Fluid Flow and Diagenesis of Fractures in the Front of the Zagros Fold Belt: Timing Constraints of Deformation

Taberner, C.¹, J. Vergés¹, P. Gillespie², I. Sharp², M. Rejas¹, R. Karpuz³, D. Morsalnejhad⁴ (1) Institute of Earth Sciences, CSIC, Barcelona, Spain (2) Norsk Hydro Research Centre, Bergen, Norway (3) Norsk Hydro Saga Petroleum, Tehran, Iran (4) National Iranian Oil Company

Fractures and faults at the front of the Zagros Fold Belt were conduits for fluid flow as recorded in fracture-fill cements (calcite and dolomite). Dolostone bodies in Cenomanian­Turonian carbonate ramps display geometries and relations to faults supporting their role for the circulation of dolomitizing fluids. Paragenesis and cement stratigraphy of fracture fills, as well as cements from the dolomitized and non-dolomitized rocks provide constraints on early to burial diagenesis stages, on the role of fractures as conduits for fluid circulation and on the types of parental fluids and their relation to hydrocarbon charge or hydrocarbon seepage. Timing and depth of deformation is constrained from the diagenetic and geochem­ical evolution of fracture fill cements and dolostone bodies. Some of the normal faults that appear to be related to the evolution of anticlines are proven to have existed, at least as frac­tures, at burial stages. Deep-seated hydrothermal fluids have circulated along these frac­tures at calculated burial depths of 2000 m. At burial stages these fractures played a signif­icant role in the dolomitization and generation of secondary porosity in Cenomanian-Turonian ramp deposits prior to hydrocarbon charge. At the Late Cenozoic, during anticline formation and uplift of the frontal zone of the Zagros Fold Belt, these fractures were reacti­vated as normal faults. Hydrothermal circulation at this late stage had major implications for hydrocarbon seepage from the reservoirs.