Approximately twenty percent by volume of most sedimentary basins is pore water. Most of these pore waters are hot, salty, and under high pressure (see figures). Their chemical composition is unlike the composition of most waters formed in earth surface environments.
Interest in the origin and migration of subsurface sedimentary brines has been stimulated by the role these fluids play in a wide range of fundamentally important geological processes. For example, as complex mixtures of sedimentary materials and organic matter attempt to achieve chemical equilibrium during burial diagenesis, pore fluids permit the transfer of material from one phase to another and themselves evolve in chemical composition. Because pore fluids are mobile, they can migrate through the earth's crust, carrying with them diagenetically reactive components, heat, hydrocarbons, salts, and metals. The presence of pore fluid under pressure facilitates or may even induce tectonic deformation of sedimentary basins and continental margins. Vast quantities of hazardous wastes are injected into the subsurface each year, often with little understanding of how they may react with native pore fluids and sediment and where they will migrate. Massive salt in some hydrologically active sedimentary basins has been considered as a possible repository for the storage of high-level radioactive wastes. Is this salt actively dissolving?
It is of obvious importance to know how fluids in sedimentary basins originate, what processes determine their chemical composition, and what factors determine pathways and mechanisms of migration and solute transport.The detailed answers to these questions are far from certain, even
Figures & Tables
Origin and Migration of Subsurface Sedimentary Brines
The topics covered in these notes have been selected to provide the general sedimentary geologist with an introduction to some of the key problems and a conversational familiarity with some of the basic techniques in this important area of sedimentary geology. A number of field examples are drawn from the Louisiana Gulf Coast, buy many of the general principles will be applicable to other areas and problems.