Geomechanically Based Numerical Models of Complex Geological Structures: Stress and Strain in the Baram Delta and Deepwater Fold-Thrust Belt, NW Borneo

Guillaume Backé², Rosalind C. King¹, Christopher Morley³, Richard Hillis¹, and Mark Tingay^2
1Australian School of Petroleum, University of Adelaide, Adelaide, SA, Australia
²School of Earth & Environmental Sciences, University of Adelaide, Adelaide, SA, Australia
³Geological and Geophysical Services, PTTEP, Bangkok, Thailand

Computer-based numerical techniques based on kinematics approaches have led to better constraints of the evolution and structure of fold-thrust belts and their hydrocarbon reservoirs. However, these tools do not respect the fundamental principles of mass and momentum conservation, and do not allow mechanical interactions between faults. Modeling deformation in delta and deepwater fold-thrust belts (DDFTBs) remains challenging because compressional stresses (margin normal), in the delta toe, are coupled to extensional stresses (margin parallel), on the delta top.

Here, we use a continuum Finite Element Method code, which can accommodate large displacements and strain for a heterogeneous anisotropic and discontinuous medium, to model the behavior of the DDFTB in two dimensions.

We have reconstructed, step by step, the tectonic evolution of the Baram DDFTB in NW Borneo, and systematically compared the results of our model to independent stress analyses and field studies. We confirm that the maximum horizontal stress orientation has rotated across the region through time as a consequence of hinterland uplift resulting in forced progradation of the delta offshore. Field observations demonstrate that the maximum horizontal stress was approximately NE-SW in the Miocene (margin parallel); suggesting that the inner shelf was under active deltaic extension. At present-day the inner shelf is characterized by a NW-SE trending maximum horizontal stress (130; margin parallel) and large-scale inversion structures. The Baram DDFTB is located at the outer shelf to basin floor at present-day; and the maximum horizontal stress trends NE-SW (030; margin parallel) on the extensional delta top and NW-SE (120; margin normal) in the compressional delta toe; consistent with stress orientations calculated from well data.

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