Rift Valley Mechanism With Relation to Plate Tectonic Movement and Its Petroleum Potential Area by Siti Nursyazwani Binti Ismail (PE Dept)

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SEPTEMBER 2012 TRIMESTER

PCB1023 INTRODUCTION TO PETROLEUM GEOSCIENCE

RIFT VALLEY MECHANISM

WITH RELATION TO PLATE

TECTONIC MOVEMENTAND ITS PETROLEUM

POTENTIAL AREA.

By

Siti Nursyazwani binti Ismail,

Petroleum Engineering Department,

First year (Undergraduate).

© COPYRIGHT UNIVERSITY OF TECHNOLOGY PETRONAS (UTP),

Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia.



1.0 Definition of Rift Valley.

According to Google.com,

“ Rift valley is a large elongated depression with steep walls formed by the

downward displacement of a block of the earth's surface between nearly

parallel faults or fault systems.”

According to Dictionary.com, rift valley has two interpretations;

1. Graben.

2. Subsea chasm extending along the crest of a mid-

oceanridge, locus of the magma upwellings that accompanyseafloor spreading.

According to World English Dictionary, rift valley is;

“ A long narrow valley resulting from the subsidence of land

between two parallel faults, often associated with volcanism.”

According to Science Dictionary, rift valley is;

“ A long, narrow valley lying between two normal geologic faults.”



According to Wikipedia,

“ A rift valley is a linear-shaped lowland between highlands or mountain ranges

created by the action of a geologic rift or fault.”

There are also many other web definitions regarding rift valley, namely;

a valley with steep sides; formed by a rift in the earth's crust

wordnetweb.princeton.edu/perl/webwn

An elongated valley formed by the depression of a block of the planet's crust between two

faults or groups of faults of approximately parallel strike.

www.solarviews.com/eng/terms.htm

A valley of regional extent formed by normal faulting in which extensional stresses tend to

pull the crust apart, usually located along divergent plate margins.

http://www.lpi.usra.edu/resources/stereo_atlas/HTDOCS/GLOSSARY.HTM

A valley formed when the strip of land between two faults subsides.

www.fisicx.com/quickreference/earth/glossary.html

A fault trough formed in a divergence zone or other area of tension.

www.a-z-dictionaries.com/Geological_terms_dictionary.html

https://www.google.com.my/url?url=http://wordnetweb.princeton.edu/perl/webwn%3Fs%3Drift%2520valley&rct=j&sa=X&ei=eSyKULPhEaijiAeGzIDYBg&ved=0CDIQngkwAQ&q=rift+valley&usg=AFQjCNFMNZlG9MkbI2oAHw2fY-bShHnrFA

https://www.google.com.my/url?url=http://www.solarviews.com/eng/terms.htm&rct=j&sa=X&ei=eSyKULPhEaijiAeGzIDYBg&ved=0CDYQngkwAQ&q=rift+valley&usg=AFQjCNG9HbywOzRLq-jq_bZzQIWxEynJ8g


https://www.google.com.my/url?url=http://www.fisicx.com/quickreference/earth/glossary.html&rct=j&sa=X&ei=eSyKULPhEaijiAeGzIDYBg&ved=0CDoQngkwAQ&q=rift+valley&usg=AFQjCNHdHOpTOF4JOnlaoHuzO-KBdcJiRA

https://www.google.com.my/url?url=http://www.a-z-dictionaries.com/Geological_terms_dictionary.html&rct=j&sa=X&ei=eSyKULPhEaijiAeGzIDYBg&ved=0CD4QngkwAQ&q=rift+valley&usg=AFQjCNEQjXKjcd-Mxn8FX5SEtpCPfWGxVA

https://www.google.com.my/url?url=http://www.a-z-dictionaries.com/Geological_terms_dictionary.html&rct=j&sa=X&ei=eSyKULPhEaijiAeGzIDYBg&ved=0CD4QngkwAQ&q=rift+valley&usg=AFQjCNEQjXKjcd-Mxn8FX5SEtpCPfWGxVA

https://www.google.com.my/url?url=http://www.fisicx.com/quickreference/earth/glossary.html&rct=j&sa=X&ei=eSyKULPhEaijiAeGzIDYBg&ved=0CDoQngkwAQ&q=rift+valley&usg=AFQjCNHdHOpTOF4JOnlaoHuzO-KBdcJiRA


https://www.google.com.my/url?url=http://www.solarviews.com/eng/terms.htm&rct=j&sa=X&ei=eSyKULPhEaijiAeGzIDYBg&ved=0CDYQngkwAQ&q=rift+valley&usg=AFQjCNG9HbywOzRLq-jq_bZzQIWxEynJ8g

https://www.google.com.my/url?url=http://wordnetweb.princeton.edu/perl/webwn%3Fs%3Drift%2520valley&rct=j&sa=X&ei=eSyKULPhEaijiAeGzIDYBg&ved=0CDIQngkwAQ&q=rift+valley&usg=AFQjCNFMNZlG9MkbI2oAHw2fY-bShHnrFA



2.0 Commonly Accepted Formation of Rift Valley.

Up until now, here is no clear statement about how rift valley is exactly formed. Most

popular theory is due to the heat flow from the mental plum in the asthenosphere layer that

elevates to the earth’s crust, expands it and fractures the outer crust, which is brittle, forming a

series of faults, grabens (in German, it means ditch) and horsts.

