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Stress from Seismic
John K. Davidson, Predrill Stresses International Pty Ltd
T
he most important variable in the
conventional and unconventional
gas and oil sectors is the direction
and magnitude of the horizontal stresses
approaching and at the target horizons.
Why rely on sparsly measured data which
are insufficient to determine the optimum
direction of inclined/horizontal wells,
especially for fracture propagation?
Mapping Anderson Stress States
Deformed seismic horizons are routinely
mapped, but do the same horizon and fault
files also contain quantitative stresses? The
three Anderson stress states of normal, strike
slip and reverse faulting can be identified
on isopachs (strictly speaking ‘isochores’)
from successive pairs of structure maps. The
isochores representing periods of compression,
contain reverse and normal faults and the
‘thins’ are synchronous growth of anticlines.
The three stress states can be sub-divided
into seven, coloured pink for reverse, strong
compression through to grey for normal, and
mapped (see Figure 1). It was found on the
North Sea, Valhall 3D survey that the stress
isochores at shallow horizons had low stress
states and increased to strike slip and higher
stress states of reverse faulting with depth
(Figure 1). This is observed in all non-orogenic
basins, that is, those beyond the horizontal
‘push’ of plate tectonics.
Drilling Risk Reduction
Each of the stress states is defined in terms
of mutually perpendicular compressive stress
axes of the Earth. These axes are SV, the weight
of the rock (vertical) and the maximum and
minimum horizontal stresses, SH max (SH) and
Sh min (Sh). The strike slip stress state was
Fig. 1. A detailed quantified pressure-depth graph from a 3D stress volume derived from a 3D survey.
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written by Anderson as SH>SV>Sh. It, and each
stress state, can also be represented by two
simultaneous ratios SH/SV and Sh/‌SV. This allows
the numerical representation of gradational
stress state boundaries for software
incorporation to produce the well planning
essential, pressure-depth graphs. These can
be made for wells of any direction, and any
inclination, avoiding differential stresses and
low angle fault approach. No longer is there a
dependence on often distant and potentially
poorly related ‘offset well’ information. It is all
about drilling risk reduction.
Orogenic and Non Orogenic
Stresses
The Valhall example shows the shallow, SHD,
dark blue arrow is perpendicular to the blue
anticline. That is also the direction of SH at
the deeper horizons and is perpendicular
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to the pink reverse faults in the deepest.
These are large basin-forming and modifying
faults and they cut the crust. In the Andes
the World Stress Map shows SHD is east-west
perpendicular to the major north-south
compressional thrusts and anticlines. Less
than 1800 km to the east SHD is north-south
and perpendicular to the Amazon basement
generated trends. This means there is a
gradational boundary from plate tectonic eastwest compressional mountain building orogeny
to the non orogenic, multiple SHDs from crust
cutting structures in sedimentary basins. In the
Amazon and the North Sea graben with the
Valhall anticline, the crust must be flexing.
Vertical Stresses
Such flexing occurs on a grand scale and is due
to vertical forces, a third dimension to plate
tectonics. Australia has drifted north some
40° of latitude in the last 50 million years. The
shape of the oblate spheroidal figure of the
Earth imposes a radius change of 21 km for a
continent moving from the South Pole to the
Equator. The extension is absorbed at northerly
oriented oceanic ridges but essentially rigid
continental (and older oceanic) crust will
‘feel’ the extension at the base of the crust
with Lake Eyre sagging to 12 m below sea
level. The surface will actually bend upwards
at the eastern highlands and the east side of
the Darling Fault, Western Australia, and be
locally compressed at crust cutting faults. The
50 million year to Recent sediments in the
Cooper Basin are strongly compressed with SHD
perpendicular to crust cutting faults.
Measured Stress = Seismic Stress
Seismic lines in several basins in Australia
show the Tertiary compression has not been
increasing steadily with the northward drift
as would be anticipated. These basins have
experienced five compressional peaks: the Mid
Eocene, Early Oligocene and the Early and Mid
Miocene and Pliocene to Recent. The same
events occur in the North Sea yet the global
GPS shows Europe headed in the opposite
direction with a North Polar component. There
must be another vertical force not recognised
by 2D plate tectonics.
