Reconstruction of in situ composition of sedimentary formation waters

Abstract

Chemical equilibrium calculations on sedimentary formation waters show that the waters, as analyzed, cannot be in equilibrium with diagenetic minerals in their host rocks at the formation temperature. However, if alkalinity is corrected to account for organic acid anions, and if the pH and bicarbonate are corrected for CO 2 loss from the sample, chemical equilibrium between formation waters and host rock diagenetic minerals can be clearly shown for systems in the temperature range of 75 to 160°C. Compositional reconstruction of some formation waters from published analyses is complicated by lack of analytical data for aluminum, silica, and organic acid anions. Missing aluminum and silica can be estimated by assuming equilibrium with an aluminum silicate (K-feldspar, muscovite) and quartz or chalcedony. pH, CO 2, and organic acid anions can be reconstructed by fixing CO 2 to exactly saturate calcite at the formation temperature because the fast kinetics of calcite precipitation makes it almost certain that calcite saturation is more likely than the strong supersaturation that is otherwise observed. Results from the equilibrium calculations are evaluated by using graphs of the saturation states of diagenetic minerals vs. temperature, for each of many sedimentary brines. If the diagenetic minerals selected as diagnostic of equilibrium (from qz, chalcedony, mus, paragonite, k-sp, alb, kaol, ca, and dol) are not saturated at or near a single temperature, the missing or erroneous quantities of components are adjusted to obtain agreement in the saturation temperature. Composition data for fluids from four locations are used in the calculations: Kettleman North Dome, California, offshore Norway, the Texas Gulf Coast, and offshore Texas. The calculations suggest that in most cases, control of silica concentration shifts from chalcedony to quartz with increasing temperature near 100°C. In some fluids, silica concentration may approach chalcedony saturation to temperatures exceeding 150°C, where there are shale units containing smectite undergoing the smectite to illite reaction. Deviation in silica activity from equilibrium with chalcedony or quartz is small for most of the fluids, and may result from precipitation of silica as polymers or amorphous solids upon cooling, and either removal of precipitates upon filtering before analysis, or nonreactivity of the precipitates in the analytical method used. Four fluids containing significant iron and having apparently degassed significant CO 2 also show substantial apparent silica loss, and therefore, silica loss most probably results from the precipitation of amorphous Fe-silicate caused by pH increase due to degassing, and by cooling. The methods used here can be applied as a geothermometer to predict formation temperatures, and, when applied to Kettleman North Dome, yield a thermal gradient of 37.1°C/km. Formation temperature data for the Texas waters are in agreement with equilibrium temperatures predicted by the calculations.