Role of unconformities in controlling clastic reservoir properties : insights from adopting a multidisciplinary approach

Abstract

It is commonly thought that unconformities may both cause reservoir deterioration by being highly cemented and therefore form low permeability zones, or they promote reservoir development by being associated with coarse-grained sediments that offer high permeability pathways for fluid flow. Unconformity surfaces play a significant role in sequence stratigraphy and correlation of parasequences. However, they are also of fundamental importance for understanding petroleum prospectivity in many sedimentary basins. They commonly promote diagenetic change and either enhance reservoir porosity in subcropping sedimentary layers through leaching or promote cementation to create low permeability, poorer quality reservoirs. This thesis reports the results of a systematic analysis at different scales of the Caledonian and Variscan Unconformities, the two most prominent unconformities affecting British Stratigraphy, to provide new insights for our understanding that the subcrop and supracrop of unconformities are important in controlling reservoir properties. The Base Devonian Unconformity outcropping onshore in the Siccar Point, Scotland, represents the most famous angular unconformity ("Hutton’s Unconformity") and provides an exceptionally well exposed, hitherto unrecognized, wadi channel. Application of a new technique - LiDAR laser scanner, shed new light on this world famous unconformity. By generating a three-dimensional model representing the surface, highly angular character of the unconformity and its controlling factor in the deposition and distribution of the overlying sediments could be reflected. Furthermore, evaluation of the Base Permian Unconformity (BPU) through integration of seismic, electrical well-log, outcrop and core data has afforded the opportunity to determine the effects that it has on highly prospective Carboniferous gas-reservoirs which have been sealed beneath its overlying sealing Lower Permian, Rotliegend Group, Silverpit Claystone Formation cover in the UK Southern North Sea (SNS). Conventional wisdom has polarised views and has been a part of an on-going debate with opinion divided as to whether reservoir properties are enhanced or not by the unconformity. Given the significance for exploration, appraisal and development of the prospective Carboniferous play fairway in the SNS, the research has attempted to resolve this issue through seismic interpretation of the BPU, stratigraphic assessment of supracropping horizons and the systematic sampling and analysis of all relevant field exposures and cored sections. The interpretation of high-fidelity 3D seismic data has also permitted the identification of areas of structural inversion and the presence of a suite of WNW-ESE striking, sub-vertical Tertiary igneous dykes. The zones of structural inversion and the transecting dykes both affect the Carboniferous sediments and the BPU affecting the reservoir quality by instigating additional, overprinting diagenetic changes and the compartmentalisation of the reservoirs.