Reactive Transport Modeling of Diagenetic Reactions in Carbonate Bedrocks: Carbonate Corrosion and/or Dolomitization

Salas, Joaquin¹, Conxita Taberner¹, Mateu Esteban², Carlos Ayora¹ (1) Institute of Earth Sciences “Jaume Almera” (CSIC), Barcelona, Spain (2) Carbonates International Iberia, S.L, Caimari, Mallorca, Spain

Major corrosion and dolomitization were detected to occur at depth, in carbonate reser­voirs from different geologic provinces. This pattern, commonly associated with fractures zones, is coupled with generation of secondary porosity and precipitation of post-corrosion mineral paragenesis (fluorite, quartz and kaolinite-dickite). It may even destroy the newly­formed porosity.

In order to improve our understanding of observations, the available information on the lithology and groundwater flow was integrated in reactive transport models. Different hydro­geochemical scenarios were considered and evaluated: cooling of brines along fracture zones and mixing of different salinity solutions. Chemistry of the modeled brines was based on microthermometry of primary fluid inclusions, equilibrium with respect to different min­erals (quartz, K-feldspar, muscovite and calcite) and chemistry of basinal brines from oil fields, described by different authors.

Cooling of brines (from 135 to 80ºC) was modeled considering on different flow rates. Mixing of different salinity brines was also tested at different temperatures. Through the variability of the parameters imposed in the models (fluid flow rates, temperatures, salinity, pH and Ca/Mg ratio of the brines), results show as different petrographical scenarios may be reproduced. Cooling of low salinity solutions is not a very effective mechanism to dolomi­tize and to generate significant volumes of porosity and secondary minerals. For high salin­ity solutions, the flow rate of the fluids ascending along the fracture and the Mg/Ca ratio are the main control to develop dolomitization. On the contrary, mixing of two brines with dif­ferent salinities generates significant volumes of dolomitized rock-bodies and post-corro-sion mineral paragenesis.