The five Tertiary compressional pulses are
not alone. Thirty globally synchronous
Fig. 2. An initial regional stress from seismic map using widely spaced 2D seismic.
compressional pulses have been
documented from the Carboniferous in up
to 20 non orogenic continental and oceanic
border areas. In all cases the stresses are
perpendicular to crust cutting faults. The
strike of these faults has not changed and
the Earth is experiencing a compressional
pulse today. This repetition of globally small
but basin scale significant compression is
like a heartbeat, they are all the same and
predictable. Hence past isochores record
today’s SHD’s and SHMagnitudes provided the
amount of synchronous crustal flexing has
been approximately unchanged in magnitude.
Past stresses are the key to the Present.
but SH is estimated or modelled from wellbore
breakouts. Knowing, or reasonably estimating
SV, also gives a quantified value of SH and Sh
from seismic. The result is a 3D stress volume
comprising quantified stress isochores derived
entirely from the interpretation of reflection
seismic. Four horizons are shown at Valhall
but 12 were interpreted from the 3D survey
resulting in detailed pressure-depth graphs.
More horizons not only implies more dense
3D stress, but in this instance, quite accurate
overpressure can be produced. All can be
created pre-drill, from seismic and a known
stratigraphy, alone.
Scales of Applications
Stress Consistent Seismic
Interpretation
Some of the ‘interpretation’ has now gone from
seismic interpretation. The measured SHDs
on a field should be equal to the total survey
interpretation for ‘stress consistent seismic
interpretation’ provided by available software
(Pat.). Sh can be determined from mini fracs
Not all areas have 3D seismic. A regional
stress map of the Permian employing a 2.5 to
25 km 2D line spacing is shown (Figure 2).
When the number of lines in the area of
the yellow rectangle was trebled to a 2 to
5 km line spacing, the enlarged yellow area
map resulted (Figure 3, left). Two almost
perpendicular reverse fault trends generate 4
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a more detailed stress map of SHDs and stress
states. SHDs between crust cutting faults are
calculated on the basis of the relative lengths
and throws of the faults, not by averaging
between diverging measured SHDs. The
seismic method can cause SHD to vary up to
90° over distances as small as 2 km which has
been measured on at least two fields.
Fraccing
All components of the stress map SHD
= SH/‌SV, SH, ShD = Sh/SV and Sh can be
mapped. Of particular interest is a map of
the Earth’s minimum horizontal stress or
fracture gradient (ShM = Sh/SV) which equals
the fracture strength for soft rocks with
essentially zero tensile strength, TO. The ShM
map (Figure 3, right) was derived from the
yellow area. The red areas indicate Sh/SV>1,
a reverse fault stress state in which fracs
would propagate horizontally and have low
Quantified stress
has numerous
applications in
exploration ...
productivity. Vertical fracs will occur in the
green and amber areas where Sh/‌SV<1. Green
is the desirable area for fracs and the amber
is within 3 % of the red undesirable reverse
faults and should be treated with caution.
Fault Seal
In fault seal analysis, consider the geometry
and history of stresses on the fault plane.
Extensional normal faulting is well known for
creating fault gouge and clay smear, thereby
enhancing seal. Intermittent periods of
compression may reverse the throw of the fault
and considerably increase fault gouge, often
sealing sand against sand. Late, steep normal
faulting across post migration thief zones or to
the surface can leak, yet lower angle normal
faults can seal. All these physical aspects of a
fault can be handled by stress from seismic
software but geological models of timing of
migration need to be considered.
Quantified stress has numerous applications
in exploration for fault seal, in drilling for
planning multidirectional and inclined wells
and in production for fraccing old fields where
initial field stress state can only be achieved
accurately with the ‘stress from seismic’ method.
Structural history is now more detailed by
incorporating the fundamentals and new
aspects of global tectonics. Over the past 20
years a wealth of measured stresses, GPS and
geoidal data have added to our early 1990s
concepts of all ridges pushing when the
Atlantic and Indian clearly do not. 
Fig. 3. Left side, a regional to prospect stress state map using treble the number of 2D lines than in the yellow rectangle in Figure 2. Right side, fracture gradient
map derived from the map at left.
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