Unconformities and Subsurface Fluid Flow
Dr Helen Lever
Vision
Unconformities are surfaces or narrow zones between rocks of different ages. They represent missing time in the stratigraphic record, and are often used as correlation surfaces in basin studies. Unconformities can be categorised in several different ways and are created by a wide variety of processes and in many different environments. The variety of processes involved in their formation also means that there is a range of physical properties possible for an unconformity surface. There are two main ways in which unconformities will affect petroleum exploration and reservoir geology: as bounding surfaces or as a barrier or conduit for fluid flow. Stratigraphic traps are often created by erosion underneath unconformities, and unconformity surfaces are also often associated with the accumulation of coarse sediment in a basin, so that potential reservoir rocks can be bounded at the top or the bottom by an unconformity surface. Unconformities are often uneven or irregular surfaces, especially if the mode of formation of the unconformity is through erosion of previously deposited material. The irregularity of the surface, often not represented in basin models, can lead to under-or over-estimates of reservoir volume and also to changes in facies that will affect reservoir quality. With respect to fluid flow, unconformities may be barriers or seals (for example when the surface is well cemented) or the unconformity may act as a conduit, focussing fluid flow now or in the past. It is recognized that fluids in basins containing unconformities can either be prevented from migrating and be trapped by unconformities (e.g. Accaino 2005; Demirel 2004), or that unconformity surfaces and associated coarse sediments can form conduits for fluid flow (e.g. Rogers et al. 2006; Cao 2005; Zhang 2004; Fengjun 2001). Both the geometrical shape, stratigraphic location and permeability of an unconformity surface can vary laterally as well - especially from basin margin to basin centre. However there is currently no real way of predicting what type of flow regime will occur along any specific unconformity.