Valley starts rifting through the spreading of

the surface or crust of the earth’s plate. This spreading

process is aggravated by erosion force. Up to optimum

tensional force, the plate will start to split apart and

fault lines are vertically produced, causing plates to slip between each other, forming grabens and

horsts. Grabens slip downwards while horsts are pushed upwards.

The sliding goes on and on. In the meantime, surrounding sediments are deposited on the

valley. Granules from the fault lines of rift walls are eroded and become sediments as well, and

further deposited on the valley too.

Rift Valley can be formed at any elevations, be it in oceanic crust or continental crust.

Besides vertical faults, there are rift valleys formed through moving horizontal faults. Some rift

valley formations even involve the movement of the plate itself that results in splitting of

continents, which mostly occur in oceanic crust. As the plates drift away from each other, molten

rock from the mantel layer may propagates upwards and hardens upon contact with seawater to

form new oceanic crust. Most rift valleys are found along the oceanic ridges.



3.0 Proposed Models of Rift Formation.

J. Tuzo Wilson proposed 5 possible models for formation of rift. This is described in the Wilson-

cycle.

3.0.1 Mantle Plumes and Hot Spots

Magma plumes rise from the mantle to the earth’s surface and spread evenly. Most arise

under continental or ocean basin. Some at the plate boundaries.

The travelling magma heats the overlying lithosphere and its paths and the surrounding.

The heated medium swells and a hot spot is created.

The plumes that create the hot spots stay still while the plate started shifting across each

other. The volcanic and tectonic activities on the surface keep shifting as well.



3.0.2 Hot Spot and Thermal Doming

Mental plume reaches the continental lithosphere’s base.

The mental plume spreads and a magma pond is formed.

The overlying lithosphere heats and swells upward, forming hot spot.

The dome of the hot spot swells, its upper part expands.

The brittle crust cracks, forming faults along a series of three rift

valleys (triple junction, radiating at 120o

from the centre of the hot

spot) radiating away from the centre of the hot spot.

When several hotspots are formed in series, a long rift valley will be

formed.



3.0.3 Foundering of Rift Valley and Marine Invasion

The hot spots cause the earth to swell upward, being pulled away from each other.

When the plates are pulled apart, a space is

left.

The grabens slide down through the space; the

horsts stay and become higher than the

grabens.

The process takes a lot of time, occurs at many

times.

Each time, only a small crack of fault is produced but the result is a significant rift valley

structure after thousands of years.



3.0.4 Early Divergent Margin

Axial rift is formed during the active rifting of the continental crust.

The sea floods the axial rift.

Mafic volcanic activities begin along the sides of the axial rift.

The magma is injected into the granitic continental crust as uncountable basaltic dikes.

Transition crust is formed from the mixture of injected basalt and granite.

Continental divergence forms.

Volcanic activities resumes, the divergent continental margins drift apart.

Oceanic lithosphere formation begins.



3.0.5 Full Divergent Margin

The crust loses heat (thermal decay) and gain an increment in its density.

Subsidence takes place.

Sediments accumulate the most at the ocean with greater subsidence and the least at the

continental.



4.0 Potential Petroleum Area

Continental rift has its own economic value in terms of source rock, mineral and

hydrocarbon deposition. Oil and gas are found in the North Sea, Albert Graben, Viking Graben

and Gulf of Suez Rift. There are many potential petroleum sites among rift valley structures,

namely The Great Rift Valley (lies through Burundi, Ethiopia, Kenya, Malawi, Mozambique,

Uganda, and Tanzania) and Northwest Kenya Rift basin. During formation of rift valley,

sedimentations are formed. This sediment may contain fossil fuels and other minerals from

various reactions; be it detrital, chemical or biogenic. As time passes by, these sedimentations are

pressurised due to compression from new coming layers of sediments. Under the oceanic crust,

they become rigid blocks of different depositions, salts and sand. Continental collision and

tectonic activities metamorphosed the rigid blocks. The hydrocarbons are mostly formed during

Triassic or Jurassic age whereby the metamorphosed rocks are heated through intrusion of

magma. Most hydrocarbons are found in Mesozoic strata of the structure.

