School of Earth and Environmental Sciences

In-Situ Stress and Natural Fractures in Australian Basins

Ros King

Hani Abul Khair, Khalid Amrouch, Guillaume Backé, Adam Bailey, Bradley Grosser, Richard Hillis, Simon Holford, Marie Neubauer,

Mojtaba Rajabi, Josh Sage, Ernest Swierczek, David Tassone and Mark Tingay

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S3 Stress Structure Seismic

S3 Stress Structure Seismic

In-Situ Stress and Natural Fractures

“A fracture is considered open to fluid flow when it is optimally aligned with the in-situ stress field”

• Provide secondary permeability and pathways for fluid flow

• Behave as baffles to fluid flow, if sealing

• Hence, understanding natural fracture networks and the character of different fracture sets is crucial to exploration (and production) of unconventional resources

In-Situ Stress Natural Fractures Prediction Conclusions Background

S3 Stress Structure Seismic

In-Situ Stress and Natural Fractures

Three principals stresses within the Earth’s Crust.

Three types of fractures

• Andersonian Faulting Theory (1951) states that:

o Shear Fractures (red) form ~30° to σ1

o Tensile Fractures (green) form parallel to σ1

o Stylolites (compaction bands) form perpendicular to σ1

In-Situ Stress Natural Fractures Prediction Conclusions Background

S3 Stress Structure Seismic

In-Situ Stress and Natural Fractures

Three principals stresses within the Earth’s Crust.

Three types of fractures

• Andersonian Faulting Theory (1951) states that:

o Shear Fractures (red) form ~30° to σ1

o Tensile Fractures (green) form parallel to σ1

o Stylolites (compaction bands) form perpendicular to σ1

In-Situ Stress Natural Fractures Prediction Conclusions Background

σ1

σ3 σ2

σ2

σ3 σ1

σ3

σ1 σ2

Normal Fault Stress Regime

Thrust Fault Stress Regime

Strike-slip Fault Stress Regime

S3 Stress Structure Seismic

In-Situ Stress and Natural Fractures

“A fracture is considered open to fluid flow when it is optimally aligned with the in-situ stress field”

In-Situ Stress Natural Fractures Prediction Conclusions Background

Carnarvon Basin (Bailey in progress)

S3 Stress Structure Seismic

In-Situ Stress and Natural Fractures

“A fracture is considered open to fluid flow when it is ~30° or parallel to the maximum principal stress”

• Mapping the in-situ stresses allows to predict:

– The permeability of existing fracture networks

– The orientations of fracture open to fluid flow

– The orientations of new fractures

In-Situ Stress Natural Fractures Prediction Conclusions Background

Carnarvon Basin (Bailey in progress)

S3 Stress Structure Seismic

In-Situ Stress in Australian Basins

• The Australian Stress Map is well controlled by data

• The pattern of stresses observed are coincident with the complex plate boundaries of the Indo-Australian Plate

In-Situ Stress Natural Fractures Prediction Conclusions Background

Courtesy: The World Stress Map Project

Australian Stress Map (Hillis & Reynolds, 2000

S3 Stress Structure Seismic

In-Situ Stress in Australian Basins

• The Australian Stress Map, indeed the World Stress Map, is composed of multiple data types:

o Earthquake Focal Mechanisms

o Boreholes Breakouts and Drilling-Induced Tensile Fractures (petroleum well data)

o Borehole Slotter

o Overcoring

o Hydraulic Fractures

o Geological Indicators

EQ Mags above 3.0M Yellow 5.0M Orange 4.0-4.9M Pink 3.0-3.9M Jan 1990 to August 2005 GA

SE Australia (King et al., 2012)

In-Situ Stress Natural Fractures Prediction Conclusions Background

S3 Stress Structure Seismic

In-Situ Stress in Australian Basins

• Vertical Stress (σv) – Assumed to be vertical

– Magnitude calculated from density logs

• Maximum Horizontal Stress (σH) – Orientation from drilling-induced

tensile fractures (DITFs)

