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NORMAL FAULTS, ASSOCIATED
STRUCTURES AND
HYDROCARBON TRAPS.
GROUP 3
•RACHEL EVANS
•FRANCIS EZEH
•CHU’KA CHIZEA
•NICK PAPANICOLAOU
•Supervisor: Dr Noelle Odling
School of Earth Science
University of Leeds.
December, 2005
PRESENTATION OUTLINE
• NORMAL FAULTS
• TYPES
• CHARACTERISTICS
• GEOMETRIES
• FORMATION OF NORMAL FAULTS
• STRESS AND STRAIN REGIMES
• BASINS & ASSOCIATED STRUCTURES
• HYDROCARBON STRUCTURES AND
PROSPECTIVITY
• CASE EXAMPLES
• DISCUSSION AND CONCLUSIONS
Definition of normal faults
• Hanging wall moves down relative to the
footwall.
o.
• Fault surface dips more steeply than 45
• Created under tensional stress
ASSOCIATED NOMENCLATURE
•
•
FOOTWALL IS THE BLOCK
BELOW THE FAULT PLANE
HANGING WALL IS THE BLOCK
ABOVE THE FAULT PLANE
• HEAVE IS THE MAXIMUM
HORIZONTAL DISPLACEMENT
•
•
THROW IS THE MAXIMUM
VERTICAL DISPLACEMENT
DIP IS THE ANGLE BETWEEN
THE FAULT PLANE AND
HORISONTAL
(Butler, R. 2003)
TYPES OF NORMAL FAULTS
•
•
•
•
LOW ANGLE NORMAL FAULTS
LISTRIC
GROWTH FAULTS
DOMINO AND IMBRICATE NORMAL
FAULTS.
• CONJUGATE NORMAL FAULTS.
LOW ANGLE NORMAL FAULTS.
• Detachment confined to crust: Extension
Balanced by compression.
• Gulf Coast and Perido Fold Belt.
• Detachment Fault cuts whole lithosphere:
Ductile shear zone at 10-15km.
• Basin and Range Pronvince.
(Twiss, R.J & Moore, E.M.
1992)
GULF COAST EXAMPLE
(Twiss, R.J & Moore, E.M. 1992)
BASIN AND RANGE PROVINCE
(Twiss, R.J & Moore, E.M. 1992)
(Butler, R. 2003)
GROWTH FAULTS
• Form at same time as sedimentation (synsedimentary).
• Sediment thickness decreases away from normal
faults.
• Fault dip shallows with increasing depth.
• Associated with roll-over anticlines in syndepositional settings.
• Also associated with synthetic and antithetic faults.
• Forms collapsed crest structures when detached
faults can’t accommodate sediment load.
• Growth index (ratio of sediments on both sides of
major growth faults).
(Twiss, R.J & Moore, E.M. 1992)
CONJUGATE NORMAL FAULTS
Fault planes dip towards each other.
STRESSES
• Stress is a pair of equal forces acting on a unit
area of a body.
– The magnitude of the stress is:
Stress = Force / Area
• The force of gravity can give rise to a stress
• Gravity makes an important contribution to the
stress field governing the formation of faults and
folds.
• A force F acting on a surface within a body can be
resolved into:
– a normal stress (σ)
– a shear stress (τ).
• Principal stress planes,
• Principal stress axes,
– σ1 greatest
– σ2 intermediate
– σ3 least.
• Where the principal stresses are equal, the state of
stress is said to be hydrostatic.
– at depth, hydrostatic pressure is termed lithostatic.
STRESS AXES AND FAULTS
σ1
σ3
σ3
σ2
STRESS AXES AND FAULTS
σ1
σ3
σ3
σ2
MOHR DIAGRAM
• Can obtain:
– Maximum & minimum stresses
– Orientation of principal axes (note that the angle is 2Θ)
– The angle in which failure occurs
FAILURE CRITERIA
• τ = c + μ * σ ( Coulomb criterion )
• τ2 = I 4σt (σt + σ) I ( Griffith’s criterion )
STRAIN
• Is the geometrical expression of the amount of
deformation caused by the action of a system of
stresses on a body.
