Chemistry of Interstitial Solutions in Shales Versus that in Associated Sandstones

Abstract

Abstract Experimental results obtained by the writers show that the salinity of squeezed-out solutions from muds progressively changes with increasing overburden pressure. Both marine muds and clays plus sea water mixtures were used. Experimental results also show that the salinity of water remaining in shales should be considerably lower than that in associated sandstones. Yet, log analysts in their calculations assume that salinity of interstitial waters in shales and in sands is the same. This may result in erroneous interpretations. Field data show a typical pattern of decreasing salinity of produced water with time in zones which receive most of their water from associated shales. This supports the above-mentioned conclusion based on laboratory data. Introduction Large amounts of water are squeezed out of the continental and marine sediments during compaction and lithification. The overburden pressures on these sediments may reach pressures on these sediments may reach magnitudes of 14,000 to 36,000 psi in geosynclinal basins. Most of the salts present in the waters, which are trapped during sedimentation,, are squeezed out during the initial stages of compaction. The laboratory results obtained by Buneeva et al. Kryukov and Komarova, Kryukov and Zhuchkova, Rieke et al. and Chilingarian and Rieke showed that mineralization of squeezed-out solutions progressively decreases with increasing overburden pressure. Very little is known of the actual porosity of shale and the actual salinity of the contained water. It seems that if one could be determined the other probably could be calculated. The actual salinity of formation waters of the sands usually ranges from three times the salinity of sea water to one-fourth times the salinity of sea water [average salinity of sea water = 35 degrees/oo], and may range from greater than 10 times salinity of sea water to virtually fresh water. Recently, Foster and Whalen presented a technique for estimating formation pressures from electrical logs. This technique has been found especially applicable in the offshore Louisiana area in detecting abnormally high reservoir pressures. These authors, however, assumed that pressures. These authors, however, assumed that the water resistivity values in shales are equal to, or of the same magnitude, as that calculated for nearby sands. Consequently, solution Of this problem [chemistry of interstitial waters in shales vs chemistry of waters in associated sandstones] is of great practical importance.