Salt Tectonics, Associated sedimentary structures and hydrocarbon Traps

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Salt Tectonics, Associated sedimentary
structures and hydrocarbon Traps
presented byAdeniyi Sanyaolu, Dan Sopher,Nick
Shane Cormac OReilly
MSc Exploration Geophysics School of Earth and
Environment University of Leeds, Leeds LS2 9JT
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Topics to be covered

• Depositional environments of Evaporites
• Physical properties of salt
• Salt related structures
• Sedimentary structures associated with salt
• Role of salt in generation of hydrocarbons
• Salt related hydrocarbon traps
• Case study Persian Gulf

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What are Evaporites?
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How are Evaporites Deposited

• Two principle modes of
• Deposition
• Subaqueous Precipitation
• Shallow to deep water
• Evaporating dish process
• Periodic replenishment
• Subaerial Precipitation
• Subkhas
• Sediments around salt lakes
• Oases
• Evaporite minerals include gypsum, sylvite,
  polyhalite, anhydrite, etc.

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Where are Evaporites Deposited?
After Tucker ,1991
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Physical Properties of Salt

• Denisty 0.00215Kg/cm3
• Hardness 2.5 (Mohs)
• Colour clear to white
• Soluble in water
• High Ductility
• High Thermal conductivity
• Flows easily under pressure and at geological
  timescales by either
• Pressure solution
• Dislocation Creep

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SALT TECTONICS

• Salt, which is weak and buoyant is found in
  many sedimentary basins where it occur as a weak
  layer between other lithologies, as such it
  behaves like a pressured viscous fluid during
  deformation and tends to flow.
• Key factors in salt tectonics are
• Buoyancy (density contrast)
• Differential Loading
• Regional Tilt
• The weakness of salt

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SALT FLOW

• A tabular layer of salt can deform either by
  poiseuille flow or couette flow.
• Poiseulli flow involves the vertical thinning of
  overburden and the lateral extrusion of salt from
  under sediment depocenters.
• Couette flow on the other hand corresponds to
  layer parallel simple shear as overlying
  sediments translate seaward

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Extrusion of Salt
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SALT STRUCTURES

• Salt flow or movement results in the formation
  of structures. Salt forms two main types of
  structures
• Salt pillows here the movement of salt results
  in the uplift of overlying lithologies.
• Salt diapirs here the overlying sediments are
  pierced by the moving salt and diarpirs can be
  of different shapes (Walls, columns, bulbs and
  mushrooms).
• The geometry of salt structures is dependent on
  the rate of sedimentation and the rate at which
  the salt flows.

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Salt Dome Growth Stages

• Salt Dome Growth Stages
• Seni Jackson (1984)

Seni Jackson (1984)
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Other processes that enhance salt flow

• A number of processes are known to thin or
  weaken overburden thereby creating paths or
  spaces for salts to move into. These processes
  include
• PASSIVE DIAPIRISM
• MOVEMENT TRIGGERED BY DIFFERENTIAL LOADING
• MOVEMENT TRIGERRED BY EXTENSION
• MOVEMENT TRIGERRED BY CONTRACTION
• MOVEMENT CAUSED BY STRIKE-SLIP FAULTING
• NEAR DIAPIR DEFORMATION
• ALLOCHTHONOUS SALT

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Salt diapirs in seismic section
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Associated Sedimentary Structures
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Peripheral Sinks

• Basins developed due to flow of salt layer.
• Primary Peripheral sink generated far from diapir
  early in development.
• Secondary Peripheral sink generated on
  penetration of the upper layers

After Halbouty, 1967
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Turtles

• Form Between two adjacent Salt diapirs
• Salt flow generates accommodation in the centre
  of the basin
• Continued salt flow leaves anticlinal structures
  that pinch out towards the diapirs Turtles.

After Ordling, 2005
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Unconformities and lateral changes
After Allen ,1992
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Effects of salt on h/c maturation

• Geothermal heat flow is the product of 2 factors
• Thermal gradient
• Thermal conductivity variation with depth
• Thermal conductivity of salt is 3 to 4 times
  greater than that of other sedimentary rocks.
• Salt body will funnel geothermal heat and cause a
  higher temperature anomaly in the surrounding
  rocks.
• Anomaly can be up to 2 to 3 times greater than
  what would normally be expected.