One example is the North Sea, where hydrocarbons are found under the burial of rift valley.

During the Jurassic to the early Cretaceous time, some of the seabed repeatedly sank, and the

graben subsidised rapidly. The oceanic crust fractured along big faults, huge blocks dropped and

tilted, forming long ridges along the seabed. All of these triggered slumping of soft sediments into

deeper parts of troughs. Unstable parts of oceanic crust shifted until rock particles were

transported away. Coarse rubble was deposited near steep slope of seabed. Channels and fans of

sand and silt spread out across the seabed and thick layers were built. Some of these sandy rocks

are highly permeable. Now, these rocks contain oil and gas, as those trapped in the Brae, Galley,

Claymore and Magnus fields. The traps are well formed and then the oil and gas migrated from

the Kimmeridge Clay (the most hydrocarbon-rich spot that surrounds the trap) into the trap.





“The Kimmeridge Clay is particularly rich in hydrocarbons along the line

of the rift valley. This is because the slow subsidence of the rift helped to

set up the right environment for a rapid build-up of thick mud layers,

rich in planktonic algal remains, on the deepest parts of the seabed.

Climate and sea conditions were ideal for the massive growth of

'blooms' of plankton. Dead plankton sank in vast numbers, and the

seabed bacteria feeding on their remains made the mud stagnant, so

that particles from the plankton cells were preserved in it and slowly

buried. The buried mud became compressed to form the Kimmeridge

Clay. The thickest mud layers were deposited over the rift and have

since subsided deep within the rift heating up slowly as they became

more deeply buried.”

(The United Kingdom Offshore Oil and Gas Industry Association, 2010)

In the Gulf of Suez, shallow sea depositing carbonates are formed before the rifting

formation (pre-rift), in the late Cretaceous to Eocene. Most of its best reservoir is the Lower

Cretaceous Malha Formation, also known as Nubia or Nubian A. Its porosity is between 19 to 29 %

while the permeability ranges from 70 – 400 mD. The area of potential reservoir is 19 000 km2.



Generalised structural cross-section through the Gulf of Suez, just south of the Morgan Accommodation Zone.

PZ-LK = Paleozoic to lower Cretaceous Nubia (reservoir rock)

UK-EO = Upper Cretaceous to Eocene pre-rift carbonate (source rock)

N, R, K, and B = syn- and post-rift Nukhul, Rudeis, Kareem and Belayim formation (sources, reservoirs, seals and

overburden)

SG = South Gharib salt (seal and overburden)

Z=Zeit (seals and overburden)

PP = Plio-Pleistocene( overburden)

”The lithostratigraphic units in the Gulf of Suez can be subdivided into

three megasequences: a prerift succession (pre-Miocene or Paleozoic –

Eocene), a synrift succession (Oligocene –Miocene), and a postrift

succession (post-Miocene or Pliocene –Holocene). These units vary in

lithology, thickness, areal distribution, depositional environment, and

hydrocarbon importance. Geological and geophysical data show that

the northern and central Gulf of Suez consist of several narrow,

http://en.wikipedia.org/wiki/Paleozoic

http://en.wikipedia.org/wiki/Cretaceous

http://en.wikipedia.org/wiki/Eocene

http://en.wikipedia.org/wiki/Plio-Pleistocene

http://en.wikipedia.org/wiki/Plio-Pleistocene

http://en.wikipedia.org/wiki/Eocene

http://en.wikipedia.org/wiki/Cretaceous

http://en.wikipedia.org/wiki/Paleozoic



elongated depositional troughs, whereas the southern part is

dominated by a tilt-block terrane, containing numerous offset linear

highs “

(Alsharhan, 2003)

The Great Rift Valley was formed between two tectonic plates, namely,

Asia and Africa. It lies from Mozambique through south of Mozambique until east

of Africa, whereas in the east of Africa, the rift was formed through initialisation

of splitting of the Arabian and African plates. Fractures were formed at the

boundary. As they drift further, the fracture turned into a sloping fault line,

leaving a land separated from these two plates. This land (graben) sank due to

divergent movement of the tectonic plates. The Great Rift valley is known as the

tree – armed rift valley. Tectonically, its formation is not just about the movement

of the plate. Before the plates split, there is mental plume in eastern Africa that

causes the earth’s crust to become thin. This causes a part of the crust to bulge

into a dome. As the centre of doming arises, the landmass around it is stressed

into three ridges. These three arms of dome collapsed to form valleys due to

stretching of the earth’s crust. The three arms are Jordan Rift Valley, Red Sea Rift

Valley and East African Rift Valley.



5.0 References

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Documents

Rift Valley Mechanism With Relation to Plate Tectonic Movement and Its Petroleum Potential Area by Siti Nursyazwani Binti Ismail (PE Dept)