Structural geology:

Structural geology is the study of the architecture of the rocks(geometry of the

rocks) with respect to their deformational histories.

Use and importance:

The study of geologic structures has been of prime importance in economic geology,

both petroleum geology and mining geology. Folded and faulted rock strata

commonly form traps for the accumulation and concentration of fluids such as

petroleum and natural gas. Faulted and structurally complex areas are notable as

permeable zones for hydrothermal fluids and the resulting concentration areas for

base and precious metal ore deposits. Veins of minerals containing various metals

commonly occupy faults and fractures in structurally complex areas. These

structurally fractured and faulted zones often occur in association with intrusive

igneous rocks. They often also occur around geologic reef complexes and collapse

features such as ancient sinkholes. Deposits of gold, silver, copper, lead, zinc, and

other metals, are commonly located in structurally complex areas.

Structural geology is a critical part of engineering geology, which is concerned with

the physical and mechanical properties of natural rocks. Structural fabrics and defects

such as faults, folds, foliations and joints are internal weaknesses of rocks which may

affect the stability of human engineered structures such as dams, road cuts, open pit

mines and underground mines or road tunnels.

Geotechnical risk, including earthquake risk can only be investigated by inspecting a

combination of structural geology and geomorphology. In addition areas of karst

landscapes which are underlain by underground caverns and potential sinkholes or

collapse features are of importance for these scientists. In addition, areas of steep

slopes are potential collapse or landslide hazards.

Environmental geologists and hydrogeologists or hydrologists need to understand

structural geology because structures are sites of groundwater flow and penetration,

which may affect, for instance, seepage of toxic substances from waste dumps, or

seepage of salty water into aquifers.

Plate tectonics is a theory developed during the 1960s which describes the movement

of continents by way of the separation and collision of crustal plates. It is in a sense

structural geology on a planet scale, and is used throughout structural geology as a

framework to analyze and understand global, regional, and local scale features.

Measurement conventions(Strike&dip):The inclination of a planar structure

in geology is measured by strike and dip. The strike is the line of intersection between

the surface of dipping bed and a horizontal plane, , and the dip is the magnitude of

inclination between bedding plane and horizontal plane.. For example; striking 25

degrees East of North, dipping 45 degrees Southeast, recorded as N25E,45SE.

Stress Fields:

Stress and Strain: The concepts of stress, strain and material behavior are fundamental to the understanding of geological structures including faults and folds.

Stress: Stress is the force applied to each unit area in particular direction.Measured in pascals, N/m2

Type of Stresses: 1-Normal stress:Perpendicular to plane a-Extensional stress b-Compressional stress 2-Shear stress:Parallel to plane

Strain: When rocks deform they are said to strain.A strain is a change in size, shape, or volumeof a material in response to applied stress. Strain is given infraction, no unit. Linear strain = ΔL / L.

Principal Stress Directions: The stress state at any given point can be described by a system of three principal stresses,normal to each other and along which there are noshear stress components. σ1 maximum principal stress. σ2 intermediate principal stress. σ3 minimum principal stress.

By understanding the constitutive relationships between stress and strain in rocks,

geologists can translate the observed patterns of rock deformation into a stress field

during the geologic past.

[

Why change in stress: Tectonic processes are responsible for the change in stress.

Three stages of deformation (strain) affected on the rocks:

1-Elastic deformation: the deformation of a body in proportion to the applied stress and its recovery once the stress is removed.

2-Elastic limit: it is limiting stress,if this exceeded, the body does not return to its original shape.

3-Plastic deformation:after the stress is released,the body not return to its original shape & size.

Two types of substances (rocks) affected by deformation:

Ductile rocks: The permanent deformation, without fracture in the shape of a solid.

Brittle rocks: The fracturing of a rock in response to stress with little or no permanent deformation prior to its rupture.

Geological Structures:

Common structures: 1. Faults 2. Folds 3. Joints 4. Unconformities

Implications: 1. Tectonic history 2. Mineral exploration 3. Gas and oil exploration 4. Geotechnical engineering

Fold :

The term fold is bent or curved of strata as a result of pressure and high temperature.

The basic cause is likely to be some aspect of plate tectonics.

Structure of a fold:

The upfold is called an anticline. The downfold is called a syncline.

The imaginary line joining the highest points along the upfold is called the crest line.

The flanks of a fold are known as the limbs.

The central line from which the rock strata dip away in opposing directions is called

the axis of fold.

Describing folds:

Fold terminology. For more general fold shapes, a hinge curve replaces the hinge

line, and a non-planar axial surface replaces the axial plane.

Folds are classified by their size, fold shape, tightness, dip of the axial plane.

Strike and Dip Diagram

STRIKE: The direction of the line formed by the intersection of a horizontal plane with a bedding or fault plane. The trend of the rock/fault outcrop.

DIP: The angle formed by the intersection of a bedding or fault plane and the horizontal plane; measured in a vertical plane perpendicular to the strike.

Fold types:

Anticline: linear, strata normally dip away from axial center, oldest strata in center.

Syncline: linear, strata normally dip toward axial center, youngest strata in center.

Dome: nonlinear, strata dip away from center in all directions, oldest strata in center.

Basin: nonlinear, strata dip toward center in all directions, youngest strata in center.

Recumbent: linear, fold axial plane oriented at low angle resulting in overturned strata in one limb of the fold.

symmetrical fold: two limbs are of equal steepness Assymmetrical fold: one limb is steeper than the other Recumbent fold: two limbs are nearly parallel

Causes of folding:

1-Compressive stress.

2-Shearing stress.

Syncline and Anticline

This diagram depicts an adjacent ANTICLINE and SYNCLINE with their representative FOLD AXIS and AXIAL PLANES.

Fault:

A fault is a crack in the Earth's crust with movement. Typically, faults are associated

with, or form, the boundaries between Earth's tectonic plates.

The two sides of a non-vertical fault are known as the hanging wall and footwall. By

definition, the hanging wall occurs above the fault plane and the footwall occurs

below the fault.

Slip, heave, throw:

Slip is defined as the relative movement of geological features present on either side

of a fault plane, and is a displacement vector. A fault's sense of slip is defined as the

relative motion of the rock on each side of the fault with respect to the other side. In

measuring the horizontal or vertical separation, the throw of the fault is the vertical

component of the dip separation and the heave of the fault is the horizontal

component, as in "throw up and heave out".

(Fault Nomenclature)

Fault types:

Geologists can categorize faults into three groups based on the sense of slip:

Normal dip-slip fault

Reverse dip-slip fault

Transform (strike-slip) faults

Causes of faulting:

1-Compressive stress cause reverse fault.

2-Tention stress cause normal fault.

3-Shear stress cause transform fault.

Evidence of faulting:

1-Slickenside:striation on the fault plane.

2-Breccia:fault breccia(Mylonite)

3-Cliff at aparticular region.

4-Discontinuity of the strata by displacement.

5-Repetition&omissionof strata due to an unconformity or fault.

Joint :

The term joint refers to a fracture in rock without movement.Joint sets are commonly

observed to have relatively constant spacing, which is roughly proportional to the

thickness of the layer.

The criteria of joints: 1- It is fracture without movement.

2-Irregular or polygonal shape.

3-Takeplace by shrinkage of the rock mass.

4-It has been observed that most rock are brittle and tend to fail by fracture.

5-Rock are subjected to tensile or shearing stresses.

6-Joints may be vertical, horizontal or inclined depending upon the direction of the

stress and the resistance of the rocks.

Types of joints:

Joints are classified by the processes responsible for their formation, or their

geometry.

Types with respect to formation:

Tectonic joints:

Tectonic joints are formed during deformation episodes whenever the differential

stress is high enough to induce tensile failure of the rock,

Unloading joints (Release joints):

Joints are most commonly formed when uplift and erosion removes the overlying

rocks there by reducing the compressive load and allowing the rock to expand

laterally.

Types with respect to attitude and geometry:

Joints can be classified into three groups depending on their geometrical relationship

with the country rock:

Strike joints – Joints which run parallel to the direction of strike of country rocks are called "strike joints"

Dip joints – Joints which run parallel to the direction of dip of country rocks are called "dip joints"

Oblique joints – Joints which run oblique to the dip and strike directions of the

country rocks are called "oblique

Disadvantage of the joints:

In engineering a joint forms a discontinuity that may have a large influence on the

mechanical behavior (strength, deformation, etc.) of soil and rock masses in, for

example, tunnel, foundation, or slope construction.