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Effects of salt on h/c maturation

• Geothermal gradients created by salt structures
  may move surrounding rocks into the maturation
  window.
• Factors which effect the geothermal gradient of
  salt are
• (1) size of the salt structure
• (2) geometrical shape of the salt structure
• (3) depth of burial
• Salt structures can produce both positive and
  negative anomalies.
• Oil maturation window Temperatures of 80 c -
  120 c
• Gas maturation window Temperatures of 120 c -
  150 c
• If heat flow anomaly is characterised in detail,
  this can help to better define the geometry of
  the salt body

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Positive and negative anomalies
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Hydrocarbon Traps in Salt Provinces
Salt diapirs were the first diapiric structures
to be recognised and best understood due to their
economic importance.
Doming
Graben
The upturned sediments, truncated against the
impermeable salts, provide excellent traps for
hydrocarbons.
Pinch out
Cap rock
Walling
Walling
Unconformity
Flank Faults
Flank Faults
Figure from Allen Allen (1992)
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Widespread in USA, Mexico, SW Russia, West
Central Africa and Canadian Arctic
   
     
Priority province
     
U.S. province that is ranked among the world
priority provinces
  
Boutique province
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Case Study Persian Gulf
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Case Study Persian Gulf
The dark circular patches represent the surface
expression of salt domes that have risen
diapirically from the Cambrian Hormuz salt
horizon through the younger sediments to reach
the surface. Only in a hot arid environment such
as that of the Gulf can the soluble salt escape
rapid erosion.
Source Landsat 7, NASA (2002)
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Case Study Persian Gulf

• Extensional rifting of Arabic Plate gt basin
  development gt evaporites deposited up to 2.5km
  thick (Hormuz Series) and up to 4km (Oman Salt
  Basin)
• Diapiric movement initiated by extensional and
  strike slip movements of Precambrian basement
  block
• Pathways for salt movement
• - basement faults cut overlying seds (doming
  walls)
• - pull apart from wrench fault deflections
• - reactivation of extensional grabens with salt
  deposits
• - instability of thick salt beds at the foot of
  tilt blocks (gravity glides)
• Pillows.. Rim anticlines.. Turtlebacks..

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Case Study - Persian Gulf
NE
SW
Zagros Reverse Fault
Precambrian Basement
Neoproterozoic Evaporite Basins Develop
Sedimentation continuous Upper Jurassic
evaporite deposits
TIME
Overburden thickens, basement block movements
rejuvenated
Diapirism Upper Jurassic Miocene Cap rocks
faulting and folding
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Turtleback Structures in the Persian Gulf

• Marmul Field, South Oman Salt Basin formed by
  initial salt withdrawal and shallow dissolution.
• Near surface and subsurface meteoric waters
  caused dissolution, evidenced by unconformities

Structural inversion after shallow dissolution
Ara Pillow dissolution
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Reasons for Prolific Hydrocarbons

• Uplift of the Zagros ranges in the Pliocene
• Thick sedimentary sequence (gt18000m) with
  occasional anaerobic intervals, and large basin
• Rich source rocks at several levels
  (Neoproterozoic, Palaeozoic, Jurassic, Lower
  Cretaceous and Lower Tertiary.
• Excellent carbonate (faulted) and sandstone
  reservoir rocks with high permeability and
  porosity
• Cap rocks of salt, anhydrite and shale sealing
  the reservoirs providing multiple stacked
  reservoirs
• Continuous structural growth of growth of major
  folds, due to salt diapirism or basement block
  uplift
• Deep seated diapirism, providing 60 of oil
  field structures in the Basin

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Conclusions

• Salts deform as a viscous fluid with little or
  no ultimate stress and will flow if subjected to
  minimal shear stress. Flow of salt imposes strain
  on other lithologies they are associated with
  forming different structures
• Different salt styles control trap styles in
  supra- and subsalt environments and have varying
  effects on sediment transport, deposition, and on
  hydrocarbon generation and migration. Better
  predictive models for reservoirs will be based on
  improved knowledge of mechanisms of salt
• The presence of salt also effects the maturation
  process of hydrocarbons due to its very high
  thermal conductivity.
• Some 60 of the ultimate recoverable oil
  reserves of the Persian Gulf Basin originate from
  Salt tectonism, and 40 of the known world oil
  reserves are due to salt diapirism in this basin

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References

• Alsop, G. I., Blundell, D. J. Davidson, I.
  (eds), Salt Tectonics, Geological Society Special
  Publications No. 100, 129-151 (1996)
• Jackson, M. P .A Talbot, C. J., Advances in
  Salt Tectonics. In Continental Deformation
  (Edited by Hancock, P. L.), Pergamon Press,
  159-179, (1994)
• Allen, P. A., Allen, J. R., Basin Analysis,
  Blackwell (1992)
• Tucker, M. E., Sedimentary Petrology, Geoscience
  Texts (1991)
• Halbouty, M. T., Salt Domes, Gulf publishing
  company (1967)
• Odling, N., EARS5131 course notes, University of
  Leeds, MSC Exploration Geophysics (2005)
• Nagihara, S., Application of marine heat flow
  data important in oil and gas exploration, (2005)
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• Letouzey, J., Salt movement, tectonic events, and
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  thrust belt. Institut Francais du petrole.(2004)
• Nagihara, S., Regional synthesis of the
  sedimentary thermal history and hydrocarbon
  maturation in the deepwater Gulf of Mexico.
  Department of Geosciences, Texas State University
  (2003)
• Mello, U.T., The role of salt in restraining the
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