Analyses of oil field brines from the Pleasant Bayou in the upper Texas Gulf Coast and the Rayleigh Field in central Mississippi were used to calculate the pH of the brines under reservoir conditions by assuming equilibrium with the reservoir mineral assemblages. The calculated pH values are less than 5.7 for the Pleasant Bayou brines at 138 degrees C and 787 bars and 4.3 (+ or -0.3) for the Rayleigh Field brines at 130 degrees C and 390 bars. Typical water-rock interactions have been investigated by theoretical evaluation of the thermodynamic consequences of the reactions of a hypothetical low-pH oilfield brine with quartz sandstone and dolostone aquifers that contain about 3 percent K-feldspar, 0.2 percent muscovite, and 0.01 percent pyrite by volume. The correlation of lead-rich (Southeast Missouri and Laisvall, Sweden) and zinc-rich (upper Mississippi Valley and East Tennessee) deposits with sandstone and dolostone aquifers, respectively, suggests that the reaction capacity of sandstone is more efficiently exhausted than that of dolostone. Importance in explaining geochemical characteristics of base-metal sulfide ore deposits. --Modified journal abstract.