Importance in the production of geofluids:

It is long been recognized that joints (fractures) play a major role in the subsurface

fluid flow of water in aquifers and petroleum in oil fields. Major industry research

projects have been dedicated during the last decades to the study of faulted and

fractured reservoirs.

Unconformity:

Unconformities are defined as temporal breaks in a stratigraphic sequence.

(i.e. A surface that represents a very significant gap in the geologic

rock record due to erosion or long periods of non deposition).

There are 4 main types of unconformities:

1) Disconformity – A contact representing missing rock between sedimentary layers that are parallel to each other. Since

disconformities are parallel to bedding planes, they are difficult to see in nature(i.e.between sedimentary rocks showing visible indication of

depositional hiatus).

2) Angular Unconformity – A contact in which younger strata overlie an erosional surface on tilted or folded rock layers. This type of unconformity is easy to identify in nature(i.e. between tilted and

undeformed sediments).

Image provided by FCIT. Original image from Textbook of Geology by Sir Archibald

Geikie (1893).

3) Nonconformity – A contact in which an erosion surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock.

4) Paraconformity- A contact between parallel layers formed by extended

periods of non-deposition (as opposed to being formed by erosion). These

are sometimes called "pseudounconformities") (i.e. a cryptic disconformity

for which there is not immediate evidence of missing sediment, but abrupt

changes in fossil fauna indicate adjacent beds are of significantly different

ages

.

What does the Grand Canyon tell us about unconformities and the base level

of erosion?

The time missing in an unconformity is known as a hiatus or lacuna. The

origin and history of an unconformity may be revealed by the study of cross

sections from the edge of sedimentary basins.

Since no single section likely records continuous sedimentation it is

important to demonstrate equivalence of widely separated sections through

the process of correlation to piece together a complete history of the planet.

Evidence of unconformities:

1- Sedimentary criteria in the continental environments:

a-Basal conglomerate.

b- Residual or weathered chert nodulus (chert in chalky Lst.)

c-Burried soil profile.

2- Sedimentary criteria in the non continental(marine) environments:

a-Glauconite:Green colour mineral(Fe.KSio2).

b-Phosphatized pebbles:Shell&bones of animals &fishes.

c-Manganized zone.

3-Structural criteria:bove

a-Different in the dip angle of the bed a bove &under the unconformity(eg.angular

unconformity).

b-Occurs of igneous rocks such as dyke(eg.nonconformity).

c-Fault at the contact between two bed suddenly.

4-Stratigraphic maps:By study the maps in the fields.

Engineering considerations of structural features:

1- Engineering considerations of fold structure:

a-Disadvantage of the folds are disturbed by some external force,this force may

damage the site in many ways,depending upon the nature &intensityof the

deformational stresses as well as nature of the rocks such as(dam ,

tunnel,araileway…etc).

b-Advantage of the folds are important to awater supply(aquifer),oil

trap(anticlinal or salt dome trap).

2- Engineering considerations of fault structure:

a- Disadvantage of the faults are disturbed the site project &cause leakage in the

water &petroleum from oil trap if the cap rock is affected by the fault.

b-Advantage of the fault may consideras oil trap if the cap rock is not affected by

the fault.

3-Engineering considerations of joint structure:

It has been experienced that the joints always permit considerable leakage of

water(eg. Lst formation joint)from aquifer &petroleum from trap as well as may

have a large influence on the mechanical behavior (strength, deformation, etc.) of soil

and rock masses in, for example, tunnel, foundation, or slope construction.

PLATE TECTONICS

There are three hypothesis or theory for tectonic movements of the earth:

1-Seafloor spreading(1935-1940):

Age of oceanic crust; youngest (red) is along spreading centers.

Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic

crust is formed through volcanic activity and then gradually moves away from the

ridge. Seafloor spreading helps explain continental drift in the theory of plate

tectonics. When oceanic plates diverge, tensional stress causes fractures to occur in

the lithosphere. Basaltic magma rises up the fractures and cools on the ocean floor to

form new sea floor. Older rocks will be found further away from the spreading zone

while younger rocks will be found nearer to the spreading zone

2-Continental drift(Wegener 1912):

Continental drift is the movement of the Earth's continents relative to each other by

appearing to drift across the ocean bed. The concept was independently (and more

fully) developed by Alfred Wegener in 1912. The theory of continental drift was

superseded by the theory of plate tectonics, which builds upon and better explains

why the continents move.

Alfred Wegener first proposes Continental Drift in his book published in 1915. Suggests that 200 million years ago there existed one large supercontinent

which he called Pangaea (All Land)(Figure). This was not really a new idea, but Wegener offered several lines of evidence in support of his proposal.

1. Fit of the Continents - Noted the similarity in the coastlines of North and South America and Europe and Africa. Today the fit is done at the continental shelf and it is nearly a perfect match.

2. Fossil Similarities - Mesosaurus, (Figure) reptile similar to modern alligator which lived in shallow waters of South America and Africa.

3. Rock Similarities a. Rocks of same age juxtaposed across ocean basins. (Figure)

b. Termination of mountain chains. (Figure)

4. Paleoclimatic Evidence a. Glacial deposits at equator b. Coral reefs in Antarctica

Idea was rejected by North American geologists because Wegener couldn't come up with a mechanism for continental drift. Suggested tidal forces, but physicists showed this to be impossible. Wegener dies in 1930 and his idea dies with him.

Pangea:

Pangea was a supercontinent which existed during the Permian Period about 225 million years ago.

Diagram of five maps of the Earth showing Pangea and the positions of the continents as they split apart over time, from the U.S.

Geological Survey. According to the continental drift theory, the supercontinent Pangaea began to break up about 225-200 million

years ago, eventually fragmenting into the continents as we know them today. Continental drift was the forerunner of the theory of

plate tectonics.

3- Plate Tectonics theory(1960-1970):

Map of the Earth's tectonic plates from the US Geological Survey.

The lithosphere consists of the Earth's crust and part of the uppermost mantle. The Earth's surface or lithosphere is divided into

about 7 large plates and 20 smaller ones.

Seven Major Plates (See Figure)

1. Pacific 2. North American 3. South American 4. African 5. Eurasian 6. Antarctic 7. Indo-Australian 8. There are dozens of smaller plates, 9. the seven largest of which are:

Arabian Plate

Caribbean Plate

Juan de Fuca Plate

Cocos Plate

Nazca Plate

Philippine Sea Plate

Scotia Plate.

Asthenosphere is a partially molten part of the mantle, below the

lithosphere.

Two types of crust are present in the lithospheric plates:

1. Thin (5 - 7 km), dense (3.0 g/cm3), basaltic oceanic crust (dark, fine-grained igneous rock)

2. Thick (35 - 40 km, ranging to 60-70 km in some mountain

ranges), low density (2.7 g/cm3), granitic continental crust (light-colored, coarse-grained igneous rock)

The tectonic plates move very slowly relative to one another. The

rates and directions of plate movements vary. The rate of movement has been determined to be approximately 5 - 10 cm per

year (2 - 6 inches per year), depending on location.

Tectonic plate boundaries tend to be sites of relatively intense

geologic activity. Earthquakes and volcanic eruptions occur predominantly along plate boundaries. The interiors of plates tend

to be less geologically active than the boundaries.

Earth structures in Plate Tectonics:

The story of plate tectonics really starts deep within the Earth, so lets take a

look inside first. Although the Earth appears to be made up of solid rock to

us surface-dwelling humans, it’s actually made up of three distinct layers:

the crust, mantle, and core. Each layer has its own unique properties and

chemical composition.

Crust

The crust is the thin, solid, outermost layer of the Earth. The crust is

thinnest beneath the oceans, averaging only 5 kilometers thick, and thickest

beneath large mountain ranges. Continental crust (the crust that makes up

the continents, of course!) is much more variable in thickness but averages

about 30-35 km. Beneath large mountain ranges, such as the Himalayas or

the Sierra Nevada, the crust reaches a thickness of up to 100 km.

Mantle

The layer below the crust is the mantle. The mantle has more iron and

magnesium than the crust, making it more dense. The uppermost part of the

mantle is solid and, along with the crust, forms the lithosphere. The rocky

lithosphere is brittle and can fracture. This is the zone where earthquakes

occur. It’s the lithosphere that breaks into the thick, moving slabs of rock

that geologist’s call tectonic plates.