– Magnitude calculated from relationship of circumferential stress and minimum horizontal stress

• Minimum Horizontal Stress (σh) – Orientation from borehole

breakouts

– Magnitude calculated from leak-off tests (LOTs)

σh

σH

σv

In-Situ Stress Natural Fractures Prediction Conclusions Background

S3 Stress Structure Seismic

In-Situ Stress in Australian Basins: Orientations

In-Situ Stress Natural Fractures Prediction Conclusions Background

Carnarvon Basin (Bailey in progress)

S3 Stress Structure Seismic

In-Situ Stress in Australian Basins: Magnitudes

Carnarvon Basin (Bailey in progress)

σh

σH

σv

Normal σv > σH > σh

Strike-Slip σH > σv > σh

In-Situ Stress Natural Fractures Prediction Conclusions Background

S3 Stress Structure Seismic

In-Situ Stress in Australian Basins: Magnitudes

σh

σH

σv

In-Situ Stress Natural Fractures Prediction Conclusions Background

Normal σv > σH > σh

Carnarvon Basin (Bailey in progress)

S3 Stress Structure Seismic

In-Situ Stress and Natural Fractures

“A fracture is considered open to fluid flow when it is ~30° or parallel to the maximum principal stress”

• Mapping the in-situ stresses allows to predict:

The permeability of existing fracture networks

The orientations of fracture open to fluid flow

The orientations of new fractures

In-Situ Stress Natural Fractures Prediction Conclusions Background

Otway Basin (Sage, 2013)

Otway Basin (Bailey et al., 2013)

Perth Basin (King et al., 2008)

S3 Stress Structure Seismic

Natural Fractures

• Most basins have been subject to multiple tectonic and uplift events that have resulted in several generations and sets of fractures.

• BUT, which of these behave as fluid pathways?

In-Situ Stress Natural Fractures Prediction Conclusions Background

Otway Basin (Sage, 2013) Otway Basin (Sage, 2013)

S3 Stress Structure Seismic

Characterising Natural Fractures

• Fractures observed in image logs: Open vs. Closed

o Open fractures, dark and conductive (filled with drilling mud)

o Closed fractures, light and resistive (filled with cement)

In-Situ Stress Natural Fractures Prediction Conclusions Background

Closed Fractures Open Fractures

S3 Stress Structure Seismic

Characterising Natural Fractures

• Are open fractures really open?

In-Situ Stress Natural Fractures Prediction Conclusions Background

Well Senecio-1, North Perth Basin (Bailey, 2011)

S3 Stress Structure Seismic

Characterising Natural Fractures

• Are open fractures really open?

In-Situ Stress Natural Fractures Prediction Conclusions Background

Otway Basin (Tassone et al., 2012)

S3 Stress Structure Seismic

Characterising Natural Fractures

• Some common cements may be electrically conductive (e.g. siderite) in image logs

In-Situ Stress Natural Fractures Prediction Conclusions Background

Well Jacaranda Ridge-1, Otway Basin (Bailey et al., in review)

S3 Stress Structure Seismic

Relationship Between In-Situ Stress and Fractures

• Intimate relationship between stress and fractures

In-Situ Stress Natural Fractures Prediction Conclusions Background

Perth Basin (Bailey, 2011)

3D Mohr Circle used for Fault Reactivation Diagrams (Courtesy M. Tingay)

S3 Stress Structure Seismic

Relationship Between In-Situ Stress and Fractures

In-Situ Stress Natural Fractures Prediction Conclusions Background

Perth Basin (King et al., 2008)

Fracture

S3 Stress Structure Seismic

Conclusions

• Fractures required for production of unconventionals, so an important factor in exploration

• The in-situ stress field is a major control on fracture formation and reactivation

• A fracture is considered open to fluid flow when it is ~30° or parallel to the maximum principal stress

o The orientations of fracture open to fluid flow

o The orientations of new fractures

• We map and characterise fractures using image logs, core and outcrop

• Open fractures can be both open and siderite filled

In-Situ Stress Natural Fractures Prediction Conclusions Background