Geomechanical Characterisation of Fault and Top Seal - Offshore Northern Perth Basin

Langhi, Laurent¹; Zhang, Yanhua¹; Nicholson, Chris²; Rollet, Nadege²; Bernardel, George²; Kempton, Richard¹; Kennard, John²; Schaubs, Peter¹; Strand, Julian¹
¹CSIRO Earth Science and Resource Engineering, Kensington, WA, Australia.
²Geoscience Australia, Canberra, ACT, Australia.

The geomechanical and structural risk factors that could lead to trap breach were assessed in a series of Permo-Triassic prospects across the offshore northern Perth Basin (continental margin off southwest Western Australia). 3D geomodels of four prospects with different structural styles and charge/retention histories were constructed and used to simulate the response of the trap-bounding faults to extensional reactivation. We modelled the stress and strain distribution at, and around, the trap-bounding faults to evaluate the likelihood of fault planes to conduct fluid and bypass the regional top seal.

Calibration of the modelling results with current and palaeo oil columns demonstrates that along-fault fluid flow correlates with areas of local high shear and volumetric strains. The concentration of this deformation leads to: (i) an increase of structural permeability promoting fluid flow and, (ii) the development of hard-linkage between reactivated Permian reservoir faults and Jurassic faults producing top seal bypass.

Applying the modelling outcomes to a larger set of fields and dry prospects across the basin we show that the main structural factors controlling the distribution of leak-prone fault segments are: (i) fault strikes of 350N to 110N (NNW-SSE to ESE-WNW) that are more likely to be forced into failure; and (ii) intersections of fault planes that generated zones of high shear deformation and dilation. Although top seal thickness partly controls the development of hard-linked top seal bypass, the thickness variations recorded for the offshore northern Perth Basin do not appear to represent a first order risk factor.

Results from this work is a step towards the development of a regional predictive approach for assessing trap integrity in the offshore northern Perth Basin, and addresses a key exploration risk.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012