Effects of Salt Dome Dissolution on Sediment Diagenesis: An Experimental and Field Study.

Abstract

The introduction of high-salinity or high-ionic strength fluids into sedimentary formations has been hypothesized to induce diagenetic reactions. Such reactions are potentially important because they alter porosity and affect the migration of fluids such as petroleum, or hazardous liquid wastes disposed of by deep-well injection. An experimental and field investigation of the controls on diagenesis in high and variable salinity environments was undertaken regarding Gulf Coast salt domes because ambient formation waters and sediments around salt domes are influenced by the dissolution of evaporites. Laboratory experiments were performed with carbonate-free and carbonate-bearing siliciclastic sediments in aqueous NaCl solutions with concentrations from 1 mg/L to halite saturation (ca. 350,000 mg/L). The experiments were performed at $\rm 25\sp\circ C$ and $\rm 90\sp\circ C$ at approximately 1 bar in runs up to 90 and 270 days. In the reacted fluids, pH and alkalinity decreased with increasing salinity, and the concentrations of Ca, Mg, and Sr increased with increasing salinity. K increased with salinity only in the carbonate-free experiments. Cation exchange reactions between dissolved Na and adsorbed species on mineral surfaces account for much of the Ca, Mg, and Sr released to solution, however, the presence of dissolved Si and K after reaction is attributable to hydrolysis reactions. X-ray diffraction data show the development of Na-rectorite in the high-salinity carbonate-free experiments and also suggest that quartz is increasingly attacked at high salinities. Sediments were examined from the Iberia salt dome, south Louisiana, and the Eugene Island 128 salt dome, offshore Louisiana. Diagenetic pyrite, calcite cements, and analcime are present in flank sediments of both. At EI-128, barite pseudomorphs after anhydrite develop where halite is dissolved. Evidence was found at both domes for sediment and grain fracturing which can directly influence fluid transmission, solute transport, and mineral dissolution in flank sediments. At the Iberia dome, spatial variations in pore-fluid composition suggest that fluids are transmitted through faults and fractures as well as through the intergranular porosity of detrital sediments. Thus, fracturing, fluid movements, and redox reactions influence diagenesis in addition to evaporite dissolution.