Dissolution experiment and transformation condition analysis of Paleogene aragonite in the Jiyang Depression, China

In general, aragonite exists as a metastable carbonate mineral under near-surface conditions, and is commonly transformed into calcite under the subsurface and during diagenesis. It is thus seldom found in sedimentary rocks, but aragonite is common in the Paleogene lacustrine shales in the Jiyang Depression in eastern China. Dissolution experiments were conducted on the Paleogene aragonite-enriched and calcite-enriched shales at different temperatures, pressures and acetic acid concentrations, and in different types of solution. The results show that aragonite is insoluble in the in situ formation water but dissolved more readily under acetic acid conditions than calcite with the degree of dissolution increasing with increasing temperature, pressure and acetic acid concentrations. During the shallow burial diagenesis of the Paleogene sediment sequence in the Jiyang Depression, aragonite was relatively stable and was not dissolved by the connate pore water in the shales. Increasing burial (temperature) and maturity of the organic matter produced large amounts of organic acids that accelerated the dissolution of aragonite. In the late stage, as the organic matter became over-matured, the pore water changed from acidic to alkaline, and calcite precipitated from the carbonate-rich solution. Therefore, the conditions provided by organic acids enabled the conversion of aragonite to calcite during sedimentary diagenesis in the Paleogene lacustrine shales in the Jiyang Depression. This transformation corresponded to the thermal evolution of the organic matter within the shale sequence.