AAPG/GSTT HEDBERG CONFERENCE

“Mobile Shale Basins – Genesis, Evolution and Hydrocarbon Systems”

June 4-7, 2006 – Port of Spain, Trinidad & Tobago

caprock bypass systems: seismic Evidence for highly focused fluid expulsion on a basin scale

Joe Cartwright¹, Mads Huuse², Andrew Aplin³

¹3D Lab, Department of Earth Sciences, Cardiff University, Cardiff CF10 3YE Wales, UK, [email protected]

²Department of Petroleum Geology, Kings College Aberdeen University, UK

³Department of Civil Engineering and Geoscience, Newcastle University, UK

We present 3D seismic evidence for widespread development of vertical fluid expulsion features transecting otherwise highly impermeable sequences in many different types of sedimentary basin. Features that are indicative of highly focused fluid expulsion include mud diapirs, hydrothermal diatremes feeding surface vents, sandstone intrusions, and blowout pipes feeding surface pockmarks. We also show evidence of flow focusing along different kinds of fault system. These observations suggest that many relatively thick and impermeable sedimentary sequences 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 impermeable 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 sequence, then predictions of sealing capacity or gross fluid flow 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.

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 fluid flow behaviour on a basin scale. 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.