Abstract - Methods for evaluating fault properties, in both production and exploration settings, are typically based on algorithms which are some expression of the shale content of the faulted sequence and the fault throw, combined in a simple model for internal fault structure (for example Shale Gouge Ratio, Clay Smear Potential, etc.). Here we describe the structure of small faults in soft-sediment, with a view to providing better constraints on fault property algorithms appropriate to these types of faults.
The faults studied are from 3 outcrops along the Danish coast. The 3D geometry of the faults demonstrates that complexities on fault surfaces are common and arise at an early stage of fault growth, causing faults to be composed of multiple fault strands and enhancing across fault connectivity between beds. Furthermore, the burial depth at which the faults formed controls the internal fault zone structure and properties. At very shallow burial depths (<50 m), clays may form very irregular smears and even deform in a brittle manner, so that they do not contribute material to the fault zone. Sands on the other hand, deform by particulate flow, without grain breakage, creating sand smears along faults and therefore conduits for fluids along fault surfaces. Clays and sands may both behave in a ductile manner to create a composite fault zone comprising discrete sheared beds with no grain scale mixing between the sand and clay layers. At deeper burial depth (>1 km), or higher confining pressure, the internal fault zone structure is no longer composite, and instead, fault slip is concentrated along discrete narrow shear bands, commonly anastomosing along larger (10-20 cm) faults, enclosing lenses of less deformed host sediment. Extensive grain scale mixing takes place within these shear bands and the sands deform by grain breakage and cataclasis. The exact depth at which the transition between these deformation styles occurs is ill defined; yet the variability in fault zone structure and properties necessitates constraining the depth of faulting when estimating fault properties in the subsurface. Smear (sand and clay) continuity was investigated for shallow burial (<50m) depth faults, where it was found that clays become discontinuous at throw/layer thickness ratios of 3 and sands may be continuous up to throw/layer thickness ratios of 4-5. The 3D smear distribution and continuity of beds along fault surfaces was mapped and shows how fault surface complexities may enhance the connectivity of sand beds in 3D. These observations show that the assumption that juxtaposition seal is a generally “safe” fault seal cannot be applied at shallow burial depths. Our observations also suggest that no one fault seal potential algorithm is appropriate to all soft-sediment faults, and that different algorithms are required for faults over relatively narrow depth ranges.
Abstract of talk given to:
Structurally Complex Reservoirs, Geological Society of London, February 2006.