Diagenesis and fluid system evolution in the northern Oman Mountains, United Arab Emirates: Implications for petroleum exploration

Abstract

The diagenesis and fluid system evolution of outcrop analogues of potential sub-thrust Cretaceous carbonate reservoirs in the Musandam Peninsula, northern United Arab Emirates, is reconstructed during the successive stages of the Oman Mountains development. Detailed petrographic and geochemical analyses were carried out on fracture cements in limestones and dolomites mostly situated close to the main faults, which were the locations of major fluid fluxes. The main result of this study is a generalised paragenesis subdivided into four diagenetic time periods. Based on analyses of syn-tectonic veins and dolomites a large-scale fluid system is inferred with migration of hot brines with H2O-NaCl-CaCl2 composition along Cenozoic reverse faults. These brines were sourced from deeper formations or even from the basal decollement and infiltrated in the footwall. These results are compared with similar studies, which were carried out in other regions worldwide. Furthermore some implications for reservoir characteristics and hydrocarbon scenarios could be postulated. It must be noted that the majority of the analysed rocks do not have sufficiently high porosities to be regarded as reservoir rocks. However, some diagenetic processes that can improve the reservoir quality were observed. For example dolomite recrystallisation occurred in patches at the carbonate platform border, which created poorly connected reservoirs. Other possible exploration targets could be the footwall blocks of the Cenozoic reverse fault zones. When the migration of hot brines along these faults and into the footwall would be combined with petroleum migration, the footwall block could act as a potential hycrocarbon trap sealed by the fault. The fluid system evolution is incorporated in a schematic model of the geodynamic framework of the region in order to summarise the different diagenetic and fluid events, which took place during the northern Oman Mountains evolution up to now.