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In Situ Stress and Fracture Characterisation in the Victorian Otway Basin, South-eastern Australia: Implications for the Unconventional Plays

Tassone, David¹; Holford, Simon P.¹; King, Rosalind²; Backe, Guillaume¹
¹The Australian School of Petroleum, The University of Adelaide, Adelaide, SA, Australia.
²Discipline of Geology & Geophysics, The University of Adelaide, Adelaide, SA, Australia.

The south-eastern Australian Otway Basin has proven economically-viable, conventional hydrocarbon systems and with advances in drilling completions technology, a renewed interest in unconventional plays, such as tight and shale gas, has emerged in recent years. In addition to source rock properties, source rock maturation and reservoir characteristic, a robust understanding of the in situ stress state and secondary porosity (fracture distribution and orientation) is vital in order to optimize wellbore trajectories in relation to hydraulic fracture stimulation and existing nature fracture networks in low permeability reservoirs.

This study combines petrophysical wireline data, drilling data and image log data from recently drilled wells in the Victorian Otway Basin to characterise the orientations of faults and fracture systems within conventional and unconventional reservoirs.

Borehole breakouts and drilling induced tensile fractures interpreted from dipmeter and image log data reveal a NW-SE maximum horizontal stress orientation, consistent with published estimates, and formation pressure tests have confirmed near-hydrostatic conditions. Preliminary analysis of newly available leak-off test result using a new method for analysing leak-off test data constrains the minimal horizontal stress magnitude to be ~20 MPa/km, which is similar to the vertical stress magnitude derived from wireline data for depths shallower than ~2-2.5 km. The alternate interpretation of leak-off test data has constrained the upper limit of the maximum horizontal stress magnitude to be the greatest principal stress, implying a reverse to strike-slip faulting stress regime that is consistent with neotectonic faulting evidence. Preliminary interpretation indicates extant natural fractures strike parallel with the maximum horizontal stress orientation and generally sub vertical in the eastern Victorian Otway Basin and strike generally N-S to NE-SW at variable dips angles in the western Victorian Otway Basin.

This study represents the first of its kind in the Otway Basin, and we suggest combined analysis of the in situ stress tensor and fracture density and geometries provides a powerful workflow for constraining fracture-related fluid flow pathways in potential unconventional reservoirs.


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