Syn-rift basin fill architecture - constraints from forward stratigraphic modelling

 

PhD Candidate: Eóin O’ Donnell

Supervisor: Prof. Peter Haughton, Dr. Lawrence Amy & Dr. Conrad Childs

Funded by: Science Foundation Ireland and co-funded under the European Regional Development Fund with further funding from the Petroleum Infrastructure Programme and its member companies

Abstract:

Triassic and Upper Jurassic rift-basin plays in the Irish offshore are relatively poorly imaged by seismic datasets and reflector geometry and seismic attributes provide only limited constraints on depositional architecture and potential reservoir distribution. In addition, some basins are only partly preserved due to subsequent uplift and erosion. Fault systems that were active during or immediately prior to deposition are better imaged, better preserved and are being studied in a number of related iCRAG projects. An important question is to what extent knowledge of the fault geometries and the inferred fault system evolution can be used to predict aspects of the stratigraphic geometry and reservoir development that are otherwise poorly resolved? These insights could be key to identifying and risking prospects at syn-rift level in the Irish basins.

In actively-extending areas, there is a strong coupling between tectonic deformation and factors that control sediment transport and accumulation, such as surface gradients, hinterland drainage basin extent and the location of local sources and sinks for sediment. This interplay has been extensively documented in modern extending terrains where basin asymmetry, surface tilting, fault displacement gradients and the pattern of fault linkage have all been related at least in a general way to basin-fill character (e.g. Gawthorpe & Leeder, 2000). The issue is whether these relationships can be tested, more effectively linked to the structural evolution, and better ‘tuned’ to conditions in the Irish rift basins. A challenge is that tectonic deformation is not the only factor influencing depositional response in rift basins, with variable sediment supply, water balance and base level also key issues. Hence, climate and climate cycles also can play a key role in determining overall sedimentary response, particularly within internally-drained basins where water balance is so critical. Forward stratigraphic modelling is an approach that can be used to explore competing controls on deposition and to address the implications for stratigraphy.

This project will couple forward stratigraphic modelling with output from structural studies aimed at characterising fault zone evolution, including results from Discrete Element Method (DEM) modelling of fault systems undertaken in TP4.2PhD5. The structural studies will be used to predict the progressive deformation of the free surface as a fault system evolves during extension. The resulting subsidence and uplift rates and displacement gradients will then be fed to a forward stratigraphic model that examines the coupling between tectonic and other factors during different stages in the extension cycle, up to and including fault death. The Triassic and Jurassic in the Irish offshore provide interesting end-members in terms of climate (Pangaean mega-monsoonal vs late Jurassic equatorial arid zone), timing and character of rifting (early vs late syn-rift, Triassic wide extension vs narrow Jurassic rifting with strong lateral strain gradients) and sand delivery systems (Triassic continental/regional drainage versus rivers draining late Jurassic local footwall uplifts – provenance data demonstrate a significant change in sediment composition and drainage re-organisation between the two systems).