• Strain is the change
– in shape (distortion) and
– in volume (dilation),
– or a combination.
• If the amount of strain in all parts of a body is
equal then it is called homogenous strain
• In the case of heterogeneous strain, straight
lines become curved and parallel lines nonparallel.
• Strain can be measured in two ways:
– Either by a change in length of a line (linear
strain or extension)
– Or by a change in the angle between two
lines (angular strain or shear strain)
• The principal strain axes are projected in
an ellipsoid, the strain ellipsoid, which
can be regarded as a deformed sphere.
• e1,max, e2,inter and e3,min represent the
stretches along the axes and are known
as principal strains.
MATERIAL BEHAVIOR
• It is dependant on several factors.
Amongst them are:
– Temperature
– Confining Pressure
– Strain rate (time)
– Composition
– Another aspect is the presence or absence
of water.
When a rock is subjected to increasing stress it passes
through 3 successive stages of deformation.
• Elastic Deformation –
– wherein the strain is
reversible,
• Ductile Deformation –
– wherein the strain is
irreversible,
• Fracture –
– irreversible strain,
wherein the material
breaks.
STRENGTH OF THE MATERIALS
• Two limiting stresses can be mentioned
here:
• Yield strength = above which permanent
deformation occurs
• Failure strength = above which failure
occurs
STRAIN RATE
• In laboratory experiments the effect of the
applied stress is instantaneous,
• Whereas in nature, the same effect will
probably take more than tenths, hundreds,
millions of years…?
• So aside of the failure of the materials
there will be elastic flow, deformation,
e.t.c.
ASSOCIATED BASINS AND REGIONAL
STRUCTURES
• How all the previous statements correlate to
basins?
• BASINS ASSOCIATED WITH EXTENSIONAL
DYNAMICS
• RIFT BASINS
• MID-OCEAN RIDGES
STRUCTURES ASSOCIATED WITH
NORMAL FAULTS.
• FOLDS.
• Rollover Anticlines
• Drag Folds
• COMMONLY PRESENT AS SYSTEMS OF
MANY ASSOCIATED FAULTS.
• Synthetic faults
– Usually smaller and parallel to the major fault and have same
direction of dip.
• Antithetic faults
– In conjugate orientation to major faults and have opposite dip.
• Ring faults
– Concentric normal faults developed as surficial rock collapse
into subsurface cavity: Calderas.
• Strike-Slip faults
• HORSTS AND GRABENS
SYNTHETIC AND ANTITHETIC
(Allen, P.A. & Allen, J.R. 1990)
HORSTS AND GRABEN
• Due to the tensional stress responsible for
normal faults, they often occur in a series, with
adjacent faults dipping in opposite directions.
• In such a case the down-dropped blocks form
grabens and the uplifted blocks form horsts.
• In areas where tensional stress has recently
affected the crust, the grabens may form rift
basins and the horsts may form linear mountain
ranges.
HORST AND GRABEN TOPOGRAPHY
• The East African Rift Valley is an example of an area
where continental extension has created such a rift.
• The basin and range province of the Western U.S. is
also an area that has recently undergone crustal
extension.
ASSOCIATION OF HYDROCARBONS
WITH NORMAL FAULTS
• Settings for normal fault traps occur in two
principal geological settings:
• Fault-bounded grabens or half grabens
• Forelands of compressional basins
• Faults are rarely on their own a trapping
mechanism but can have intrinsic association
with other trap geometries.
• Percentage of potential structures decrease
rapidly with increasing distance from faults.
TRAPPING STRUCTURES
ASSOCIATED WITH NORMAL FAULTS
(North, F.K. 1985)
Fluid seals or conduits?
• Faults can act to both inhibit and enhance
fluid flow.
