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THRUST FAULTS: ASSOCIATED

STRUCTURES AND IMPLICATIONS IN

HYDROCARBONS TRAPS

James MooreAlex NyombiChristian HidalgoAdekunle Odutola

STRUCTURE AND HYDROCARBON PROSPECTIVITY OF BASINS

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1. A brief introduction to Thrust Faults

2. Define Tectonics Settings involving Thrust Faults formation

3. Review structures associated with Thrust Faults

4. Highlight two examples of Hydrocarbon Traps associated with Thrust Faults

5. Summarise

Aims & Objectives

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• The are very damaging. Host of the largest and potentially most destructive earthquakes

• Associated with mountain building and collisional tectonics

• Influence positions of ore deposits and hydrocarbons

Why Study Thrust Faults ?

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• They are reverse faults with low dip that form in layered sequences where horizontal stress is greater than vertical stress.

• They are formed by compressive stresses

• Thrust faults are dip-slip faults

• They have dip less than 45°

What Are Thrust Faults ?

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What Are Thrust Faults ?

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• Thin skinned thrusts developed within sedimentary basins

• Basement may act as a basal detachment• Basement is not involved• Typical of leading edges of compressive orogens

1. Thin-Skinned Thrusts

Major Types of Thrust Faults

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Major Types of Thrust Faults

2. Thick-Skinned Thrusts

• Thick skinned thrusts cut upward through the basement and propagate through the sediment

• Thick skinned thrusts usually steepen upwards and are covered by drape folds

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Rules For Thrust Development

• Thrust always cut up-section in the direction of transport

• Thrust always place older over younger rocks

• Higher thrust develop first• Thrusting proceeds from higher to lower and

from hinterland towards foreland

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Dynamics of Thrust Motion

Conditions for thrust movement include overcoming:

• Frictional resistance to sliding on the basal fault surface

• Gravitational forces resulting from the mass of the sheet

Hypothesis supporting thrust movement include:

• Gravity sliding hypothesis

• Fluid pressure hypothesis

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Mechanisms of Thrusting

• They are formed by compressive tectonic stresses and occur at convergent plate boundaries.

• Ϭ1 is horizontal and Ϭ3 is vertical.

• The two sets of thrust faults intersect along the horizontal Ϭ2 axis with dip less that 45°.

• Horizontal contraction parallel to Ϭ1

and vertical extension parallel to Ϭ3

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Components of a Thrust System

• Foreland is the area in front of the thrust belt

• Hinterland is the area behind the thrust belt

• Imbricate fan: Individual thrust sheets overlap like roofing tiles

• Duplex: system of imbricate thrusts that branch off from a single fault below and merge with a thrust fault above. The rock body bounded by faults above and below is called a horse

• Ramps & Flats: Faults consist of flats parallel to bedding (surface of weakness) and ramps where the fault cuts across the bedding

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Components Of Thrust Faults

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• Thrust faults accommodate shortening of the earth's crust

• Most big mountain ranges are fold-and-thrust belts

• Consists of a set of folds and thrust faults that extend for 10s to 100s of kilometres along strike

• Usually shortening takes place near the plate boundary in convergent margins: (Himalayas, Alps, Appalachians, Urals)

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Tectonic Setting of Thrust Fault

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Formation of Duplex Structure

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Formation of Thrust Duplex

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Structures Associated with Thrust Faults

Foreland thrust and fold belts

• Belts of deformed sedimentary rock in which the layers are folded and duplicated by thrust faults are common at the edges of orogens

• Folds and thrust faults are the dominant structures

• Results in the formation of foreland basin and “piggy- back basin”

• Classic examples occur in: the Main Ranges, Valley and Ridge province of the Appalachians, and the Jura Mountains in the Alps

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Foreland Thrust And Fold Belt

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• Fault bend folds

• Detachment folds

• Fault propagation fold

Folds Associated with Thrust Faults

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Thrust Faults:

This example shows sediment deposited by a glacier 10,000 years ago.

When the glacier moved back over the sediments, faulting occurred. A glacial ‘readvance.’

Thrust Faults as Hydrocarbon Traps: Hypothetical Example

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Thrust Faults as Hydrocarbon Traps: Hypothetical Example

Rock sequence is thrusted: creates a possible hydrocarbon trap.

Sandstone: source rock, Shale: seal rock.

Hypothetical situation set up below.

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Thrust Faults as Hydrocarbon Traps: Nepal

Location of Nepal

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Thrust Faults as Hydrocarbon Traps: Nepal

Traps can be seen in the Siwalk Fold Belt.

A blind thrust is also seen. This can also act as a trapping mechanism.

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Thrust Faults as Hydrocarbon Traps: Moose Mountain, Canada

McConnell Thrust

Moose Mountain

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Thrust Faults as Hydrocarbon Traps: Moose Mountain, Canada

“Moose Mountain” thrust sheet is defined as the uppermost thrust sheet which strata are exposed at the surface.

This thrust essentially placed a single repetition of the entire Cambrian through Cretaceous sequence in the hanging wall, over the footwall.

Strata in the footwall of the Moose Mountain thrust range from Paleozoic to the Lower Cretaceous.

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Summary

• They are found in compressional settings like orogenic belts, salt domes and diapiric structures in general

• Repeat stratigraphy, puts older rocks on top of younger rocks and contracts layering

• They closely associated with folds and thrust belts

• 2 major types of Thrust faults: Thin-Skinned Thrusts and Thick-Skinned Thrusts.

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Summary

• Components of a Thrust System : Foreland, Hinterland, Imbricate fans, Duplexes, Ramps & Flats

• When rock sequence is trusted: creates a possible hydrocarbon trap

• Two examples of thrust faults forming a structural trap for hydrocarbons: Nepal, Moose Mountains

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Questions ?

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References

• Park, R.G. Foundations of Structural Geology, Chapman & Hall. pp 95 – 96, 1997.

• Twiss, R. J. and E. M. Moores, Structural Geology, W. H. Freeman & Co., New York, 256p, 1992.

• http://vle.leeds.ac.uk/site/nbodington/earthscience/fifth/msc_struc/• http://ic.ucsc.edu/~casey/eart150/Lectures/4ThrustFlts/

4thrustfaults.htm• http://courses.eas.ualberta.ca/eas421/lecturepages/thrust.html• http://earth.leeds.ac.uk/faults/thrust/tipline/fbfold.htm• http://www.colorado.edu/geolsci/courses/GEOL3120/3120lect_13.pdf (Taken from

http://www.cseg.ca/conferences/2000/2000abstracts/505.PDF)