As we descend into the Earth temperature rises and we reach part of the

mantle that is partially molten, the asthenosphere. As rock heats up, it

becomes pliable or ‘plastic’. Rock here is hot enough to fold, stretch,

compress, and flow very slowly without fracturing. Think about the

behavior of Silly Putty® and you have the general idea. The plates, made

up of the relatively light, rigid rock of the lithosphere actually ‘float’ on the

more dense, flowing asthenosphere!

Core

At the center of the Earth lies the super-dense core. With a diameter of

3486 kilometers, the core is larger than the planet Mars! The core of the

Earth is made up of two distinct layers: a liquid outer layer and a solid

inner core. Unlike the Earth’s outer layers with rocky compositions, the

core is made up of metallic iron-nickel alloy. It’s hard to imagine, but the

core is about 5 times as dense as the rock we walk on at the surface!

The Origin of Plate Tectonics:

What is the origin of plate tectonics? The continents drift slowly (the timescale

for substantial change is 10-100 million years), but that they drift at all is

remarkable. The following figure illustrates the structure of the first 100-200

kilometers of the Earth's interior, and provides an answer to this question.

The lithosphere and the aesthenosphere

The crust is thin, varying from a few tens of kilometers thick beneath the

continents to less than 10 km thick beneath the many of the oceans. The crust

and upper mantle together constitute the lithosphere, which is typically 50-100

km thick and is broken into large plates (not illustrated). These plates sit on the

aesthenosphere.

The aesthenosphere is kept plastic (deformable) largely through heat generated

by radioactive decay. The material that is decaying is primarily radioactive

isotopes of light elements like aluminum and magnesium. This heat source is

small on an absolute scale (the corresponding heat flow at the surface out of the

Earth is only about 1/6000 of the Solar energy falling on the surface).

What forces drive plate tectonics?

Basically, the driving forces that are advocated at the moment, can be divided in three

categories:1- mantle dynamics related, 2- gravity related (mostly secondary forces),

3- Earth rotation related (secondary forces).

1-Mantle dynamics related driving forces:

Convection Currents The mantle is made of much denser, thicker material, because of this the plates

"float" on it like oil floats on water.

Many geologists believe that the mantle "flows" because of convection

currents. Convection currents are caused by the very hot material at the

deepest part of the mantle rising, then cooling, sinking again and then heating,

rising and repeating the cycle over and over. The next time you heat anything

like soup or pudding in a pan you can watch the convection currents move in

the liquid. When the convection currents flow in the mantle they also move the

crust. The crust gets a free ride with these currents. A conveyor belt in a

factory moves boxess like the convection currents in the mantle moves the

plates of the Earth.

Convection cells. Roughly circular.Mantle heat probably due to radioactive decay .

Plate tectonics is driven by the convection in the asthenosphere (part of the Earth's mantle).

Conceptual drawing of assumed convection cells in the mantle. Below a depth of about 700 km, the des cending slab begins to soften and flow, losing its form

Types of plate tectonic boundaries

There are three major types of plate tectonic boundaries. These include:

1. Divergent plate boundaries where plates move apart from one another.

2. Convergent plate boundaries where plates move toward one another.

3. Transform plate boundaries whe re plates slide past one another.

Artist's cross section illustrating the main types of plate boundaries. Cross section by José F. Vigil from This Dynamic Planet -- a wall

map produced jointly by the U.S. Geological Survey, the Smithsonian Institution, and the U.S. Naval Research Laboratory.

Image courtesy of U. S. Geological Survey.

1. Divergent - where the plates are moving apart.

Animation of divergent plate motion at a mid-ocean ridge..

Examples: mid-ocean ridges such as the Mid-Atlantic Ridge (the site of sea-floor spreading), and continental rifts such as

the east African Rift system.

Animation of divergent plate motion. (Constructive margin).

2. Convergent - where the plates are moving toward one

another.

Example of a convergent plate boundary. In this case, ocean-

to-continent convergence is shown. The deep sea trench is the site of a subduction zone, where oceanic crust is being

carried down into the mantle, where it begins to melt. The magma rises to form volcanoes along the edge of the

continent. Image courtesy of U.S. Geological Survey.

Example of a convergent plate boundary. In this case, ocean-

to-ocean convergence is shown. The deep sea trench is the site of a subduction zone, where oceanic crust is being carried

down into the mantle, where it begins to melt. The magma

rises to form a volcanic island arc. Image courtesy of U.S. Geological Survey.

Example of a convergent plate boundary. In this case,

continent-to-continent convergence is shown. Continental

crust collides with and slides over other continental crust, forming high mountain ranges like the Himalayas(Asia&India).

Image courtesy of U.S. Geological Survey.

Examples: subduction zones which occur at deep sea

trenches such as the Marianas Trench, and sites of continental collision forming mountain belts, such as the

Himalaya Mountains, the Ural Mountains, the Appalachian Mountains(America &Europe), and the Alps.

Animation of convergent plate motion. (Destructive margin) (or

active margins)

3. Transform - where the plates are sliding past one another,

such as one sliding to the north and the adjacent plate sliding to the south.

Examples: transform faults(strike-slip fault), (easily seen where they cut at right angles to the mid-ocean ridges);

includes the San Andreas fault. Levant fault in black sea.

Animation of transform plate motion. (Conservative)

Types of plate tectonic collision:

1-C.C. versus O.C. with subduction zone(eg.Andes mountain).

2-C.C. versus O.C. with obduction zone (eg.Arabianplate(o.c.)with Iranian plate(c.c.)(ophiolites zone).

3-C.C. versus C.C. collision (eg.Asia+India=Himalayas mountain type).

4-O.C.versus O.C. collision(eg. Andunsian arc &Carebian arc).

Evidence in support of the Theory of Plate Tectonics:

1. Shape of the coastlines - Africa and South America would fit together like jigsaw puzzle pieces.

2. Fossil evidence 1. Glossopteris flora - a type of Late Paleozoic seed ferns

(plant fossils) that were found in Gondwanaland

(India, Africa, Australia, S. America, Antarctica)

2. Mesosaurus, a freshwater aquatic reptile whose fossils

were found in South America and Africa

The locations of certain fossil plants and animals on present-

day, widely separated continents form definite patterns (shown by the bands of colors), if the continents are put back

together. Click image for a larger version. Diagram courtesy

U.S. Geological Survey.

3. Rift Valleys of Africa - (evidence for a continent breaking up)

Map of the East African Rift Valley showing plate boundaries

(solid black lines), rift zones (dashed black lines), and

volcanoes (red triangles). Several deep lakes are present in the rift valleys. Saudi Arabia (the Arabian Plate) has rifted

away from the African Plate, forming a rift valley which has been flooded by the Red Sea. The African and Arabian Plates

meet in a "triple junction", where the Red Sea meets the Gulf of Aden. This area (shaded orange) is called the Afar Triangle.

A new spreading center appears to be developing along the East African Rift Valley. As rifting occurs, normal faults or

tensional cracks form. Magma rises from below into the cracks or faults, and in places erupts onto the earth's surface

forming volcanoes.

4. Geologic similarities between S. America and Africa

1. Same stratigraphic sequence (i.e. same sequence of types of layered sedimentary rocks)

2. Mountain belts and folded rocks would line up if you could push the continents back together

5. Paleoclimatic evidence (Paleo = "ancient", climatic = "climate")

If you could push the continents back together, the ancient climatic zones, as indicated by the rock types, would match

up. Layers of glacial deposits are found at same place in

sequence of rocks Note directions of glacial ice movement as indicated by

striations or grooves in the rock.

Glaciers start to form on continents from the buildup of snow.

As they grow through snow accumulation, they begin to move outwards. They do not form in the sea and move onto the

land.

6. Sediment thickness is greatest along the edges of continents.

When deep sea drilling began, scientists expected to see a

complete record of sediment deposition over hundreds of millions of years or more. Contrary to expectations, they

found that there was only a thin layer of sediment on the

ocean floors, and that the ocean crust beneath was quite young.

7. Hot spots - thermal plumes (heat rising in mantle).

Plates move over hot spots creating a chain of volcanoes.

Hawaiian Islands, Emperor Sea Mounts

Map of part of the Pacific basin showing the volcanic trail of

the Hawaiian hotspot-- 6,000-km-long Hawaiian Ridge-Emperor Seamounts chain. (Base map.

