Salt Tectonics, Associated sedimentary Salt Tectonics, Associated sedimentary structures and hydrocarbon Trapsstructures and hydrocarbon Traps

MSc Exploration GeophysicsSchool of Earth and Environment

University of Leeds, LeedsLS2 9JT

presented by:presented by:

Adeniyi Sanyaolu, Dan Sopher,Adeniyi Sanyaolu, Dan Sopher,Nick Shane & Cormac O’ReillyNick Shane & Cormac O’Reilly

Topics to be coveredTopics to be covered

Depositional environments of EvaporitesDepositional environments of Evaporites Physical properties of saltPhysical properties of salt Salt related structuresSalt related structures Sedimentary structures associated with Sedimentary structures associated with

saltsalt Role of salt in generation of hydrocarbonsRole of salt in generation of hydrocarbons Salt related hydrocarbon trapsSalt related hydrocarbon traps Case study: Persian GulfCase study: Persian Gulf

What are Evaporites?What are Evaporites?

How are Evaporites How are Evaporites DepositedDeposited

Two principle modes ofTwo principle modes ofDeposition:Deposition:

Subaqueous PrecipitationSubaqueous Precipitation Shallow to deep waterShallow to deep water Evaporating dish processEvaporating dish process Periodic replenishmentPeriodic replenishment

Subaerial PrecipitationSubaerial Precipitation SubkhasSubkhas Sediments around salt Sediments around salt

lakeslakes OasesOases

Evaporite minerals include Evaporite minerals include gypsum, sylvite, polyhalite, gypsum, sylvite, polyhalite, anhydrite, etc.anhydrite, etc.

Where are Evaporites Where are Evaporites Deposited?Deposited?

After Tucker ,1991

Physical Properties of Physical Properties of SaltSalt

Denisty: Denisty: 0.00215Kg/cm0.00215Kg/cm33

Hardness: 2.5 Hardness: 2.5 (Moh’s)(Moh’s)

Colour: clear to whiteColour: clear to white Soluble in waterSoluble in water High DuctilityHigh Ductility High Thermal High Thermal

conductivityconductivity Flows easily under Flows easily under

pressure and at pressure and at geological geological timescales by either:timescales by either: Pressure solutionPressure solution Dislocation CreepDislocation Creep

SALT TECTONICSSALT TECTONICS Salt, which is weak and buoyant is found in Salt, which is weak and buoyant is found in

many sedimentary basins where it occur as a many sedimentary basins where it occur as a weak layer between other lithologies, as such weak layer between other lithologies, as such it behaves like a pressured viscous fluid it behaves like a pressured viscous fluid during deformation and tends to flow.during deformation and tends to flow.

Key factors in salt tectonics are:Key factors in salt tectonics are:

Buoyancy (density contrast)Buoyancy (density contrast) Differential LoadingDifferential Loading Regional TiltRegional Tilt The weakness of salt The weakness of salt

SALT FLOWSALT FLOW

A tabular layer of salt can deform either A tabular layer of salt can deform either by poiseuille flow or couette flow.by poiseuille flow or couette flow.

Poiseulli flow involves the vertical Poiseulli flow involves the vertical thinning of overburden and the lateral thinning of overburden and the lateral extrusion of salt from under sediment extrusion of salt from under sediment depocenters.depocenters.

Couette flow on the other hand Couette flow on the other hand corresponds to layer parallel simple shear corresponds to layer parallel simple shear as overlying sediments translate seawardas overlying sediments translate seaward

Extrusion of SaltExtrusion of Salt

SALT STRUCTURESSALT STRUCTURESSalt flow or movement results in the formation of Salt flow or movement results in the formation of structures. Salt forms two main types of structures:structures. Salt forms two main types of structures:

Salt pillows: here the movement of salt results in Salt pillows: here the movement of salt results in the uplift of overlying lithologies.the uplift of overlying lithologies.

