1. Salt Tectonics, Associated sedimentary structures and hydrocarbon Traps
presented by: Adeniyi Sanyaolu, Dan Sopher, Nick Shane & Cormac O’Reilly
MSc Exploration Geophysics
School of Earth and Environment
University of Leeds, Leeds
LS2 9JT

2. 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

3. What are Evaporites?

4. 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.

5. Where are Evaporites Deposited?
After Tucker ,1991

6. Physical Properties of Salt
Denisty: Kg/cm3
Hardness: 2.5 (Moh’s)
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

7. 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

8. 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

9. Extrusion of Salt

10. 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.

11. Salt Dome Growth Stages
Seni & Jackson (1984)
Seni & Jackson (1984)

12. 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

13. Salt diapirs in seismic section

14. Associated Sedimentary Structures

15. 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

16. 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

17. Unconformities and lateral changes
After Allen ,1992

18. 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.

19. 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 °c
Gas maturation window : Temperatures of 120 °c °c
If heat flow anomaly is characterised in detail, this can help to better define the geometry of the salt body

20. Positive and negative anomalies

21. 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)

22. 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

23. Case Study – Persian Gulf

24. 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)

25. Case Study – Persian Gulf
Extensional rifting of Arabic Plate > basin development > 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..

26. Case Study - Persian GulfSW NE
Precambrian Basement
Zagros Reverse Fault
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

27. 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

28. Reasons for Prolific Hydrocarbons
Uplift of the Zagros ranges in the Pliocene
Thick sedimentary sequence (>18000m) 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

29. 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

30. References
Alsop, G. I., Blundell, D. J. & Davidson, I. (eds), Salt Tectonics, Geological Society Special Publications No. 100, (1996)
Jackson, M. P .A & Talbot, C. J., Advances in Salt Tectonics. In: Continental Deformation (Edited by Hancock, P. L.), Pergamon Press, , (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)
Shaker, S.S., Geopressure compartmentilization in salt basins: their assessement for hydrocarbon entrapment in the gulf of Mexico, Geopressure Analysis Services (2004)
Letouzey, J., Salt movement, tectonic events, and structural style in the central Zagros fold and 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 maturation of subsalt source rocks (2000)