8. Evidence for subsidence in oceans - Guyots - flat-topped sea

mounts (erosion when at or above sea level) 9. Mid-ocean ridges located near ocean centers

Benioff Zones - inclined zone of earthquake foci (plural of focus)

near deep sea trenches

Application of plate tectonic theory:

Earth movements &volcanic activities:

Earthquakes:A vibration,which may be feeble or severe set up in the earths

crust.

Epicenter:Apoint at the surface of the earth which is immediately above the

focus or the origin of the earthquake(i.e.center of the earthquake).

Seismograph:Instrument which is recorded the waves of an earthquake.

Causes of earthquakes:

1-Superficial movements.(eg:dashing waves cause vibrations along the sea-

shores,such as Tsunamis travel rapidly & are very destructive when they reach

land with 750 Km|hour.

2-Volcanic eruptions.

3-Folding &faulting.

Classification of earthquakes:

The earthquakes have been classified on the basis of their:

1-Intensity,2-Causes of origin,3-Depth of the shock originated.

But the classification based on the depth of the shock originated is widely used

into the following three categories:

1-Shallow earthquake less than 50Km depth.

2-Intermediate earthquake 50-300 Km depth.

3-Deep earthquake :shock is originated from more than 300 Km depth.

(eg:earthquake in indian,1905 Penjab,1934Bihar).

Engineering consideration of earthquake:

He only solution that can be done at the best is to provide additional factors in

the design of structure to minimize the losses due to shock of an earthquake, this

can be done in the following way:

1-To collect sufficient data,regarding the previous seismic activity in the area.

2-To provide factors of safety, to stop or minimize the loss due to severe

earthquake.

(eg:the foundation of building,dam should rest on asolid rock bed,best materials

should be used in the petroleum engineering projects.)

Petroleum geology:

Petroleum geology is the study of origin, occurrence, movement, accumulation, and

exploration of hydrocarbon fuels. It refers to the specific set of geological disciplines

that are applied to the search for hydrocarbons (oil exploration).

Introduction and Summary

PETROLEUM (rock-oil, from the Latin petra, rock or stone, and oleum, oil) occurs widely in the earth as gas, liquid, semisolid, or solid, or in more than one of these states at a single place. Chemically any petroleum is an extremely complex mixture of hydrocarbon (hydrogen and carbon) compounds, with minor amounts of nitrogen, oxygen, and sulfur as impurities. Liquid petroleum, which is called crude oil to distinguish it from refined oil, is the most important commercially. It consists chiefly of the liquid hydrocarbons, with varying amounts of dissolved gases, bitumens, and impurities. It has an oily appearance and feel; in fact, it resembles the ordinary lubricating oil sold at filling stations, is immiscible with water and floats on it, but is soluble in naphtha, carbon disulfide, ether, and benzene. Petroleum gas, commonly called natural gas to distinguish it from manufactured gas, consists of the lighter paraffin hydrocarbons, of which the most abundant is methane gas (CH4). The semisolid and solid forms of petroleum consist of the heavy hydrocarbons and bitumens. They are called asphalt, tar, pitch, albertite, gilsonite, or grahamite, or by any one of many other terms, depending on their individual characteristics and local usage. The general term "bitumen" has long been used interchangeably with "petroleum" for both the liquid and the solid forms. Hydrocarbon is a term often used interchangeably with "petroleum" for any of its forms.

A porous and permeable body of rock, called the reservoir rock, which is overlain by an impervious rock, called the roof rock, contains oil or gas or both, and is deformed or obstructed in such a manner that the oil and gas are trapped.

Commercial deposits of crude oil and natural gas are always found underground, where they nearly always occur in the water-coated pore spaces of sedimentary rocks. Being lighter than water, the gas and oil rise and are concentrated in the highest part of the container; in order to prevent their escape, the upper contact of the porous rock with an impervious cover must be concave, as viewed from below. Such a container is called a trap, and the portion of the trap that holds the pool of oil or gas is called the reservoir. The significant thing is that reservoirs can be of various shapes, sizes, origins, and rock compositions.

Any rock that is porous and permeable may become a reservoir, but those properties are most commonly found in sedimentary rocks, especially sandstones and carbonates. A trap may be formed, either ' wholly or partly, by the deformation of the reservoir rock, which may be accomplished by folding, faulting, or both, and in either a single episode or in several episodes.

Or a trap may be formed, either wholly or partly, by stratigraphic variations in the reservoir rock. These may be primary, such as original facies changes,

Petroleum-Bearing Rocks: Sedimentary rocks are the most important and interesting type of rock to the

petroleum industry because most oil and gas accumulations occur in them; igneous

and metamorphic rocks rarely contain oil and gas.

All petroleum source rocks are sedimentary.

Furthermore, most of the world’s oil lies in sedimentary rock formed from marine

sediments deposited on the edges of continents. For example, there are many large

deposits that lie along the Gulf of Mexico and the Persian Gulf.

Origin of the petroleum:

Abiogenic petroleum origin

Abiogenic petroleum origin is a hypothesis that was proposed as an alternative

mechanism of petroleum origin. Geologists now consider the abiogenic formation of

petroleum scientifically unsupported.

According to the abiogenic hypothesis, petroleum was formed from deep carbon

deposits, perhaps dating to the formation of the Earth. Supporters of the abiogenic

hypothesis suggest that a great deal more petroleum exists on Earth than commonly

thought, and that petroleum may originate from carbon-bearing fluids that migrate

upward from the mantle. The presence (oceans) of methane on Saturn's moon Titan

and in the atmospheres of Jupiter, Saturn, Uranus and Neptune is cited as evidence of

the formation of hydrocarbons without biology.

These abiogenic origin theories are:

1-Cosmic theory:Suppose that most of planet of solar system have saturated

hydrocarbons gases as semiliquid state.When the earth is cold the hydrocarbon

materials accumulated on the earth surface rocks &form accumulation of

hydrocarbon deposits.

2-Volcanic theory:Eruption of hydrocarbon gases from volcanic activity.

3-Magmatic theory:

4-Chemical theory:Ethane from polymerization.

Biogenic petroleum origin:

Oil forms from the decay and transformation of dead organisms buried in

sedimentary rocks .Petroleum geologists agree that oil originates from to vast

quantities of dead marine plankton or plant material that sank into the mud of shallow

seas. Under the resulting anaerobic conditions, organic compounds remained in a

reduced state where anaerobic bacteria converted to hydrocarbon materials,

1-The vast amount of organic matter and hydrocarbons now found in the sediments of

the earth(95% oil crude& natural gas in sedimentary rocks).

2-The fact that many crude oils have been found to contain porphyrius

pigments(complex hydrocarbon compounds)and the fact that nearly all petroleum

contain nitrogene,are more or less direct evidence of the animal or vegetable origin or

both.

3-Awide variety of petroleum hydrocarbon and even crude oil have been found in

nearly sources rocks such as (shale, carbonate),

((The occurrence of petroleum))

Mode of occurrence:

A-surface occurrence: such as seepages-mud volcanoes-springs-exudate

bitumen-vein and caving deposits-various kinds of oil kerogen ( kerogen shale),may

reach the surface along fractures, joints, fault, unconformities and bedding plane.

Some surface occur may be thought of as currently:

a/active or live such as (1)those that form active seeqage example

(Quaiyarah,Kirkuk,Naft Khaneh,Naft Shah and Masjid-al- Sulaiman oil fields .

2-those associated with springs, mud volcanoes and mud flows.

b/other may be considered as fossils or dead occurrences such as inspissated

deposits ,dikes and vein filling of solid bitumen, Vug and cavity filling ,disseminated

hydrocarbon.

B-Subsurface occurrence: Underground or subsurface occurrence petroleum

may be divided according to their size as:

a-Minor showing of oil&gas:It distinguish by naked eyes or by using Ultraviolent lamp

through the examine of rock cutting during the drilling processes.

b-Oil &gas accumulation: Commercial petroleum deposits are classified as oil pool,

oil field, oil province.

1-Oil pool: The simplest unit of hydrocarbon according to small size &commercial or

economic value.

2-Oil field:When several pools are related to a single geographic features either

structural& stratigraphic(i.e field consist of two or more oil pools).

3-Oil province: Is a region in which a number of oil &gas pools and field occur in a

similar or related geographic environments

Sedimentary basin analysis

Petroleum geology is principally concerned with the evaluation of seven key elements

in sedimentary basins:

A structural trap, where a fault has juxtaposed a porous and permeable reservoir

against an impermeable seal. Oil (shown in red) accumulates against the seal, to the

depth of the base of the seal. Any further oil migrating in from the source will escape

to the surface and seep.