Salt diapirs: here the overlying sediments are Salt diapirs: here the overlying sediments are pierced by the moving salt and diarpirs can be of pierced by the moving salt and diarpirs can be of different shapes (Walls, columns, bulbs and different shapes (Walls, columns, bulbs and mushrooms).mushrooms).

The geometry of salt structures is dependent on the The geometry of salt structures is dependent on the rate of sedimentation and the rate at which the salt rate of sedimentation and the rate at which the salt flows.flows.

Salt Dome Growth Stages

Salt Dome Growth StagesSalt Dome Growth Stages Seni & Jackson (1984)Seni & Jackson (1984)

Seni & Jackson (1984)

A number of processes are known to thin or A number of processes are known to thin or weaken overburden thereby creating paths or weaken overburden thereby creating paths or spaces for salts to move into. These processes spaces for salts to move into. These processes include :include :

PASSIVE DIAPIRISMPASSIVE DIAPIRISMMOVEMENT TRIGGERED BY DIFFERENTIAL MOVEMENT TRIGGERED BY DIFFERENTIAL LOADINGLOADINGMOVEMENT TRIGERRED BY EXTENSIONMOVEMENT TRIGERRED BY EXTENSIONMOVEMENT TRIGERRED BY CONTRACTIONMOVEMENT TRIGERRED BY CONTRACTIONMOVEMENT CAUSED BY STRIKE-SLIP FAULTINGMOVEMENT CAUSED BY STRIKE-SLIP FAULTINGNEAR DIAPIR DEFORMATIONNEAR DIAPIR DEFORMATIONALLOCHTHONOUS SALTALLOCHTHONOUS SALT

Other processes that enhance salt flow

Salt diapirs in seismic Salt diapirs in seismic sectionsection

Associated Sedimentary Associated Sedimentary StructuresStructures

Peripheral SinksPeripheral Sinks Basins developed due to flow of salt Basins developed due to flow of salt

layer.layer. Primary Peripheral sink generated far Primary Peripheral sink generated far

from diapir early in development.from diapir early in development. Secondary Peripheral sink generated on Secondary Peripheral sink generated on

penetration of the upper layerspenetration of the upper layers

After Halbouty, 1967

TurtlesTurtles Form Between two adjacent Salt diapirsForm Between two adjacent Salt diapirs Salt flow generates accommodation in the Salt flow generates accommodation in the

centre of the basincentre of the basin Continued salt flow leaves anticlinal Continued salt flow leaves anticlinal

structures that pinch out towards the diapirs structures that pinch out towards the diapirs “Turtles”.“Turtles”.

After Ordling, 2005

Unconformities and lateral Unconformities and lateral changeschanges

After Allen ,1992

Effects of salt on h/c maturationEffects of salt on h/c maturation

• Geothermal heat flow is the product of 2 factors:

(1) Thermal gradient

(2) 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.

Effects of salt on h/c maturationEffects of salt on h/c maturation

Geothermal gradients created by salt structures may move Geothermal gradients created by salt structures may move surrounding rocks into the maturation window.surrounding rocks into the maturation window.

Factors which effect the geothermal gradient of salt are:Factors which effect the geothermal gradient of salt are:

(1) size of the salt structure(1) size of the salt structure

(2) geometrical shape of the salt structure(2) geometrical shape of the salt structure

(3) depth of burial(3) depth of burial Salt structures can produce both positive and negative anomalies.Salt structures can produce both positive and negative anomalies. Oil maturation window : Temperatures of 80 Oil maturation window : Temperatures of 80 °c - 120°c - 120 °c°c

Gas maturation window : Temperatures of 120 Gas maturation window : Temperatures of 120 °c - 150°c - 150 °c°c

If heat flow anomaly is characterised in detail, this can help to If heat flow anomaly is characterised in detail, this can help to

better define the geometry of the salt body better define the geometry of the salt body

Positive and negative anomaliesPositive and negative anomalies

Salt diapirs were the first diapiric structures to be recognised and best understood due to their economic importance.

The upturned sediments, truncated against the impermeable salts, provide excellent traps for hydrocarbons.

