Physical and chemical controls on the composition of waters in sedimentary basins

Abstract

The composition of subsurface waters is determined not only by diagenetic reactions but also by physical processes of fluid convection and hydrodynamic dispersion. The south-west Louisiana Gulf Coast provides an instructive field example of the net effects of diagenetic reaction and solute transport on pore water compositions in a regional siliciclastic sequence. Most formation waters here have compositions totally unlike the compositions of the connate meteoric and marine fluids that were buried with their host sediments at the time of deposition. Dissolved chloride has been generated by the subsurface dissolution of salt domes and has been pervasively transported by fluid flow throughout most of the upper 3 km of the sedimentary section. The simultaneous systematic variation in dissolved Na, K, Mg, Ca and alkalinity with chloride in these waters supports the hypothesis that metastable thermodynamic buffering by silicate-carbonate mineral assemblages is a first-order control on fluid compositions. The chemical potential of chloride, or alternatively, total anionic charge, appears to be a master variable which ranks in importance with such other variables as pressure and temperature in driving diagenesis in this region. This variable is controlled largely by physical processes of advection and dispersion in the upper 3 km of the section and by dehydration reactions in deeper, mudstone-dominated sediments. Where the composition of the fluid is largely rock-buffered, the ultimate origin of the fluid and its pathway of chemical evolution may be obscured, at least in terms of major solute composition. Non-buffered components, such as Cl and Br, or isotopic compositions are more likely to retain information on original end-member fluid compositions and reaction pathways.