Source

Reservoir

Seal

Trap

Timing

Maturation

Migration

Evaluation of the source uses the methods of geochemistry to quantify the nature of

organic-rich rocks which contain the precursors to hydrocarbons, such that the type

and quality of expelled hydrocarbon can be assessed.

The reservoir is a porous and permeable lithological unit or set of units that holds the

hydrocarbon reserves. Analysis of reservoirs at the simplest level requires an

assessment of their porosity (to calculate the volume of in situ hydrocarbons) and

their permeability (to calculate how easily hydrocarbons will flow out of them). Some

of the key disciplines used in reservoir analysis are the fields of structural analysis,

stratigraphy, sedimentology, and reservoir engineering.

The seal, or cap rock, is a unit with low permeability that impedes the escape of

hydrocarbons from the reservoir rock. Common seals include evaporites, chalks and

shales. Analysis of seals involves assessment of their thickness and extent, such that

their effectiveness can be quantified.

The trap is the stratigraphic or structural feature that ensures the juxtaposition of

reservoir and seal such that hydrocarbons remain trapped in the subsurface, rather

than escaping (due to their natural buoyancy) and being lost.

Analysis of maturation involves assessing the thermal history of the source rock in

order to make predictions of the amount and timing of hydrocarbon generation and

expulsion.

Finally, careful studies of migration reveal information on how hydrocarbons move

from source to reservoir and help quantify the source (or kitchen) of hydrocarbons in a

particular area.

Factors required to make an Oil deposit :

• Source rock- rich in organic matter

• Burial heating- > maturation

• Reservoir rock- porous and permeable

• Trap-

• structural trap

• stratigraphic trap.

Source rock:

Characters of Source rock

� Black organic-rich marine shales

� Organic matter is preserved low-oxygen water

� Restricted marine basins and zones were water rises from the deep

In petroleum geology, source rock refers to rocks from which hydrocarbons have

been generated or are capable of being generated. They are organic-rich sediments

that may have been deposited in a variety of environments including deep water

marine, lacustrine and deltaic. Oil shale can be regarded as an organic-rich but

immature source rock from which little or no oil has been generated and expelled.

Types of source rock

Source rocks are classified from the types of kerogen that they contain, which in turn

governs the type of hydrocarbons that will be generated.

Type 1 source rocks are formed from algal remains deposited under anoxic conditions in deep lakes: they tend to generate waxy crude oils when submitted to thermal stress during deep burial.

Type 2 source rocks are formed from marine planktonic and bacterial remains preserved under anoxic conditions in marine environments: they produce both oil and gas when thermally cracked during deep burial.

Type 3 source rocks are formed from terrestrial plant material that has been decomposed by bacteria and fungi under oxic or sub-oxic conditions: they tend to generate mostly gas with associated light oils when thermally cracked during deep burial. Most coals and coaly shales are generally Type 3 source rocks.

Maturation and expulsion

With increasing burial by later sediments and increase in temperature, the kerogen

within the rock begins to break down. This thermal degradation or cracking releases

shorter chain hydrocarbons from the original large and complex molecules found in

the kerogen.

The hydrocarbons generated from thermally mature source rock are first expelled,

along with other pore fluids, due to the effects of internal source rock over pressuring

caused by hydrocarbon generation as well as by compaction. Once released into

porous and permeable carrier beds or into faults planes, oil and gas then move

upwards towards the surface, an overall buoyancy driven process known as secondary

migration.

Petroleum reservoir:

A petroleum reservoir, or oil and gas reservoir, is a subsurface pool of

hydrocarbons contained in porous or fractured rock formations. The naturally

occurring hydrocarbons, such as crude oil or natural gas, are trapped by overlying

rock formations with lower permeability.

Formation:

Crude oil found in all oil reservoirs formed in the Earth's crust from the remains of

once-living things. Crude oil is properly known as petroleum, and is used as fossil

fuel. Evidence indicates that millions of years of heat and pressure changed the

remains of microscopic plant and animal into oil and natural gas.

Although the process is generally the same, various environmental factors lead to the

creation of a wide variety of reservoirs. Reservoirs exist anywhere from the land

surface to 30,000 ft (9,000 m) below the surface and are a variety of shapes, sizes and

ages.

Classifiation of reservoir rocks: A-Primary classification( simple& broad):

1-Fragments (clastic) –sandstone 59%(gray wacke,arkose,conglomerate)

2-Limestone-40%(organic &argillaceous limestone)

3-Miscellaneous- 1%(igneous rocks -granite &basalt,metamorphic rocks-slate

&marble). But these condition are rare and anomalous.

B-Genetic classification-It is some times useful to class a reservoir rocks as of marine

or non marine origin may be combined with a lithologic classification(marine Lst,

continental Sst, non marine conglomerate).

TT

Trap structure:

A trap forms when the buoyancy forces driving the upward migration of hydrocarbons

through a permeable rock cannot overcome the capillary forces of a sealing medium.

The timing of trap formation relative to that of petroleum generation and migration is

crucial to ensuring a reservoir can form.

Petroleum geologists broadly classify traps into three categories that are based on

their geological characteristics: the structural trap, the stratigraphic trap and the

combination trap..

1-Structural traps

Fold (structural) trap

Fault (structural) trap

Structural traps are formed as a result of changes in the structure of the subsurface

due to processes such as folding and faulting, leading to the formation of domes,

anticlines, and folds.. Examples of this kind of trap are an anti-cline trap . a fault trap

and a salt dome trap.

They are more easily delineated and more prospective than their stratigraphic

counterparts, with the majority of the world's petroleum reserves being found in

structural traps.

2-Stratigraphic traps

Stratigraphic traps are formed as a result of lateral and vertical variations in the

thickness, texture, porosity or lithology of the reservoir rock(i.e ,are the result of

changes in the continuity of the rocks). Examples of this type of trap are an

unconformity trap, a lens trap and a reef trap.

3-Combination traps

Combination of structral elements (fold, fault)&stratigraphic elements(unconformity).

The Two Types of Traps

1-Structural Traps

These traps hold oil and gas because the earth has been bent and deformed in some

way. The trap may be a simple dome (or big bump), just a “crease” in the rocks, or it

may be a more complex fault

trap like the one shown at the right. All pore spaces in the rocks are filled with fluid,

either water, gas, or oil. Gas, being the lightest, moves to the top. Oil locates right

beneath the gas, and water stays lower.

Once the oil and gas reach an impenetrable layer, a layer that is very dense or non-

permeable, the movement stops. The impenetrable layer is called a “cap rock.”

2-Stratigraphic Traps

Stratigraphic traps are depositional in nature. This means they are formed in place,

often by a body of porous sandstone or limestone becoming enclosed in shale. The

shale keeps the oil and gas from

escaping the trap, as it is generally very difficult for fluids (either oil or gas) to

migrate through shales. In essence, this kind of stratigraphic trap is surrounded by

“cap rock.”

Here are four traps. The anticline is a structural type of trap, as is the fault trap and

the salt dome trap.

Four Types Of Structural and Stratigraphic Traps

The stratigraphic trap shown at the lower left is a cool one. It was formed when rock

layers at the bottom were tilted, then eroded flat. Then more layers were formed

horizontally on top of the tilted ones. The oil moved up through the tilted porous rock

and was trapped underneath the horizontal, nonporous (cap) rocks.

Structural Traps

There are three basic forms of a structural trap in petroleum geology:

Anticline Trap Fault Trap Salt Dome Trap

The common link between these three is simple: some part of the earth has

moved in the past, creating an impedence to oil flow.

Anticline Trap

An anticline is an example of rocks which were previously flat, but have been

bent into an arch. Oil that finds its way into a reservoir rock that has been bent

into an arch will flow to the crest of the arch, and get stuck (provided, of course,

that there is a trap rock above the arch to seal the oil in place).

A cross section of the Earth showing

typical Anticline Traps. Reseroir rock

that isn't completely filled with oil

also contains large amounts of salt

water.

Fault Trap

Fault traps are formed by movement of rock along a fault line. In some cases, the

reservoir rock has moved opposite a layer of impermeable rock. The

impermeable rock thus prevents the oil from escaping. In other cases, the fault

itself can be a very effective trap. Clays within the fault zone are smeared as the

layers of rock slip past one another. This is known as fault gouge.

