Scott D. Reynolds

The Australian Stress Map

Knowledge of the in situ stress field of the Australian continent has increased greatly since compilation of the World Stress Map in 1992, principally by analysis of borehole breakouts and drilling-induced tensile fractures in petroleum wells. Stress orientations are variable across the Australian continent as a whole. However, within 15 of 16 individual stress provinces defined in the Australian continent (of one to a few hundred kilometres scale), mean stress orientations are statistically significant. The stress provinces, and stress trajectory mapping, reveal that there are systematic, continental-scale rotations of stress orientation within Australia. Unlike many other continental areas, stress orientations do not parallel the direction of absolute plate motion. Nonetheless, the regional pattern of stress orientation is consistent with control by plate boundary forces, if the complex nature of the convergent northeastern boundary of the Indo-Australian plate, and stress focusing by collisional segments of the boundary, is recognized.

Present-day stresses, seismicity and Neogene-to-Recent tectonics of Australia's 'passive' margins: intraplate deformation controlled by plate boundary forces

Abstract Neogene-to-Recent deformation is widespread on and adjacent to Australias ‘passive’ margins. Elevated historical seismic activity and relatively high levels of Neogene-to-Recent tectonic activity are recognized in the Flinders and Mount Lofty Ranges, the SE Australian Passive Margin, SW Western Australia and the North West Shelf. In all cases the orientation of palaeostresses inferred from Neogene-to-Recent structures is consistent with independent determinations of the orientation of the present-day stress field. Present-day stress orientations (and neotectonic palaeostress trends) vary across the Australian continent. Plate-scale stress modelling that incorporates the complex nature of the convergent plate boundary of the Indo-Australian Plate (with segments of continent–continent collision, continent–arc collision and subduction) indicates that present-day stress orientations in the Australian continent are consistent with a first-order control by plate-boundary forces. The consistency between the present-day, plate-boundary-sourced stress orientations and the record of deformation deduced from neotectonic structures implicates plate boundary forces in the ongoing intraplate deformation of the Australian continent. Deformation rates inferred from seismicity and neotectonics (as high as 10−16 s−1) are faster than seismic strain rates in many other ‘stable’ intraplate regions, suggestive of unusually high stress levels imposed on the Australian intraplate environment from plate boundary interactions many thousands of kilometres distant. The spatial overlap of neotectonic structures and zones of concentrated historical seismicity with ancient fault zones and/or regions of enhanced crustal heat flow indicates that patterns of active deformation in Australia are in part, governed, by prior tectonic structuring and are also related to structural and thermal weakening of continental crust. Neogene-to-Recent intraplate deformation within the Australian continent has had profound and under-recognized effects on hydrocarbon occurrence, both by amplifying some hydrocarbon-hosting structures and by inducing leakage from pre-existing traps due to fault reactivation or tilting.

In situ stress field of Australia

The Australian stress map comprises 331 reliable indicators of the orientation of horizontal, tectonic stresses in the Australian crust. In order to elucidate regional trends in stress orientation across the Australian continent, stress provinces have been defined and stress trajectories mapped, based on these indicators. Unlike most other continental areas, stress orientations in the Australian continent as a whole are variable and do not parallel the north to north-northeast absolute motion direction of the Indo-Australian Plate. The stress provinces and stress trajectories reveal systematic, continental-scale rotations in stress orientation. Maximum horizontal stress is oriented east-west in western Australia. The east-west orientation rotates to northeast-southwest moving eastwards along the northern Australian margin and in central Australia. The east-west maximum horizontal stress orientation rotates to northwest-southeast moving eastwards along the southern Australian margin. The area of divergence between northeast-southwest and northwest-southeast maximum horizontal stress trajectories in central eastern Australia is characterised by east-west or poorly defined (low horizontal stress anisotropy) trends. Regional stress orientations in the Australian continent are not affected to a first-order by either tectonic province, regional structural trends, geological age, or by the depth at which orientations are sampled. The regional pattern of stress orientation in the Australian continent is consistent with a first-order control being exerted by plate-boundary forces, if the complex nature of the northeastern boundary of the Indo-Australian Plate, and stress focusing by collislonal segments of the boundary, is recognised. A number of locally anomalous stress orientations appear influenced by second-order sources of stress such as structure, topography and density heterogeneities. In situ stress orientations show a strong correlation with the direction of seismic anisotropy in the lithosphere. It is suggested that both datasets are predominantly controlled by present-day plate dynamics.

