The relationship between structural evolution and diagenesis: Some thoughts with respect to the Arabian Plate

Although it is widely accepted that carbonate rocks are commonly fractured, the relationship between structural evolution and post-depositional fluid migration remains relatively poorly constrained. Nevertheless, through the combination of detailed field observations, robust petrographical characterisation and geochemical analysis, it is possible to relate cement stratigraphy to fracture evolution and thereby constrain the timing of structuration and fault/fracture controlled fluid migration. Such an approach has been successfully conducted in the Carboniferous of the UK, where a kinematic model that relates structural evolution to post-depositional, fault-controlled fluid flow has been constructed. Similar studies are now underway in a range of locations on the Arabian Plate. Three multi-scale, integrated field and laboratory studies, all initiated in the last year and incorporating state-of-the art imaging technology, will be examined. In the first example, a thin, laterally extensive exposure of Turonian carbonates in Tunisia, diagenetic modification is perceived to have taken place in the shallow subsurface through matrix-controlled fluid circulation. This resulted in patterns of differential dolomitisation, which could potentially lead to heterogeneity in fracture density and spacing during subsequent structuration. In the second case study, in western Sinai (Egypt), pre-rift, deep water carbonates of Eocene age have undergone partial dolomitisation. Vertical to sub-vertical discontinuous dolomite halos form in both the hangingwall and the footwall of major NE-SW and NW-SE trending (syn-rift) normal fault zone that today supports hydrothermal groundwater flow. Away from the fault, stratabound dolomite bodies extend as connected and isolated lenses of one to tens of metres diameter and appear to occur preferentially within higher permeability debris flows and skeletal grainstones. Throughout the study area, fracture density is significantly higher in the dolomitised beds compared to the host limestone. The third study is from middle Cretaceous (Aptian) carbonates of the Shuaiba Formation, North Oman. Deposition took place within a passive margin, with structuration during the subsequent Alpine Orogeny. In the easternmost part of the study area, located closest to the orogenic belt, there is good evidence for leaching and porosity generation along fractures and within the adjacent matrix.Future work will concentrate on relating the detailed paragenetic evolution in each of the study areas with the history of structuration in order to assess the role of basin-scale tectonics on basin- and platform-scale fluid flow. Detailed petrographical observations and geochemical analysis will be used to relate specific dolomitisation, cementation and pore-generating events to the burial history. The key to these studies is a process-based approach that considers the interplay of depositional rock properties, fault propagation, fracturing, fluid source and migration mechanisms and resultant patterns of cementation and porosity distribution. Some of these conceptual models could ideally be tested using reactive transport models in order to assess their robustness and constrain uncertainties. All stand to output valuable geometrical data for input into subsurface reservoir models and to improve our understanding of fracture-matrix interaction during post-depositional fluid flow.