A cross section of rock showing a

fault trap - in this case, an example of

gouge. This is because the reservoir

rock on both sides of the fault would

be connected, if not for the fault

seperating the two. In this example, it

is the fault itself that is trapping the

oil.

Click here to see an example of

another fault trap

Salt Dome Trap

Salt is a peculiar substance. If you put enough heat and pressure on it, the salt

will slowly flow, much like a glacier that slowly but continually moves downhill.

Unlike glaciers, salt which is buried kilometers below the surface of the Earth

can move upward until it breaks through to the Earth's surface, where it is then

dissolved by ground- and rain-water. To get all the way to the Earth's surface,

salt has to push aside and break through many layers of rock in its path. This is

what ultimately will create the oil trap.

Here we see salt that has moved up

through the Earth, punching through

and bending rock along the way. Oil

can come to rest right up against the

salt, which makes salt an effective trap

rock. However, many times, the salt

chemically changes the rocks next to it

in such a way that oil will no longer

seep into them. In a sense, it destroys

the porosity of a reservoir rock.

2-Stratigraphic Traps

A stratigraphic trap accumulates oil due to changes of rock character rather than

faulting or folding of the rock. The term "stratigraphy" basically means "the study of

the rocks and their variations". One thing stratigraphy has shown us is that many

layers of rock change, sometimes over short distances, even within the same rock

layer. As an example, it is possible that a layer of rock which is a sandstone at one

location is a siltstone or a shale at another location. In between, the rock grades

between the two rock types. From the section on reservoir rocks, we learned that

sandstones make a good reservoir because of the many pore spaces contained within.

On the other hand, shale, made up of clay particles, does NOT make a good reservoir,

because it does not contain large pore spaces. Therefore, if oil migrates into the

sandstone, it will flow along this rock layer until it hits the low-porosity shale. Voilà,

a stratigraphic trap is born!

An example of a stratigraphic trap

The above series of diagrams is an attempt to illustrate a type of stratigraphic trap. In

the diagram at the upper left, we see a river that is meandering. As it does so, it

deposits sand along its bank. Further away from the river is the floodplain, where

broad layers of mud are deposited during a flood. Though they seem fairly constant,

rivers actually change course frequently, eventually moving to new locations.

Sometimes these new locations are miles away from their former path. In the diagram

at the upper right, we show what happens when a river changes its course. The sand

bars that were deposited earlier are now covered by the mud of the new floodplain.

These lenses of sand, when looked at from the side many years later (the bottom

diagram), become cut off from each other, and are surrounded by the mud of the

river's floodplain - which will eventually turn to shale. This makes for a perfect

stratigraphic trap.

Types of Hydrocarbon Trap

Diagrams of structural and stratigraphic traps

A trap is a geologic structure or a stratigraphic feature capable of retaining hydrocarbons. Hydrocarbon traps that result from changes in rock type or pinch-outs, unconformities, or other sedimentary features such as reefs or buildups are called stratigraphic traps. Hydrocarbon traps that form in geologic structures such as folds and faults are called structural traps. Any mixture of structural and stratigraphic elements is called a combination trap.

Seals

The seal is a fundamental part of the trap that prevents hydrocarbons from further

upward migration(anhydrite, gypsum)

A capillary seal is formed when the capillary pressure across the pore throats is

greater than or equal to the buoyancy pressure of the migrating hydrocarbons. They

do not allow fluids to migrate across them until their integrity is disrupted, causing

them to leak.

Oil and Gas Traps

All oil and gas deposits are found in structural or stratigraphic traps. You may have

heard that oil is found underground in “pools,” “lakes,” or “rivers.” Maybe someone

told you there was a “sea” or “ocean” of oil underground. This is all completely

wrong, so don’t believe everything you hear.

Oil Moving Through Pore Space In Sandstone

Most oil and gas deposits are found in sandstones and coarse-grained limestones. A

piece of sandstone or limestone is very much like a hard sponge, full of holes, but not

compressible. These holes, or pores, can contain water or oil or gas, and the rock will

be saturated with one of the three. The holes are much tinier than sponge holes, but

they are still holes, and they are called porosity.

The oil and gas become trapped in these holes, stays there, for millions of years, until

petroleum geologists come to find it and extract it.

When you hold a piece of sandstone containing oil in your hand, the rock may look

and smell oily, but the oil usually won’t run out, and you can’t squeeze sandstone like

a sponge! The oil is trapped inside the rock’s porosity.

Migration of the petroleum:

Essential features of the petroleum migration:

� Oil is less dense than water

� Oil will move up by buoyancy

� Oil needs a permeable bed to move

� It will stop when it reaches an impermeable bed

The last section deals with migration: how and why fluid hydrocarbons

migrate from a source rock (rock material where they formed) to the reservoir rock (rock material where they are found). We continue to use

the basic formula:

Petroleum End Product =

[RawMaterial+Accumulation+Transformation+Migration] + Geologic Time

There are two types of migration when discussing the movement of

petroleum, primary and secondary. Primary migration refers to the movement of hydrocarbons from source rock into reservoir rock.

Secondary migration refers to the subsequent movement of hydrocarbons within reservoir rock; the oil and gas has left the source

rock and has entered the reservoir rock. This occurs when petroleum is

clearly identifiable as crude oil and gas although the gas may be dissolved in the oil. Buoyancy of the hydrocarbons occurs because of

differences in densities of respective fluids and in response to

differential pressures in reservoir rock.

There are two important concepts that must be understood and how they relate to source rocks and reservoir rocks in order to discuss

migration. They are porosity and permeability. Porosity refers to the percentage of total volume of a material that is occupied by voids or air

spaces that exist between the rock grains. The more porous a material is, the greater the amount of open space, or voids, it contains. Stored in

these voids are liquids and gases. Porosity differs from one material to another. Unconsolidated deposits of clay have the greatest porosities

because of their crystallographic structure; they are comprised of parallel sheets of clay minerals. Unconsolidated deposits of sand have

lower porosities because of the nature of the sand grains to each other.

Source rocks have high porosities; the best source materials are clays & shales, but these same materials make poor reservoir rocks.

Permeability (measured in centimeters per second) refers to the ability

of a material to transmit [fluid or gas]. The rate at which a material will transmit a fluid or gas depends upon total porosity, number of

interconnections between voids, and size of interconnections between voids.

(Exploration&prospecting of petroleum)

Essential features of exploration&prospecting of petroleum:

1-Occurrence of source rocks in sedimentary basin such as(shale,marl,organic

&fossiliferous Lst, chalk).

2- Occurrence of reservoir rocks with high porosity &permeability such as(Sst,fracture

Lst.)

3- Occurrence of subsurface structural geology such as(fault,fold,salt dome,lenses)which

are form asoil traps.

4-Area prospecting must be located faraway from metamorphism region&volcanic

activity.

5- Occurrence of cap rocks (impermeable).

6-Study the stratigraphic sequence of the area.

All of these essential features can be determined by:

(a)Study the exposed rocks(outcrops)with correlation between localities.

(b)Study the paleogegraphy ,paleoecology &topography of the area.

Stage of exploration&prospecting of petroleum:

1-Geological survey:

a-Geological map. b-Topographic map. c-Structural geology.

2-Geophysical survey:

a-Seismic method. b-Gravity method. C-Electrical method. d-Magmatic method.

3-Well logging(explorating well).

4-Geochemical prospecting(kerogene,organic materials)

5-Well drilling(bit drilling,rotary drilling).

Well drilling always associated with casing tube.There are two types of

samples:

1-Cutting samples. 2-Core samples.

Classification of petroleum wells:

1-Exploratory well . 2-Production well. 3-Injection well.

4-Observatory well. 5-Pressure release well. 6-Delimiting well.

The main factors affected on the hydrocarbons accumulation:

1-Change in the dip angle of reservoir bed.

2- Change in the hydrodynamic pressure.

3- Change in the direction of water associated with oil.

4- Change in the water density associated with oil.

5- Change in the ability of cap rock closure.

Five Major Types of Hydrocarbons of Interest to Petroleum Exploration: Kerogen/Bitumens

kerogen :A fossilized mixture of insoluble organic material that, when

heated, breaks down into petroleum and natural gas. Kerogen consists of

carbon, hydrogen, oxygen, nitrogen, and sulfur and forms from

compacted organic material, including algae, pollen, spores and spore

coats, and insects. It is usually found in sedimentary rocks, such as shale.

