Characterising and understanding fractures, karstification, and surface fissures related to active rifting: processes and implications for geothermal reservoir development
The Dinantian platform carbonates of the Netherlands formed on palaeohighs including the London-Brabant Massif and Krefeld High, separated by intervening deeper basins. From Visean to Namurian times (Mid to Late Mississippian/Early Pennsylvanian), these platforms are believed to have been emergent, leading to widespread fissuring and karstification. Present day permeability within Dinantian carbonates is a function of primary deposition (linked to facies), the natural fracture network, and secondary, diagenetic alteration that can include karstic processes.
Whilst there are differences in depositional, burial, tectonic and diagenetic histories between the different carbonate highs, fracturing contributes to present-day permeability across all platforms; during karstification, open fractures were used as fluid conduits. Characterising these fractures is important, as they have impacted the development of permeability up to the present day. Understanding the natural fracture network and how this relates to facies and diagenesis should therefore result in better prediction of subsurface fluid flow.
We use several complimentary strategies to predict the key characteristics of fracture systems in the subsurface. Fracture data from carefully chosen analogue outcrops and hydrocarbon fields can be used to derive a conceptual fracture model that describes background fracturing, including genetic links between karst and fractures. Both the choice of analogue and the degree to which it is meaningful to apply any derived insights warrant careful consideration and calibration against known attributes from the target site.
Recent studies of oil fields under development in fractured basement along the Rona Ridge (West of Shetland, UK) have demonstrated that sedimentary infill of open fractures was directly linked to extensional faulting and associated hydrothermal veining during Mesozoic rifting. The open fractures at surface, transition at depth into normal faults. The sediments became entrained in the fluids associated with faulting and were transmitted down hundreds of metres into connected open fracture cavities. These fills also serve to prop the fractures to maintain an open conduit after episodic faulting. A genetic link between faulting at depth and formation of open fractures at surface is supported by analogue modelling of fault zones in carbonates. Since infilled fault-related fractures can closely resemble karstic features at surface, these faulting processes may have direct relevance for the Dinantian platform carbonates on emergent rift shoulders at the edge of the London-Brabant shelf, southern Netherlands.