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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Structural Traps Modification Associated with Foreland Lithospheric Flexure
(1) CSIRO, Kennsington, WA, Australia.
Late Jurassic horst blocks represent the typical exploration target in the Bonaparte Basin (Australian North West Shelf). Following the Tertiary early development of a foreland basin in this region and a concomitant phase of fault activity, these potential traps are classically overlain by grabens and form so-called hourglass structures. The impact of such structures on the top seal is difficult to ascertain from seismic data due to signal degradation and varies locally following the heterogeneous accommodation of Tertiary strain; this classically affects the hydrocarbon column height, resulting in underfilled or totally reached traps.
We investigate the impact of the lithosphere flexure associated with the foreland basin initiation in order to constrain the 4D (time and 3D space) distribution of deformation and stress and their impact on reservoir faults and top seal. The flexural deformation is simulated by 2D elastic models and integrated to the basin evolution framework. The related stress distribution is quantified using multi-scale geomechanically-based forward deformation models. At basin scale, models outcomes support that the distribution of a flexural extension front triggers the Tertiary structural activity. At reservoir-scale, it appears that the flexural extension results in the reactivation of the reservoirs faults (Jurassic horsts) and the dissociated nucleation of the shallower faults. This evolution scenario of hourglass structures is supported by observed 3D vertical displacement patterns which suggest a connection predominantly established by the downward growth of the shallow faults (graben faults) and the minor upward reactivation of reservoir faults (horst faults). The spatial relationship between the reservoir faults and Tertiary stress tensor and the distribution of mechanical anisotropy in the stratigraphic column represent the key factors impacting on such fault propagation and connectivity. It is noted that the downward propagation of the shallow faults towards the hydrocarbon traps stresses the top seal integrity due to fault tip deformation front and the likely development of sub-seismic fractures, therefore even though the connection between reservoir and upper faults is not established thoroughly (i.e. hard-linkage), the time-transgresive evolution of hourglass structure has potential to threaten the top seal integrity.