Shale rock volume is composed of 99% clay minerals and 1% organic material. We

have seen that petroleum is derived mainly from lipid-rich organic material buried in

sediments. Most of this organic matter is in a form known as kerogen. Kerogen is that

part of the organic matter in a rock that is insoluble in common organic solvents. It

owes its insolubility to its large molecular size and heat is required to break it down.

Maturation of kerogen is a function of increased burial and temperature and is

accompanied by chemical changes.

As kerogen thermally matures and increases in carbon content, it changes form an

immature light greenish-yellow color to an overmature black, which is representative

of a progressively higher coal rank. Different types of kerogen can be identified, each

with different concentrations of the five primary elements, carbon, hydrogen, oxygen,

nitrogen, and sulphur, and each with a different potential for generating petroleum.

The organic content of a rock that is extractable with organic solvents is known as

bitumen. It normally forms a small proportion of the total organic carbon in a rock.

Bitumen forms largely as a result of the breaking of chemical bonds in kerogen as

temperature rises. Petroleum is the organic substance recovered from wells and found

in natural seepages. Bitumen becomes petroleum at some point during migration. Important chemical differences often exist between source rock extracts (bitumen) and

crude oils (petroleum).

Kerogen is of no commercial significance except where it is so abundant (greater than

10%) as to occur in oil shales. It is, however, of great geological importance because it is

the substance that generates hydrocarbon oil and gas. A source rock must contain

significant amounts of kerogen.

Crude Oil

Crude oil is a mixture of many hydrocarbons that are liquid at surface temperatures

and pressures, and are soluble in normal petroleum solvents. It can vary in type and

amount of hydrocarbons as well as which impurities it may contain.

Crude oil may be classified chemically (e.g. paraffinic, naphthenic) or by its density.

This is expressed as specific gravity or as API (American Petroleum Institute) gravity

according to the formula:

sp.grav.@60 F141.5

API o o - 131.5

Specific gravity is the ratio of the density of a substance to the density of water.

API gravity is a standard adopted by the American Petroleum Institute for expressing

the specific weight of oils.

The lower the specific gravity, the higher the API gravity, for example, a fluid

with a specific gravity of 1.0 g cm –3 has an API value of 10 degrees. Heavy oils are

those with API gravities of less than 20 (sp. gr. >0.93). These oils have frequently

suffered chemical alteration as a result of microbial attack (biodegradation) and other

effects. Not only are heavy oils less valuable commercially, but they are considerably

more difficult to extract. API gravities of 20 to 40 degrees (sp. gr. 0.83 to 0.93)

indicate normal oils.

Oils of API gravity greater than 40 degrees (sp. gr. < 0.83) are light.

Asphalt

Asphalt is a dark colored solid to semi-solid form of petroleum (at surface

temperatures and pressures) that consists of heavy hydrocarbons and bitumens. It can

occur naturally or as a residue in the refining of some petroleums. It generally

contains appreciable amounts of sulphur, oxygen, and nitrogen and unlike kerogen,

asphalt is soluble in normal petroleum solvents. It is produced by the partial

maturation of kerogen or by the degradation of mature crude oil. Asphalt is

particularly suitable for making high-quality gasoline and roofing and paving

materials.

Natural Gas

There are two basic types of natural gas, biogenic gas and thermogenic gas. The

difference between the two is contingent upon conditions of origin. Biogenic gas is a

natural gas formed solely as a result of bacterial activity in the early stages of

diagenesis, meaning it forms at low temperatures, at overburden depths of less than

3000 feet, and under anaerobic conditions often associated with high rates of marine

sediment accumulation. Because of these factors, biogenic gas occurs in a variety of

environments, including contemporary deltas of the Nile, Mississippi and Amazon

rivers. Currently it is estimated that approximately 20% of the worlds known natural

gas is biogenic.

Thermogenic gas is a natural gas resulting from the thermal alteration of kerogen due

to an increase in overburden pressure and temperature.

The major hydocarbon gases are: methane (CH4 ), ethane (C2H6), propane (C3H8),

and butane (C4H10).

The terms sweet and sour gas are used in the field to designate gases that are low or

high, respectively, in hydrogen sulfide.

Natural gas, as it comes from the well, is also classified as dry gas or wet gas,

according to the amount of natural gas liquid vapors it contains. A dry gas contains

less than 0.1 gallon natural gas liquid vapors per 1,000 cubic feet, and a wet gas 0.3 or

more liquid vapors per 1,000 cubic feet.

Condensates

Condensates are hydrocarbons transitional between gas and crude oil (gaseous in the

subsurface but condensing to liquid at surface temperatures and pressures).

Chemically, condensates consist largely of paraffins, such as pentane, octane, and

hexane.

There are five types of sedimentary rocks that are important in the production of hydrocarbons: Sandstones

Sandstones are clastic sedimentary rocks composed of mainly sand size particles or

grains set in a matrix of silt or clay and more or less firmly united by a cementing

material (commonly silica, iron oxide, or calcium carbonate). The sand particles

usually consist of quartz, and the term “sandstone”, when used without qualification,

indicates a rock containing about 85-90% quartz.

Carbonates, broken into two categories, limestones and dolomites.

Carbonates are sediments formed by a mineral compound characterized by a

fundamental anionic structure of CO3-2. Calcite and aragonite CaCO3, are examples

of carbonates. Limestones are sedimentary rocks consisting chiefly of the mineral

calcite (calcium carbonate, CaCO3), with or without magnesium carbonate.

Limestones are the most important and widely distributed of the carbonate rocks.

Dolomite is a common rock forming mineral with the formula CaMg(CO3)2. A

sedimentary rock will be named dolomite if that rock is composed of more than 90%

mineral dolomite and less than 10% mineral calcite.

Shales

Shale is a type of detrital sedimentary rock formed by the consolidation of fine-

grained material including clay, mud, and silt and have a layered or stratified structure

parallel to bedding. Shales are typically porous and contain hydrocarbons but

generally exhibit no permeability. Therefore, they typically do not form reservoirs but

do make excellent cap rocks. If a shale is fractured, it would have the potential to be a

reservoir.

Evaporites

Evaporites do not form reservoirs like limestone and sandstone, but are very

important to petroleum exploration because they make excellent cap rocks and

generate traps. The term “evaporite” is used for all deposits, such as salt deposits, that

are composed of minerals that precipitated from saline solutions concentrated by

evaporation. On evaporation the general sequence of precipitation is: calcite, gypsum

or anhydrite, halite, and finally bittern salts.

Evaporites make excellent cap rocks because they are impermeable and, unlike

lithified shales, they deform plastically, not by fracturing.

The formation of salt structures can produce several different types of traps. One type

is created by the folding and faulting associated with the lateral and upward

movement of salt through overlying sediments. Salt overhangs create another type of

trapping mechanism.

Exploration and Mapping Techniques

Exploration for oil and gas has long been considered an art as well as a science. It

encompasses a number of older methods in addition to new techniques. The

exploration is must combine scientific analysis and an imagination to successfully solve

the problem of finding and recovering hydrocarbons.

Subsurface Mapping Geologic maps are a representation of the distribution of rocks and other geologic

materials of different lithologies and ages over the Earth’s surface or below it. The

geologist measures and describes the rock sections and plots the different formations on a

map, which shows their distribution. Just as a surface relief map shows the presence of

mountains and valleys, subsurface mapping is a valuable tool for locating underground

features that may form traps or outline the boundaries of a possible reservoir.

Subsurface mapping is used to work out the geology of petroleum deposits. Threedimensional

subsurface mapping is made possible by the use of well data and helps to

decipher the underground geology of a large area where there are no outcrops at the

surface.

Some of the commonly prepared subsurface geological maps used for exploration and

production include; (1) geophysical surveys, (2) structural maps and sections, (3)

isopach maps, and (4) lithofacies maps.

1-Geophysical Surveys Geophysics is the study of the earth by quantitative physical methods. Geophysical

techniques such as seismic surveys, gravity surveys, and magnetic surveys provide a way

of measuring the physical properties of a subsurface formation. These measurements are

translated into geologic data such as structure, stratigraphy, depth, and position. The

practical value in geophysical surveys is in their ability to measure the physical properties

of rocks that are related to potential traps in reservoir rocks as well as documenting

regional structural trends and overall basin geometry.

a-Seismic Surveys The geophysical method that provides the most detailed picture of subsurface geology is

the seismic survey. This involves the natural or artificial generation and propagation of

seismic (elastic) waves down into Earth until they encounter a discontinuity (any

interruption in sedimentation) and are reflected back to the surface. On-land, seismic

“shooting” produces acoustic waves at or near the surface by energy sources such as

dynamite, a “Thumper” (a weight dropped on ground surface), a “Dinoseis” (a gas gun),

or a “Vibroseis” (which literally vibrates the earth’s surface).

