Integrating rapid 3D structural validation into interpretation: forward modelling of the Katnook Graben, Otway Basin

Paul John Lyon , Peter J Boult

Structural validation of a prospect is invariably applied after the interpretation is finished, by restoring one or two key lines in 2D or, rarely, by a full 3D restoration that needs finished horizons and a specialist. The authors propose that a simple 3D kinematic forward model, built in minutes using a few explicit structural rules, fully restorable and scaled onto the mapped horizons, is a highly effective way of building structural validation into the early stages of interpretation. The rules are taken from the published literature and built into a software module, referred to herein as the rules module. The rules module is made available together with the crestal collapse graben solution used in this paper. The structural rules are those of the published literature: (1) fault-parallel flow, (2) slip decaying towards the fault tips and (3) syn-rift deposition to a datum. The fourth rule is that any accommodation problem arising when using them in a restorable forward model is flagged to the user. The resolution of the accommodation problem is a decision made by the user as part of the model build. Where the hangingwall of a listric fault cannot be carried over a bend in the fault without strain, that resolution is a crestal collapse graben. How the graben scales with slip is part of scaling the model onto the data, and it may differ between geological provinces. The method is tested here on the Katnook Graben of the Penola Trough, the main gas province of the onshore Otway Basin, which the authors interpreted twenty years ago as bounded by a basement-rooted, thick-skinned fault and its antithetic. The dip of the listric Hungerford Fault and its top basement cutoff are taken from 2D regional sections and the geometry of the graben from 3D interpretation. With these inputs the authors' model fits the mapped thickness of the base Laira to top Crayfish growth interval along dip to within about 120 m, roughly 10% of the 1.2 km by which the interval thickens along dip. It also reproduces the proportions of the syn-tectonic thickening into the graben. The detachment depth is projected and two parameters are fitted, namely the proportion of the slip pre-dating the base Laira horizon and the keystone throw. No basement fault beneath the graben is needed to account for the mapped geometry. Thus the authors now interpret the Katnook Graben as a crestal collapse graben, detached within the sedimentary fill and formed as a consequence of the large displacement on the listric Hungerford Fault. This has important implications for fault seal integrity risk in the area. The model also implies that the mapped basement, poorly imaged and broken by faults carried down from the reservoir level, can reasonably be re-interpreted as a simpler and less faulted surface. The value of the approach lies in structural validation carried out while the horizons are still being picked. Several structural ideas can be tested against the fault geometry, the growth pattern and the isopachs within hours, and the scenario that survives carries a structural history into trap definition, charge timing and volumetric uncertainty. The method is at an early stage of development. The purpose of this paper is to outline a new method and demonstrate its usefulness at prospect scale for reducing structural risk and the associated risk to fault seal.