In situ stresses and natural fractures in the Northern Perth Basin, Australia

Present-day stress orientations in the Northern Perth Basin have been inferred from borehole breakouts and drilling-induced tensile fractures observed on image logs from eight wells. Stress indicators from these wells give an east – west maximum horizontal stress orientation, consistent with stress-field modelling of the Indo-Australian Plate. Previous interpretations using dipmeter logs indicated anomalous north-directed maximum horizontal stress orientations. However, higher-quality image logs indicate a consistent maximum horizontal stress orientation, perpendicular to dominant north – south and northwest – southeast fault trends in the basin. Vertical stress was calculated from density logs at 21.5 MPa at 1 km depth. Minimum horizontal stress values, estimated from leak-off tests, range from 7.4 MPa at 0.4 km to 21.0 MPa at 0.8 km depth: the greatest values are in excess of the vertical stress. The maximum horizontal stress magnitude was constrained using the relationship between the minimum and maximum horizontal stresses; it ranges from 8.7 MPa at 0.4 km to 21.3 MPa at 1 km depth. These stress magnitudes and evidence of neotectonic reverse faulting indicate a transitional reverse fault to strike-slip fault-stress regime. Two natural fracture sets were interpreted from image logs: (i) a north- to northwest-striking set; and (ii) an east-striking set. The first set is parallel to adjacent north- to northwest-striking faults in the Northern Perth Basin. Several east-striking faults are evident in seismic data, and wells adjacent to east-striking faults exhibit the second east-striking set. Hence, natural fractures are subparallel to seismically resolved faults. Fractures optimally oriented to be critically stressed in the present-day stress regime were probably the cause of fluid losses during drilling. Pre-existing north- to northwest -striking faults that dip moderately have potential for reactivation within the present-day stress regime. Faults that strike north to northwest and have subvertical dips will not reactivate. The east-striking faults and fractures are not critically stressed for reactivation in the Northern Perth Basin.

The in situ stress field of the Perth Basin, Australia

A total of 17 wells in the Perth Basin were interpreted to have 114 breakouts covering a combined length of 3.2 km and a mean σHmax orientation of 108°N. The inferred mean orientation of σHmax in the Perth Basin is broadly consistent with that in the adjacent basement, Yilgarn Block. The new data confirm that σHmax orientation in the region is consistent with other stress indicators, different depths and different geological provinces. The new data also confirm that the orientation of σHmax does not parallel the NNE direction of absolute plate velocity. Anomalous north-south σHmax orientations identified in a number of wells in the Perth Basin can be attributed to local structural effects.

In situ stress field, fault reactivation and seal integrity in the Bight Basin, South Australia

We evaluate the in situ stress field and consequent risk of fault reactivation in the Bight Basin in order to assess the risk of fault seal breach at seismically mapped prospects. Borehole breakouts interpreted from dipmeter and image logs in five wells in and around the Bight Basin indicate a 130° maximum horizontal stress orientation. The large variation in water depths across the Bight Basin requires the use of effective stress magnitudes. We use a depth-stress power relationship to define the effective vertical stress based on density log data from 10 wells. The effective minimum horizontal stress gradient is estimated at 6 MPa/km using effective pressures from leak-off tests. We determine an upper bound (18.7 MPa/km) for the effective maximum horizontal stress gradient, using frictional limits to stress. The upper bound to the effective maximum horizontal stress indicates the region is in a strike-slip faulting stress regime. However, a normal faulting stress regime cannot be ruled out. Pore pressure in wells in the region is hydrostatic except in Greenly 1 where mild overpressure occurs below a depth of 3600 m. We use the FAST technique to evaluate the risk of fault reactivation in the Bight Basin. The risk of fault reactivation and consequent seal breach is expressed in terms of the pore pressure increase that would be required to induce failure. We consider three different stress regimes. These include a strike-slip faulting stress regime, a normal faulting stress regime, and a case on the boundary of strike-slip and normal faulting stress regimes. In all three cases, faults striking 40° (±15°) of any dip are the least likely to be reactivated.

Contemporary tectonic stress pattern of the Taranaki Basin, New Zealand

The present-day stress state is a key parameter in numerous geoscientific research fields including geodynamics, seismic hazard assessment, and geomechanics of georeservoirs. The Taranaki Basin of New Zealand is located on the Australian Plate and forms the western boundary of tectonic deformation due to Pacific Plate subduction along the Hikurangi margin. This paper presents the first comprehensive wellbore-derived basin-scale in situ stress analysis in New Zealand. We analyze borehole image and oriented caliper data from 129 petroleum wells in the Taranaki Basin to interpret the shape of boreholes and determine the orientation of maximum horizontal stress (S-Hmax). We combine these data (151 S-Hmax data records) with 40 stress data records derived from individual earthquake focal mechanism solutions, 6 from stress inversions of focal mechanisms, and 1 data record using the average of several focal mechanism solutions. The resulting data set has 198 data records for the Taranaki Basin and suggests a regional S-Hmax orientation of N068 degrees E (22 degrees), which is in agreement with NW-SE extension suggested by geological data. Furthermore, this ENE-WSW average S-Hmax orientation is subparallel to the subduction trench and strike of the subducting slab (N50 degrees E) beneath the central western North Island. Hence, we suggest that the slab geometry and the associated forces due to slab rollback are the key control of crustal stress in the Taranaki Basin. In addition, we find stress perturbations with depth in the vicinity of faults in some of the studied wells, which highlight the impact of local stress sources on the present-day stress rotation.