Reconstructing fluid pathways by studying dedolomitization process: example of the Benassal Formation, Maestrat Basin, Spain

Dedolomitization i.e. replacement of dolomite by calcite, is an important fluid-mediated replacement process occurring during carbonate diagenesis in basins. Especially, dedolomitization impact local reservoir rock properties, affecting the reservoir quality and rheology. The process of dedolomitization have been the subject of several studies but still, the controlling mechanism is not fully understood.

We investigate samples from the Maestrat Basin in Spain, formed during the upper Jurassic- lower Cretaceous. Between the Eocene and the Oligocene, the bassin recorded a compressive event and a general surrection responsible of dedolomitization. The detailed investigation of the progressive replacement of the original dolostone to the newly formed rock composed by calcite provide a key example to understand the dedolomitization process. Across the replacement interface, crystallographic orientation (EBSD) of the parent dolomite crystal is preserved in the calcite and no chemical zonation for major elements (EPMA) are visible in both phases, supporting a dissolution-precipitation mechanism. In order to better constrain the chemical evolution of the reaction, quantitative trace elements mapping (fs-LA-ICP-MS) was carried out and coupled to mass balance equations to quantify the elements gained and lost during the reaction. Results show a net gain of mass (~5%) with a loss of heavy Rare Rarth Elements and a gain in light ones. The specific gain in Zn and Rb pinpoints that the infiltrated fluid flowed through MVT deposits already present in the area.