oa Fracture-related diagenesis in the carbonate carapace of a salt dome, Jebel Madar, Oman

This study, carried out in the Qatar Carbonates and Carbon Storage Research Centre (QCCSRC) at Imperial College focuses on the interplay between fractures, diagenetic fluid flow and precipitation of diagenetic minerals in an exposed carbonate carapace of a salt dome in Oman. Understanding fault-related mineralization and the differences between diagenesis in the fractures and fracture walls compared to diagenesis in the rock matrix will help prediction of the reservoir quality of such fractured carapaces in the subsurface. The research questions addressed here are a) what controls the timing, distribution and geometry of fractures in the carapace of a salt dome?, and b) what controls the timing of the precipitation of minerals in the fracture network? To answer these questions, a dual approach combining structural geology with carbonate diagenesis is being applied. The origin of the fluids and their role in fracturing and diagenesis is being investigated by means of geochemistry and petrography and the processes of fracturing, fluid flow and the migration of fluids along pathways will be determined by reconstructing the structural history by a combination of field mapping and seismic interpretation. This dual approach is powerful and can be used to determine the history of fluid flow. Ultimately, the aim is to develop conceptual models linking fracturation and fluid flow during halokinesis with diagenesis.

Jebel Madar is situated in the southern foothills of the Oman Mountains and is a folded and fractured salt dome carapace comprising Jurassic and Cretaceous limestones. The structural history of the Jebel is currently being reconstructed using fracture analysis of structural data gathered during fieldwork. This will be augmented with the interpretation of seismic profiles. Initial results indicate that the dominant fracture orientations are ∼ N-S and NE-SW. Using the cross-cutting relationships of the fractures an attempt is being made to establish their chronology.

To understand the diagenetic history of the fracture infill, several techniques are being used including petrography, major and trace elemental analysis, fluid inclusion, and stable oxygen and carbon isotope analysis. An important observation made in the fractures of the Jebel Madar is that several generations of calcite cements exist, with crystals ranging in size from several centimetres in the centre of the fracture to compact, millimetre-sized crystals close to the host rock. In addition, in some fractures barite and calcite have been co-precipitated, an observation which, with the aid of fluid inclusion studies, could yield the composition and possible origin of the parent diagenetic fluids.

The presence of minerals in fractures indicates that these latter acted as fluid pathways. Initial stable isotope results suggest that the precipitating fluids were hot, with average values of δ ¹⁸O of –9.74 ‰ PDB and δ ¹³C of 1.19 ‰ PDB. Initial results indicate that the regional stress stimulated the local development of salt diapirs which produced local stress fields that totally dominated the timing and pattern of fracturing and fluid migration.