Geometric Controls on Fault Reactivation: Implications for Leakage of Hydrocarbon Reservoirs

J.J. Walsh, C. Childs, A. Van Herk, and R. Worthington
University College Dublin, Dublin, Ireland

Assessment of the risk of hydrocarbon leakage is an essential step in the evaluation of the prospectivity of both hydrocarbon provinces and reservoirs. Numerous factors have been advocated for the leakage of hydrocarbon reservoirs, including overpressure-related top seal failure or associated fault leakage, and reactivation-related leakage of bounding faults. This presentation examines the nature of, and the principal controls on, the tectonic reactivation of normal faults from a selection of basins, including the Timor Sea and the Porcupine Basin.
Reactivation of underlying pre-existing faults is typically characterised by the upward propagation of highly segmented fault arrays through overlying cover sequences. Segmentation is attributed to fault propagation through multi-layered sequences but is, nevertheless, accentuated by departures from orthogonality of the pre-existing faults with the later extension direction. The principal factors controlling fault reactivation in the basins studied are the size, location and orientation of faults. Fault size is the pre-eminent control, with longer and larger displacement faults having a greater probability of reactivation and accommodating larger reactivated displacements. The preferred localisation of later displacements on larger and more through-going faults, is consistent with numerical models indicating that larger pre-existing faults are relatively ‘weak'. Combined with simple models for slip-associated fault leakage, the preferential movement of larger faults suggests that trap integrity should diminish with fault size, an assertion which is supported by new constraints from the Timor Sea (Gartrell et al. 2006, AAPG). We briefly consider the extent to which this simple trap integrity model could be applied to other basins.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece