Pete Bretan1, Graham Yielding1
(1) Badley Technology Ltd, Lincolnshire, United Kingdom
ABSTRACT: Fault Seal Development in Siliciclastic Sequences
Fault seals in siliciclastic sequences are controlled by the mechanical processes acting during faulting, and subsequent modification of fault rocks during burial. One critical parameter that strongly influences the mechanical generation of fault rocks is the distribution of shale through the sequence. Fault rocks generated in clay-poor parts of a sequence are disaggregation zones and cataclasites, with relatively low and moderate seal capacities respectively. At greater clay content, phyllosilicate-frameworks and clay smears are developed. These rock types have much higher seal capacities than the disaggregation and cataclastic types. However, subsequent burial below c.3km typically causes quartz cementation in clay-poor fault rocks and their seal capacity increases significantly. Fault-seal prediction must therefore take account of the pressure/temperature/geochemistry evolution of the fault as well as the mechanical faulting process.
We present a variety of methods for predicting potential development of these rock types when a given rock sequence is faulted. Algorithms such as 'Clay Smear Potential' and 'Shale Gouge Ratio' can be calculated as a sequence is slipped past itself, to indicate the potential development of different fault rocks as different parts of the sequence are subjected to different fault displacements ('Triangle plots'). Converting these results to predictions of trapped hydrocarbon columns is best achieved with local calibrations of known subsurface fault behaviour, though analogues from comparable basins may be used with care. For complete trap prediction the 3D geometry of fault/sequence intersections can be built and used to identify critical leak points in their structural context.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado