HANOR, JEFFREY S., Louisiana State University, Baton Rouge, LA

ABSTRACT: Low Temperature Buffering of Pore Water Compositions in Sedimentary Basins: Implications for Sediment Diagenesis

The salinity of pore waters in sedimentary basins worldwide varies by approximately five orders of magnitude. Although the concentrations of some solutes range by a factor of ten or more at a given salinity, both the ranges in absolute concentrations and the patterns of increase in dissolved Na, K, Mg, and Ca and decrease in pH and carbonate alkalinity with increasing salinity support the hypothesis that the approach toward thermodynamic buffering by silicate - carbonate +/- (halide) mineral assemblages is a first-order control on subsurface fluid compositions, even at temperatures well below 100 degrees C. The increasing dominance of dissolved Ca with increasing salinity, for example, can be explained simply by the stoichiometry of mineral-fluid hydrolysis and the thermodynamic non-i eality of brines. The chemical potential of chloride, or alternatively, the aqueous concentration of anionic charge, is a master variable which ranks in importance with such other variables as pressure, temperature, and CO2 fugacity in driving diagenesis and controlling bulk fluid compositions. This variable iscontrolled largely by physical processes of fluid convection and dispersion. The systematics in bulk fluid compositions support the notion that water-rock interaction in sedimentary basins can be a process in which the transfer of many, perhaps most, thermodynamic components is effected essentially simultaneously, not sequentially. This model for the controls on solute composition provides a basis for the quantitative determination of fluid-rock mass transfer during fluid flow and iagenesis.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.