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Borehole breakouts, Borehole image log, Bowen basin, Drilling induced fractures, Stress map
Abstract:
Most of in-situ stress data in the Australian continent comes from wellbore stress analysis in deep hydrocarbon
reservoirs, and earthquake focal mechanism solutions near the Australian plate boundaries, where geophysical
tools facilitate understanding of the present-day stress patterns. This resulted in a paucity of stress information in
many other regions such as the northern Bowen Basin, which is an active mining province, but with low seismicity
rates and limited deep petroleum exploration. The mining industry runs several hundred kilometres of
image logs annually to characterise geotechnical attributes. These logs provide an image from the borehole wall,
which facilitates analysis of stress-related borehole deformations for in-situ stress characterisation. This paper
examines the orientation of horizontal in-situ stress using different types of image logs in mine boreholes across
the northern Bowen Basin. Analyses of 128 km of image logs in 680 vertical boreholes resulted in the interpretation
of 9046 pairs of stress-related indicators including 735 drilling induced fractures and 8311 borehole
breakouts. Our comprehensive database comprises 890 quality-ranked data records for the orientation of
maximum horizontal stress (SHmax), which makes the Bowen Basin as a basin with the highest data density in the
world in terms of quality-ranked stress information according to the World Stress Map. Statistical analysis of
SHmax orientation reveals that the mean SHmax orientation in northern Bowen Basin is N018◦ ± 16◦. The results
show that this orientation is consistent over long distances, which is in contrast with several eastern Australian
basins. This uniform stress pattern agrees well with plate-scale geomechanical model predictions, which further
highlights the impact of plate boundary forces in the contemporary stress pattern of this region. Detailed image
log investigation did not show any systematic rotation of stress; however, some small-scale stress perturbations
were observed in the vicinity of sharp stiffness contrasts and geological structures.
