The Effect of Scale-controlled Fault Properties on Fluid Transmissibility within Carbonates (Chalks), a Case Study from Flamborough Head, UK

David Sagi
Durham University, Department of Earth Sciences Durham, County Durham, UK
[email protected]

Fault systems, developed in low porosity fine-grained rocks, can act both as conduits or barriers and control the migration and accumulation of hydrocarbons in carbonate reservoirs. We studied fault systems developed in the low-porosity, fine-grained Upper Cretaceous chalk which is exceptionally well exposed in the Flamborough Head area (UK). These outcrops represent good field analogues for many of the oil fields in the North Sea.

Detailed field-based structural observations and mesoscale data collection, integrating laser scanner technology (LIDAR) recently developed at the University of Durham (UK), were performed to study fault systems attributes for a range of scales. Preliminary results show that the flat-ramp geometry of small displacement fault systems (<1m), organized in conjugate sets, is lithologically controlled by thin, clay-rich films inter-layered within the limestone beds. Fluid circulation is favoured along fault parallel directions through dilational jogs and is inhibited along fault orthogonal direction by the development of thin layers of clay-rich gouge along the fault planes. The geometry of large displacement faults (>20m) is complex and given by the juxtaposition of multiple fault strands characterized by thick and highly porous volumes of fault breccias (up to 2m), commonly developed by hydraulic implosion processes. Large amount of fluids can migrate along such thick fault zones.

Our results show that some caution is needed when using scaling laws to link across different length scales to predict fracture system attributes, as different processes and controls can operate at different scales, resulting in scale controlled fault geometry and fault rock assemblages.

AAPG Search and Discovery Article #90094 © 2009 AAPG Foundation Grants in Aid