• Act as conduits for flow (not traps)
• During reactivation of faults (after accumulation)
• If potentiometric surface of the reservoir rock is
above the topography.
• More likely to act as fluid seals
• Gouge/natural mudcake
• Fluid pressures on either side of faults
• Juxtaposition of lithologies
Standard Juxtaposition
• Common possible juxtapositions of
lithologies across faults include:
• The reservoir rock juxtaposed across the fault
within the hydrocarbon column.
• One sandstone body juxtaposed across the fault
not within the hydrocarbon column.
• Two different reservoirs juxtaposed across the fault
within the hydrocarbon column.
• The reservoir rock juxtaposed across the fault with
another lithology.
Hydrocarbon Prospectivity
(North, F.K. 1985)
(Allen, P.A. & Allen, J.R. 1990)
Hydrocarbon Production
• Although normal faults
can be useful to act as
traps they can also
restrict the production
rate of a hydrocarbon
reservoir.
Miri Oilfield, NW Borneo. Numerous oil
accumulations, generally on the
upthrown side of antithetic faults.
(North, F.K. 1985)
CASE EXAMPLE (1): NIGER DELTA
BASIN
• COMPOSED OF THREE MAJOR LITHOLOGIC UNITS.
• GROWTH FAULTS DEVELOP DUE TO INTERPLAY OF
SUBSIDENCE AND SEDIMENTATION (Weber., et al 1978).
• LARGE REGIONAL GROWTH FAULTS DIVIDE BASIN INTO
DISCRETE DEPOBELTS.
• HYDROCARBON RESERVOIRS ARE FORMED BY THE
DEVELOPMENT OF ROLLOVER ANTICLINES.
• RESERVOIRS ARE ALSO FORMED BY THE JUXTAPOSITION OF
SANDS AGAINST SHALE AS SECONDARY GROWTH FAULTS
AND ANTITHETIC FAULTS DEVELOP.
• MAJOR GROWTH FAULTS SERVE AS MIGRATION PATHWAYS.
(North, F.K. 1985)
• RESEVOIRS BOUNDED BY MAJOR GROWTH
FAULTS.
• RESEVOIRS ALSO DEVELOPS AT CREST OF
ROLLOVER ANTICLINES.
CASE EXAMPLE (2): VIKING GRABEN,
NORTH SEA.
(Glennie, K.W. 1998)
CASE EXAMPLE (2): VIKING GRABEN,
NORTH SEA.
• Plays are grouped according to reservoir age and their relationship
to 3 main rift-related tectonic phases relative to the main Late
Jurassic rift event.
• Close relationship between play type and rifting.
• Rifting controlled the thickness and facies distribution in the upper
Jurassic syn-rift succession, including its widespread organic-rich
marine source rock (Glennie., 1998).
• Rifting also determined the distribution of mature Upper Jurassic
source rocks following post-rift thermal subsidence (Glennie., 1998).
CONCLUSIONS
Normal faults:
• Influence the distribution of sediment infill (e.g.
Viking Graben).
• Can be important in hydrocarbon exploration.
• Can form the trapping structure for hydrocarbon
reserves when acting as seals; usually in
combination with other structures.
• Can also make production of hydrocarbon
reservoirs more difficult.
REFERENCES
• Allen, P.A. & Allen, J.R. (1990) Basin Analysis Principles &
Applications. Blackwell.
• Butler, R. et al. (2003). www.earth.leeds.ac.uk/learnstructure
• Glennie, K.W. (1998) Petroleum Geology of the North Sea:basic
concepts & recent advances. Blackwell Science: 4th edition.
• North, F.K. (1985). Petroleum Geology. Allen & Unwin.
• Twiss, R.J. & Moores, E.M. (1992) Structural Geology.
W.H.Freeman
• Weber, K.J. et al. The role of faults in hydrocarbon migration and
trapping in Nigerian growth fault structures, 2643-2651, Offshore
Tech. Conf., Houston paper OTC 3356.