Lecture#9 Structural geology.ppt

7/28/2019 Lecture#9 Structural geology.ppt

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Lecture #9Structural Geology

Jasmi Ab TalibGeoscience & Petroleum Engineering Department

Physical GeologyQAB1013

Learning Outcomes

Students should be able to: Know the conceptual deformation to createvarious kind of structure.Understand the stress – strain relationship withmore emphasis on ductile and brittledeformation.Familiarize the mega structure such as fold, faultand unconformity.

Appreciate the importance and usefulness of structural geology in petroleum trap.

Structural Geology - Introduction

Structural Geology is the study of thearchitecture of the earth’s crust, itsdeformational features and their mutualrelations and origin.Structural Geology can be defined as abranch of geology concerned with theshapes, arrangements, and inter-relationships of bedrock units and theforces that cause them.

Geologic Sructures - IntroductionWhy an understanding and knowledge of Structural

Geology is important?To understand earthquake for example, one must know about faults. Appreciating how major mountain belts and the continent haveevolved calls for a comprehension of faulting and folding.Understanding plate-tectonic theory as a whole also requires aknowledge of structural geology In areas of active tectonics, the location of geologic structure is very important in selection of suitable sites for buildings, dams, highway,bridge, tunnels, nuclear power plants, etc.Understanding structural geology can help us more fully appreciatethe problem of finding more of the earth’s natural resources, such as

metal ores, petroleum & gas, rock aggregates, etc.The knowledge of structural geology is also very important ingeohazards (landslide, earthqukae, tsunami, subsidence, erosions,etc) mitigation and control measures.

Kind s o f rock b ehav io r Bri t t l e - material breaks when stressed

Duct i le - material changes shape permanentlywhen stressedElastic - material changes shape temporarilywhen stressed, then breaks (at the elastic limit)

In f luenc es o n rock behav ior Temperature - enhances ductile behavior Pressure - enhances ductile behavior Time (rate at which stress is applied) - enhances

ductile behavior Rock ch arac te r is t ics – variable

In g eneral , the upp er cru st i s m ore br i t t le than th e low er c rus t because i t i s coo ler and un der less p ressu re.

Stresses

• Compressive Stress –

• pushed together or squeezed from opposite directions.

• common along convergent plate boundaries; typically results inrocks being deformed by a sho r ten ing s t rain ;

• Tensional Stress – • Forces pulling away from one another in opposite directions;

results in a stretching or extens io nal s t ra in

• Quite rare in the earth crust

• Shear Stress –sho r ten ing s t ra in ;

• Due to movement parallel but in opposite directions along a faultor other boundary

• Results in a sh ear s train parallel to the direction of the stresses.

• Notable along transform plate boundaries and along other activelymoving faults.

extension al s t ra in

sh ear s t ra in

sho r ten ing s t rain

Forces tha t cause deform ation

Ducti le

Br i t t le

Convergent Divergent Transform

Behaviour of Rocks: Stress & Strain

Rocks behave as elastic, ductile, or brittle materials depending on theamount and rate of stress applied, the type of rock, and thetemperature and pressure under which the rock is strained.

Elastic – if a deformed body recovers its original shape after the stress isreduced or removed (e.g. rubber). Rocks can behave in an elastic way at verylow stresses, however once the stress exceeds the elastic l im it the rock willdeform permanently.

Ductile – a rock that behaves in a ductile or plastic manner will bend whileunder stress and does not return to its original shape after relaxation of thestress. Under high pressure & temperature (e.g. during regionalmetamorphism) rocks behave in a ductile manner. Ductile behaviour results in

folding or bending or rock layers.Brittle – a rock exhibiting brittle behaviour will break or fracture at stress

higher that its elastic limit. Rock typically exhibit brittle behaviour at or near theearth’s surface where pressure & temperatures are low. Faults and joints areexamples of structures that form by brittle behaviour of the crust.

Behaviour of Rocks to Stress & Strain

Behaviour of rocks with increasing stress and strain .• Elastic behaviour occurs along the straight line portions (blue)• At stresses greater than the elastic limit (red points) the rock will

either deform as a ductile material or break, as shown in thedeformed rock cylinders.

Stress - a force per unitarea at a particular point.

Strain - the change insize (volume) or shape, or both, while an object is

undergoing stress.

Brittle Deformation

Fault & joint

12Folding

Ductile Deformation

13Crenulations

Ductile Deformation

Crenulations

Assimilation

Ductile Deformation

Assimilation

Basic Geometry of Folds

Syncline and Anticline

Terms AnticlineSynclineLimb

Axial planeHinge Lines/Foldaxes

Block Diagram of Folded Rocks

Folded Rock

Note:• Plan view – geological map• Side view – geologic cross sections

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Types of Folds

Folds occur in many varieties and sizes. A number of fold classification schemescan be applied to describe folds ( refer to any

Structural Geology text books ). A simple types of folds are given in thefollowing slides…

• Plastic deformation • Sufficient stress applied will bend (not

break) rock• Folds result

Components of a fold

Axis/Nose: most sharply curved part• Limb: sides of a fold

Kind s o f fo ld se t s

Folded Rock Before Erosion

Folded Rock After Erosion

Eroded Anticline, older rocks in center. Syncline is opposite.