Hydrocarbon Traps in Salt Hydrocarbon Traps in Salt ProvincesProvinces

Doming

Graben

Pinch out

Walling

Walling

Unconformity

Cap rock

Flank Fault

sFlank Fault

s

Figure from Allen & Allen (1992)

Widespread in USA, Mexico, SW Russia, West Central Africa and Canadian Arctic…

Boutique provinceBoutique province    

U.S. province that is ranked among the world priority provincesU.S. province that is ranked among the world priority provinces            

Priority provincePriority province          

       

Case Study – Persian GulfCase Study – Persian Gulf

Case Study – Persian GulfCase 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)

Case Study – Persian GulfCase 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..

NESW

Neoproterozoic Evaporite Basins Develop

Zagros Reverse FaultPrecambrian Basement

Overburden thickens, basement block movements rejuvenated

Sedimentation continuous + Upper Jurassic evaporite deposits

Diapirism + Upper Jurassic + Miocene Cap rocks + faulting and folding

Case Study - Persian GulfCase Study - Persian Gulf

TIM

E

Turtleback Structures in the Persian Turtleback Structures in the Persian GulfGulf

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

Ara Pillow dissolution

Structural inversion after shallow dissolution

Reasons for Prolific HydrocarbonsReasons 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

ConclusionsConclusions• 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

ReferencesReferences Alsop, G. I., Blundell, D. J. & Davidson, I. (eds), Salt Tectonics, Alsop, G. I., Blundell, D. J. & Davidson, I. (eds), Salt Tectonics,

Geological Society Special Publications No. 100, 129-151 Geological Society Special Publications No. 100, 129-151 (1996)(1996)

Jackson, M. P .A & Talbot, C. J., Advances in Salt Tectonics. In: Jackson, M. P .A & Talbot, C. J., Advances in Salt Tectonics. In: Continental Deformation (Edited by Hancock, P. L.), Pergamon Continental Deformation (Edited by Hancock, P. L.), Pergamon Press, 159-179, (1994)Press, 159-179, (1994)

Allen, P. A., & Allen, J. R., Basin Analysis, Blackwell (1992)Allen, P. A., & Allen, J. R., Basin Analysis, Blackwell (1992) Tucker, M. E., Sedimentary Petrology, Geoscience Texts (1991)Tucker, M. E., Sedimentary Petrology, Geoscience Texts (1991) Halbouty, M. T., Salt Domes, Gulf publishing company (1967)Halbouty, M. T., Salt Domes, Gulf publishing company (1967) Odling, N., EARS5131 course notes, University of Leeds, MSC Odling, N., EARS5131 course notes, University of Leeds, MSC

Exploration Geophysics (2005)Exploration Geophysics (2005) Nagihara, S., Application of marine heat flow data important in Nagihara, S., Application of marine heat flow data important in

oil and gas exploration, (2005)oil and gas exploration, (2005) Shaker, S.S., Geopressure compartmentilization in salt basins: Shaker, S.S., Geopressure compartmentilization in salt basins:

their assessement for hydrocarbon entrapment in the gulf of their assessement for hydrocarbon entrapment in the gulf of Mexico, Geopressure Analysis Services (2004)Mexico, Geopressure Analysis Services (2004)

Letouzey, J., Salt movement, tectonic events, and structural Letouzey, J., Salt movement, tectonic events, and structural style in the central Zagros fold and thrust belt. Institut style in the central Zagros fold and thrust belt. Institut Francais du petrole.(2004)Francais du petrole.(2004)

Nagihara, S., Regional synthesis of the sedimentary thermal Nagihara, S., Regional synthesis of the sedimentary thermal history and hydrocarbon maturation in the deepwater Gulf of history and hydrocarbon maturation in the deepwater Gulf of Mexico. Department of Geosciences, Texas State University Mexico. Department of Geosciences, Texas State University (2003)(2003)

Mello, U.T., The role of salt in restraining the maturation of Mello, U.T., The role of salt in restraining the maturation of subsalt source rocks (2000) subsalt source rocks (2000)