Other
Scientific paper
Aug 1982
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1982gecoa..46.1431g&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 46, Issue 8, pp.1431-1448
Other
6
Scientific paper
Calculations using recently-tabulated values of density and osmotic coefficient for NaCl-H 2 O indicate that overpressuring is more than adequate to overcome chemical osmosis and drive reverse chemical osmosis in sedimentary sequences. The best-demonstrated overpressuring mechanism is the rapid deposition of fine-grained sediments. The dehydration of gypsum contributes to overpressuring for brief time intervals at shallow depths, whereas water evolved during the protracted conversion of smectite to illite is probably a subordinate, but continuing contributor to overpressuring at greater depth. Occurrences of overpressuring in sedimentary sections older than Cretaceous indicate that post-depositional mechanisms such as tectonic compression and aquathermal pressuring must also operate. The latter may be of major importance in geothermal areas with adequate low-permeability seals, and a nontrivial contributor in areas of normal geothermal gradient because of shales that sharply decrease normal fluid flow. The strongest arguments for the importance to present-day brine compositions of membrane concentration of sea-water solutes are 1. (1) the correlation of D values of water molecules of pore fluid with those of local meteoric water, 2. (2) the need for major sources of Mg 2+ and Cl - in apparently evaporite-free basins. Even where dissolution of halite is a major contributor of solute, reverse chemical osmosis still operates to leak relatively dilute water. Of the associated diagenetic chemical reactions, that of Mg 2+ with limestone to form dolomite is particularly effective in generating concentrated Cl - brines rich in Ca 2+ . It decreases the concentration of Mg 2+ , increases that of Ca 2+ , and decreases those of both SO 4 2- and CO 3 2- by precipitating CaCO 3 and CaSO 4 because of the Ca 2+ common-ion effect.
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