Transcript Group7new - Stanford Exploration Project

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
Topics to be covered
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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
What are Evaporites?
How are Evaporites Deposited
Two principle modes of
Deposition:
Subaqueous Precipitation
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Subaerial Precipitation
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Shallow to deep water
Evaporating dish process
Periodic replenishment
Subkhas
Sediments around salt lakes
Oases
Evaporite minerals include gypsum,
sylvite, polyhalite, anhydrite, etc.
Where are Evaporites Deposited?
After Tucker ,1991
Physical Properties of Salt
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Denisty:
0.00215Kg/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:
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Pressure solution
Dislocation Creep
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:
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Buoyancy (density contrast)
Differential Loading
Regional Tilt
The weakness of salt
SALT FLOW
A tabular layer of salt can deform either by
poiseuille flow or couette flow.
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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
Extrusion of Salt
SALT STRUCTURES
Salt flow or movement results in the formation of
structures. Salt forms two main types of
structures:
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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.
Salt Dome Growth Stages
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Salt Dome Growth Stages
Seni & Jackson (1984)
Seni & Jackson (1984)
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
Salt diapirs in seismic section
Associated Sedimentary
Structures
Peripheral Sinks
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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
Turtles
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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
Unconformities and lateral
changes
After Allen ,1992
Effects of salt on h/c maturation
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Geothermal heat flow is the
product of 2 factors:
(1) Thermal gradient
(2) Thermal conductivity
variation with depth
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Thermal conductivity of salt
is 3 to 4 times greater than
that of other sedimentary
rocks.
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Salt body will funnel
geothermal heat and cause
a higher temperature
anomaly in the surrounding
rocks.
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Anomaly can be up to 2 to 3
times greater than what
would normally be expected.
Effects of salt on h/c maturation
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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
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Gas maturation window : Temperatures of 120 °c - 150 °c
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If heat flow anomaly is characterised in detail, this can help to better define
the geometry of the salt body
Positive and negative anomalies
Hydrocarbon Traps in Salt Provinces
Salt diapirs were the first diapiric structures to be
recognised and best understood due to their
economic importance.
Doming
The upturned
sediments, truncated
against the
impermeable salts,
provide excellent traps
for hydrocarbons.
Graben
Pinch out
Cap rock
Walling
Flank
Faults
Walling
Unconformity
Flank
Faults
Figure from Allen & Allen (1992)
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
Case Study – Persian Gulf
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)
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..
Case Study - Persian Gulf
SW
NE
Precambrian Basement
Zagros Reverse Fault
Neoproterozoic Evaporite Basins Develop
TIME
Sedimentation continuous
+ Upper Jurassic evaporite
deposits
Overburden thickens, basement block movements rejuvenated
Diapirism + Upper Jurassic + Miocene Cap rocks + faulting and folding
Turtleback Structures in the Persian Gulf
Marmul Field, South Oman Salt Basin – formed by initial salt
withdrawal and shallow dissolution.
Structural inversion
after shallow dissolution
Ara Pillow dissolution
Near surface and subsurface meteoric waters caused dissolution,
evidenced by unconformities
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
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
References
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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)
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)