EXPERIMENTAL STUDIES OF SILICATE-CARBONATE REACTIONS .1. APPLICATIONSTO DIAGENESIS

The formation of clays and CO2 from silicate-carbonate reactions and t he effect of these products on reservoir quality and fluid chemistry h ave been well recognized in the study of sediment diagenesis. The phas e relationships for the reaction of kaolinite + dolomite + quartz to c alcite + chlorite-smectite + CO2 reactions were chemographically exami ned and experimentally evaluated. The phase boundary of this reaction on a T-X(CO2) diagram was interpolated between high temperature experi mental data and low temperature field data. The results can be applied to predict the amounts, type and timing of clay and CO2 formation dur ing sediment diagenesis. The variation of pore fluid composition (e.g. Ca2+ and Mg2+) caused by these reactions was also evaluated using a g eochemical modeling program. The major conclusions and implications ar e the following: (1) the concentration of CO2 in the reaction fluid co ntrols the reaction temperature as well as the clay type (chlorite vs smectite) formed. The equilibrium boundary provides a guideline for es timating the CO2 contents, the sealing integrity and temperatures of a diagenetic system based on the observed mineral assemblages; (2) the silicate-carbonate reactions can occur in diagenetic environments that are open to CO2, and can therefore, be an important inorganic source of CO2 which can lead to the formation of CO2-rich fields and secondar y porosity. The diagenetic reactions, however, are significantly suppr essed in a closed system (e.g. overpressure zone) and therefore cannot significantly contribute to overpressure formation nor generate enoug h pressure to create fractures; and (3) the formation of smectite or c hlorite which can serve as a sink for Mg2+ and to a lesser extent, for Ca2+. The pore fluid released from these reactions during shale diage nesis, in general, contains much less Mg2+ than Ca2+. The reactions in shales are likely sources of Ca2+ for carbonate cements but not of Mg 2+ for dolomitization.