3-D: Dome and Basin

• Other types of folds – Basin• Circular or slightlyelongated structure

• Downwarpeddisplacement of rocks• Youngest rocks are foundnear the center, oldestrocks on the flanks

Faults and Oil

Anticlines and Oil

Early USApetroleumexploration, e.g.Pennsylvaniaanticlines

Fractures in Rock

If a rock is brittle, or if the strain rate is too greatfor deformation to be accommodated by plasticbehaviour, the rock fractures.Types of fractures in rock:

Joints - Fractures without @ no movement parallel tofracture surface. Often occur in sets, e.g. prominent,minor, conjugate jointFaults - fractures along which large amounts (m - kms)of movement has occurred, e.g. horizontal, normal,

reverse fault. Shear Zones - fractures along which a small amount(cms) of movement has occurred. BUT not all shear zones are fractured depending to their behaviour.

Horst and Graben Formation

Gräben (Rift Valley)

depression formed by subsidence (dropping) of landbetween two faults due to tension

• raised block of land formed by uplift of land between two faults due to

compression

Horst (opposite of a Gräben)

Rift Valley development

Types of Rock Fractures

JointsFaults

Fractures in rock

• Joints – break in roc k a lon g w hich n o m ovement has taken place

• Columnar jointing • Sheet jointing • Joint set

versus

• Faults – break in rock along which movement has taken place

T f R k F

Types of Rock Fractures

Joints

JOINTS• Joints are fracture or crack in bedrock,

with no displacement occurs along the

fracture• Most rock at or near the surface isbrittle, so nearly all exposed bedrockis jointed to some extent.

• Joints normally occur in sets , in whichthe individual joint planes are oriented

nearly parallel to one another withinthe same set.• Common to all rocks exposed at the

surface; indicates that the causes aremany and varied

• Tectonic activity ; mountain building

(orogenics), plate interactions, etc.• Non-tectonic stresses - shrinkage

due to cooling or drying; e.g.Columnar basalt

• Expansion due to release of pressure - very common at

surface; e.g exfoliation joints

Joints

• Brittle rocks fracture in response to stress• No movement on either side of

fracture• Stresses on large scale multiplefractures

Joints

The significant of joints

Valuable ore deposits (e.g.gold, tin, etc) are oftenprecipitate within themineralised veins that filled upthe joints.

Accurate information about joints are also very important inthe planning and constructionof large engineering projects(e.g highways, dam,reservoirs, etc.).

Good for fractured petroleumreservoir. The joint sets

FAULTSFaults are fractures in bedrock alongwhich movement has taken place.The displacement may be onlyseveral cms or hundreds of kms.

Active faults – movements hastaken place during the last 11,000years. But most faults are no longer

active.Faults are identified from thedislocated beds, broken or highlyfractured or pulverized rock masssandwiched between the displacedblocks.Single breaks Fault PlaneComplex zones of shearing FaultZone Crushed rock

due to faultmovement –

“fault breccia”

Faults in the Field…..

Faults in the Field …..

Faults

Termino logy:Footwall – the underlying surface of an inclined fault

plane.

Hanging Wall – the overlying surface of an inclinefault plane.Types of dip slip faultsare distinguished basedon the relative

movement of thefootwall block and thehanging wall block.

Faults

THRUST FAULT

(reverse fault withLow angle < 30 deg.)

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Disconformity

y An unconformity between parallel layers of sedimentary rocks which represents aperiod of erosion or non-deposition

Nonconformity

A nonconformity exists betweensedimentary rocks and metamorphic or igneous rocks when the sedimentary rock

lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock

Noncon

formity

Angular unconformity

Fault & Folds Relationships

The importance of Geological

p gStructures

• Structures other than folds(anticline) can also trap oil & gas

F lt & F ld R l ti hi

• The importance of understanding faults andfold relationships.

• Many important oil fields

along the Gulf Coast of Texas and Louisiana arelocated at salt domes.

• Salt-created anticlines,coupled with faultsprovide good traps for oil

and gas accumulation inthe earth’s crust.

CONCLUSION

Students should understand the maingeological structures through thedeformation.

Students could relate the structural patternto plate tectonic.Students will appreciate the importance of structural geology for petroleum traps.

FAULT MAPPING

Legend

NW-SE Lineament

NE-SW Lineament

Observation Point

Overlay Sketch Lineaments inRGB Colour

P i 1 Ob i

Point 1 ObservationValley formed by fault intersection was clearlyobserved at Point 1 location in Semenyih Dam.

Other Side of the road

P i t 4 Ob ti

Point 4 ObservationSteam was clearly observed at Point 4 in Kampung Darul Hidayah

Point 5 Observation

Point 5 ObservationQuartz veins formation was clearly observed alongthe expressway at Point 5 in Hulu Kelang, Gombak.

Lecture#9 Structural geology.ppt

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