Caprock Bypass Systems: A New Approach to Topseal Analysis
We present a conceptual model for the analysis of the sealing potential of caprock sequences based on the recognition that many high quality seals are breached episodically or semi-permanently by a range of geological structures that act as caprock by-pass systems (CBS). We formally define CBS as seismically resolvable geological features embedded within caprock sequences that promote cross-stratal fluid migration and allow fluids to bypass the pore network. We advance the concept that if such bypass systems exist within a given caprock sequence, then predictions of sealing capacity based exclusively on rock physical properties such as capillary entry pressure/hydraulic conductivity will be largely negated by the capacity of the bypass system to breach the grain and pore network. This model is based largely on observations of caprocks using 3D seismic data, in which there is direct evidence of highly focused vertical or sub-vertical fluid flow from subsurface reservoirs up through the caprock sequence with leakage internally at higher levels or to the surface as seeps.
We classify CBS into three main classes based on seismic interpretational criteria: (1) fault related, (2) intrusion-related, and (3) pipe-related. We show how each class exhibits different modes of behaviour with different scaling relationships between flux and dimensions, and different short and long-term impacts on caprock behaviour. We conclude with an analysis of CBS and their relative impacts on petroleum systems in West Africa, the NE Atlantic margin and the Eastern Mediterranean margin to show how this model can be employed to reduce risk and aid prediction in a range of play contexts.
Seals and Overpressure in Mud-Dominated Systems
2005 AAPG Annual Convention (June 19-22, 2005) Technical Program