PHYSICAL AND CHEMICAL CONTROLS ON THE COMPOSITION OF WATERS IN SEDIMENTARY BASINS

The composition of subsurface waters is determined not only by diagene tic reactions but also by physical processes of fluid convection and h ydrodynamic dispersion. The south-west Louisiana Gulf Coast provides a n instructive field example of the net effects of diagenetic reaction and solute transport on pore water compositions in a regional silicicl astic sequence. Most formation waters here have compositions totally u nlike the compositions of the connate meteoric and marine fluids that were buried with their host sediments at the time of deposition. Disso lved chloride has been generated by the subsurface dissolution of salt domes and has been pervasively transported by fluid flow throughout m ost of the upper 3 km of the sedimentary section. The simultaneous sys tematic variation in dissolved Na, K, Mg, Ca and alkalinity with chlor ide in these waters supports the hypothesis that metastable thermodyna mic buffering by silicate-carbonate mineral assemblages is a first-ord er control on fluid compositions. The chemical potential of chloride, or alternatively, total anionic charge, appears to be a master variabl e which ranks in importance with such other variables as pressure and temperature in driving diagenesis in this region. This variable is con trolled largely by physical processes of advection and dispersion in t he upper 3 km of the section and by dehydration reactions in deeper, m udstone-dominated sediments. Where the composition of the fluid is lar gely rock-buffered, the ultimate origin of the fluid and its pathway o f chemical evolution may be obscured, at least in terms of major solut e composition. Non-buffered components, such as Cl and Br, or isotopic compositions are more likely to retain information on original end-me mber fluid compositions and reaction pathways