Fault Rock Prediction for Inverted Extensional Faults in Siliciclastic Sequences – Challenges, Methods and Risking for Hydrocarbon Exploration
Prediction of fault rocks and their properties for normal faults in siliciclastic sequences is standard practice when assessing fault controlled traps. Prediction methods such as the shale gouge ratio (SGR), for predicting the distribution of clays along a fault zone and the clay smear factor (CSF), for prediction of clay smear continuity and breakdown, are commonplace in fault seal workflows. However, for normal faults that have been subject to inversion the results from these predictive methods are not applicable. In this study we review challenges faced in attempting prediction of fault properties for inverted normal faults. We present some new predictive methods and show how these can be incorporated into risking scenarios. Inverted fault systems can exhibit anticlinal growth above pre-existing normal faults and net extension is often still present at depth. The amplitude of the inversion anticlines can provide an indication of the pre-existing normal fault offsets at depth present before inversion. This can be used along with the final post inversion extensional throw to quantify the amount of initial normal offset and subsequent inversion that has occurred. Existing fault clay content prediction methods only account for initial extensional movement, while faults subjected to inversion will not only have a fault rock clay content associated with the initial normal offset, but will require additional prediction to account for the reverse movement that has occurred through later compression. We present several prediction methods that attempt to capture this behaviour and provide a fault clay prediction that can be incorporated into more familiar workflows associated with inversion. The methods include; a) Reverse offset deformation of the initial extensional fault rock clay contents, b) reverse faulting of the host rocks starting with the stratigraphic juxtapositions associated with the earlier extensional fault offsets, c) combinations of a) and b). The analysis demonstrates the potential errors that can be carried forward into reservoir fluid flow simulations of inverted faults if inversion is ignored and also highlights the uncertainty ranges in the fault rock property calculations related to the assumptions on the geological processes involved.
AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014