of hydrocarbons and, in some cases, contain major base-metal deposits. The purpose of this investigation has been to develop a model for predicting the chemical composition of brines produced by the evaporation of sea water and a model for quantitatively predicting the changes in the composition of these brines predicting the changes in the composition of these brines as they migrate into sedimentary rocks and react with a variety of common minerals. If the brines and their associated hydrocarbons in oilfield reservoirs have migrated to their present position along the same path, the ability to determine the point of origin and the migration path of a brine from its chemical characteristics could be a valuable tool in petroleum exploration. CLASSIFICATION AND ORIGIN OF BRINES Carpenter has recommended the adoption of the water classification system shown in Table 1. This system has the advantages that it is geometric in progression, easy to remember, and coincides with lay progression, easy to remember, and coincides with lay terminology for saline water. The term brine, as defined in Table 1, has definite genetic implications. Natural aqueous solutions containing more than 100,000 mg/1 appear to originate either as a result of the crystallization of siliceous magma or as a result of some process involving evaporites (Table 2). Fluid inclusion studies of brines associated with porphyry copper deposits indicate that brines porphyry copper deposits indicate that brines produced by magmatic processes are Na-K-Cl brines. produced by magmatic processes are Na-K-Cl brines. The association of evaporites and brines in sedimentary basins has been recognized for a long time. Brines produced by the dissolution of highly soluble minerals such as halite have distinct compositions which can be used to determine the mineral or minerals which have been dissolved. Brines produced by the dissolution of halite consist almost entirely of sodium and chloride and have very low concentrations of bromide and calcium. The incongruent alteration of hydrous evaporite minerals has been described by Braitsch. These brines are usually very high in calcium and/or magnesium and relatively low in sodium. Their occurrence is generally restricted to the immediate vicinity of evaporite deposits containing potash salts. Sodium-calcium-chloride brines appear potash salts. Sodium-calcium-chloride brines appear to originate as interstitial fluids in evaporites and are then subsequently expelled as a result of the compaction which occurs in response to loading by younger sediments. Landes states that evaporite mineral accumulations usually have initial porosities in excess of 50 percent which is in considerable contrast to the percent which is in considerable contrast to the negligible porosities of ancient evaporites. The modification of subsurface brine composition can occur through a variety of mechanisms. Mangles-dorf et al. have reviewed the effects of gravitational settling of ions, ion movement in a thermal gradient, and thermocell diffusion. They concluded that these mechanisms cannot account for the enrichment of dissolved salts found in brines but may modify compositions produced by other processes. Manheim and Horn have carefully evaluated the difficulties of producing brines by shale membrane filtration. They producing brines by shale membrane filtration. They concluded that the pressure requirements for appreciable salt-filtration remain unsatisfied by known geologic environments. They also noted that a second requirement of a filtration system capable of producing brines is that "enormous volumes of fluids must be pushed through poorly permeable (membrane-active) pushed through poorly permeable (membrane-active) strata in preference to permeable channels under virtually leak-free conditions."