Clay Rich Fault Gouge in Low Clay Content Reservoirs: Implications for Predictive Fault Seal Evaluation

Dave Dewhurst¹, Ben Clennell¹, Julian Strand¹, Jim Underschultz¹, Anthony Gartrell¹, and Wayne Bailey^2
1CSIRO Petroleum, Kensington, WA, Australia
²Woodside Energy Limited, Perth, WA, Australia

In recent years, algorithms which predict the incorporation of shale/clay into fault rock, such as shale gouge ratio (SGR), have become the standard approach to predict fault rock properties for reservoir simulators. Such algorithms are also starting to be incorporated directly into basin models to simulate fault behaviour temporally, although the state of the art is somewhat embryonic. There remain significant knowledge gaps in our understanding of how faults transmit fluids over time leading to significant uncertainty in the predictive capability of these integrated basin models.
In this study microstructural evaluation has been performed on reservoir equivalent immature sandstones and associated fault rocks from Taranaki, New Zealand. The sandstones comprise angular clasts of quartz, feldspar, biotite, muscovite and numerous rock fragments, with low clay content (~1%). The sandstone is generally porous (~25%), although some compaction of rock fragments and micas and some fracturing of quartz and feldspars are noted. The fault rocks examined comprise a clay gouge from a fault with significant throw, which cuts a sandstone, though the unit remains self-juxtaposed. In outcrop, these clay gouges are laterally and vertically continuous at the scale of observation and would provide significant impediment to fluid flow within a carrier bed. In thin section the clay gouge displays significant cataclasis of rigid grains and the fault rock is extremely heterogeneous as a result of mechanical incorporation of adjacent sediment and their mixing during fault movement. Associated deformation bands lose all visible porosity, resulting from both grain size reduction and from partial diagenetic alteration to clay minerals post-deformation. It is suggested that the preferential reduction in grain size of reactive constituents such as feldspar, mica and rock fragments resulted in diagenetic alteration to clays though increase in surface area and availability of new clean surfaces for reactions to occur.
The observed diagenetic formation of clay-rich gouges in low clay reservoir rocks, if found to be a widespread phenomenon, has significant implications for current SGR-based methods used to assess both simulation model timescale fault transmissibility and basin scale fault seal capacity.

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands