Origin of Ca [sbnd]Cl brines in Devonian formations, western Canada sedimentary basin

Abstract

The majority of waters in Devonian formations of the western Canada sedimentary basin are high salinity Ca Cl brines. This relatively rare type of brine is of economic interest because of its association with both oil and hydrothermal ore deposits. These brines are important sources of Ca 2+, Mg 2+ and ore metals in sedimentary basins. It is possible to generate the Ca Cl brines by modification of residual evaporite brines. The evaporation of Devonian sea water produced large quantities of gypsum and halite such as the Prairie Formation. Residual brines were not retained in the evaporating system. Quantities of dense, halite-saturated brine, sufficient to account for the high salinity formation waters in the basin, were lost syndepositionally. The Cl − and Br − content of waters within Devonian formations in the basin can be explained by dilution of the residual evaporite brines. Brine composition was modified as a result of density driven reflux to depths of several kilometers. This placed concentrated brines in contact with Precambrian basement rocks at elevated temperatures (100–300°C). The most important reactions between the brine and basement rocks appear to involve albitization of feldspars. Further modification of the brines occured as cements formed in Devonian carbonates. Volumetrically, the most important of these is dolomite, in addition some Devonian carbonates were dolomitized. During ascent along faults Mg 2+ was lost from hot brines as dolomite formed.