Diagenetic Modeling of Fault-Related Hydrothermal Dolomitization: Constraints for the Prediction of Porosity Distribution

Taberner, C.¹, J. Salas¹, I. Sharp², P. Gillespie², M. Rejas¹, J. Vergés¹, R. Karpuz³, S. Schouten⁴, D. Morsalnejhad⁵ (1) Institute of Earth Sciences, CSIC, Barcelona, Spain;.(2) Norsk Hydro Research Centre, Bergen, Norway (3) Norsk Hydro Saga Petroleum, Tehran, Iran (4) Netherlands Institute for Sea Research, 1790 AB Den Burg, Netherlands;.(5) National Iranian Oil Company

    Diagenetic modelling of Cenomanian-Turonian carbonate ramps outcropping in the Anaran anticline (Zagros Fold and Thrust Belt) provides constraints for 1) evaluating the mechanisms and type of fluids causing their dolomitization, as well as 2) predicting the dis­tribution of porosity.

The geometry of the dolomitized bodies and their relationships to faults suggest that dolomitization was controlled by the inflow of deep-seated fluids along fractures during bur­ial. Regional temperatures at which dolomitization occurred have been constrained from molecular biomarker ratios and fluid inclusion microthermometry. Minimum temperatures at which deep-seated fluids flowed along fractures have been constrained from fluid inclu­sion microthermometry in dolomite and late calcite cements. The minimum difference between regional temperatures and deep-seated fluids was around 45ºC. Salinities and chemistries of formation waters and deep-seated fluids were constrained from fluid inclu­sions in dolomite and late calcite crystals, as well as from information on basinal brines available in the literature.

Two different scenarios were considered for fluid flow and reactive transport modelling of hydrothermal dolomitization: cooling of fluids ascending along fractures and mixing of deep-seated fluids with formation brines. Results from both modelled scenarios reveal sig­nificant differences in the volumes of dissolved calcite, dolomitized rock, dolomite and late calcite precipitated in either primary / early porosity or in late porosity resulting from dolomitization, as well as in the distribution of net porosity. Salinity, Mg/Ca ratios and flow rates of deep-seated fluids ascending along fractures appear as major constraints on net porosity volumes and porosity distribution.