Seismic waves travel at known but varying velocities depending upon the kinds of rocks

through which they pass and their depth below Earth’s surface. The speed of sound

waves through the earth’s crust varies directly with density and inversely with porosity.

Through soil, the pulses travel as slowly as 1,000 feet per second, which is comparable to

the speed of sound through air at sea level. On the other hand, some metamorphic rocks

transmit seismic waves at 20,000 feet (approximately 6 km) per second, or slightly less

than 4 miles per second. Some typical average velocities are: shale = 3.6 km/s;

sandstone = 4.2 km/s; limestone = 5.0 km/s.

b-Magnetic Surveys Magnetic surveys are methods that provide the quickest and least expensive way to study

gross subsurface geology over a broad area. A magnetometer is used to measure local

variations in the strength of the earth’s magnetic field and, indirectly, the thickness of

sedimentary rock layers where oil and gas might be found. Igneous and metamorphic

rocks usually contain some amount of magnetically susceptible iron-bearing minerals and

are frequently found as basement rock that lies beneath sedimentary rock layers.

Basement rock seldom contains hydrocarbons, but it sometimes intrudes into the

overlying sedimentary rock, creating structures such as folds and arches or anticlines that

could serve as hydrocarbon traps. Geophysicists can get a fairly good picture of the

configuration of the geological formations by studying the anomalies, or irregularities, in

the structures.

The earth’s magnetic field, although more complex, can be thought of as a bar magnet,

around which the lines of magnetic force form smooth, evenly spaced curves. If a small

piece of iron or titanium is placed within the bar magnet’s field it becomes weakly

magnetized, creating an anomaly or distortion of the field. The degree to which igneous

rocks concentrate this field is not only dependent upon the amount of iron or titanium

present but also upon the depth of the rock. An igneous rock formation 1,000 feet below

the surface will affect a magnetometer more strongly than a similar mass 10,000 feet

down.

.

c-Gravity Surveys The gravity survey method makes use of the earth’s gravitational field to determine the

presence of gravity anomalies (abnormally high or low gravity values) which can be

related to the presence of dense igneous or metamorphic rock or light sedimentary rock in

the subsurface. Dense igneous or metamorphic basement rocks close to the surface will

read much higher on a gravimeter because the gravitational force they exert is more

powerful than the lighter sedimentary rocks. The difference in mass for equal volumes of

rock is due to variations in specific gravity.

Some types of maps: 1-Structural Contour Maps

Contour maps show a series of lines drawn at regular intervals. The points on each line

represent equal values, such as depth or thickness. One type of contour map is the

structural map, which depicts the depth of a specific formation from the surface. The

principle is the same as that used in a topographic map, but instead shows the highs and

lows of the buried layers.

Contour maps for exploration may depict geologic structure as well as thickness of

formations. They can show the angle of a fault and where it intersects with formations

and other faults, as well as where formations taper off or stop abruptly. The subsurface

structural contour map is almost or fully dependent on well data for basic control

Cross-Sections Structural, stratigraphic, and topographic information can be portrayed on cross-sections

that reproduce horizontally represented map information in vertical section. Maps

represent information in the plan view and provide a graphic view of distribution. Cross

sections present the same information in the vertical view and illustrate vertical

relationships such as depth, thickness, superpostion, and lateral and vertical changes of

geologic features.

Raw data for cross-sections come from stratigraphic sections, structural data, well sample

logs, cores, wireline logs, and structural, stratigraphic, and topographic maps.

2-Isopach Maps Isopach maps are similar in appearance to contour maps but show variations in the

thickness of the bed. These maps may be either surface or subsurface depending on data

used during construction. Isopach maps are frequently color coded to assist visualization

and are very useful in following pinch outs or the courses of ancient stream beds.

Porosity or permeability variations may also be followed by such means. Geologists use

isopach maps to aid in exploration work, to calculate how much petroleum remains in a

formation, and to plan ways to recover it.

3-Lithofacies Maps Lithofacies maps show, by one means or another, changes in lithologic character and how

it varies horizontally within the formation. This type of map has contours representing

the variations in the proportion of sandstone, shale, and other kinds of rocks in the

formation.

Identification of source and reservoir rocks, their distribution, and their thickness’ are

essential in an exploration program, therefore, exploration, particularly over large areas,

requires correlation of geologic sections. Correlations produce cross-sections that give

visual information about structure, stratigraphy, porosity, lithology and thickness of

important formations. This is one of the fundamental uses of well logs for geologists.

Wells that have information collected by driller’s logs, sample logs, and wireline logs

enable the geologists to predict more precisely where similar rock formations will occur

in other subsurface locations.

Surface Geology There are several areas to look for oil. The first is the obvious, on the surface of the

ground. Oil and gas seeps are where the petroleum has migrated from its’ source

through either porous beds, faults or springs and appears at the surface. Locating seeps at

the surface was the primary method of exploration in the late 1800’s and before.

Seeps are abundant and well documented worldwide. Oil or gas on the surface, however,

does not give an indication of what lies in the subsurface. It is the combination of data

that gives the indication of what lies below the surface. Geologic mapping, geophysics,

geochemistry and aerial photography are all crucial aspects in the exploration for oil and

gas.

Subsurface Geology and Formation Evaluation Subsurface geology and formation evaluation covers a large range of measurement and

analytic techniques. To complete the task of defining a reservoir’s limits, storage

capacity, hydrocarbon content, produce ability, and economic value, all measurements

must be taken into account and analyzed.

First, a potential reservoir must be discovered before it can be evaluated. The initial

discovery of a reservoir lies squarely in the hands of the exploration is it using seismic

records, gravity, and magnetics.

There are a number of parameters that are needed by the exploration and evaluation team

to determine the economic value and production possibilities of a formation. These

parameters are provided from a number of different sources including, seismic records,

coring, mud logging, and wireline logging.

Log measurements, when properly calibrated, can give the majority of the parameters

required. Specifically, logs can provide a direct measurement or give a good indication

of:

1-Porosity, both primary and secondary.2-Permeability.3-Water saturation and hydrocarbon

movability.4-Hydrocarbon type (oil, gas, or condensate).5-Lithology.6-Formation dip and

structure.7-Sedimentary environment.Travel times of elastic waves in a formation

These parameters can provide good estimates of the reservoir size and the hydrocarbons

in place.

Logging techniques in cased holes can provide much of the data needed to monitor

primary production and also to gauge the applicability of waterflooding and monitor its

progress when installed. In producing wells, logging can provide measurements of :

Flow rates, Fluid type,Pressure,Residual oil saturations.

Logging can answer many questions on topics ranging from basic geology to economics;

however, logging by itself cannot answer all the formation evaluation problems. Coring,

core analysis, and formation testing are all integral parts of any formation evaluation

effort.

Well Cuttings Well samples are produced from drilling operations, by the drill bit penetrating the formation

encountered in the subsurface. Samples are taken at regular intervals. They are used to

establish a lithologic record of the well and are plotted on a strip sample log.

Cores Cores are cut where specific lithologic and rock parameter data are required. They are cut by

a hollow core barrel, which goes down around the rock core as drilling proceeds. When the

core barrel is full and the length of the core occupies the entire interior of the core barrel, it is

brought to the surface, and the core is removed and laid out in stratigraphic sequence. It is

important to note that the sample may undergo physical changes on its journey from the

bottom of the well, where it is cut, to the surface, where it is analyzed.

Logging While Drilling Formation properties can be measured at the time the formation is drilled by use of

special drill collars that house measuring devices. These logging-while-drilling (LWD)

tools are particularly valuable in deviated, offshore, or horizontally drilled wells.

Although not as complete as open-hole logs, the measurements obtained by MWD are

rapidly becoming just as accurate and usable in log analysis procedures.

Formation Testing Formation testing, commonly referred to as drill stem testing (DST), is a technique for

delivering, to the surface, samples of fluids and recorded gas, oil, and water pressures

from subsurface formations; such data allows satisfactory completion of a well. This

type of testing provides more direct evidence of formation fluids and gases, the capacity

of the reservoir and its ability to produce in the long term, than any other method except

established production from a completed well.

Wireline formation testers complement drill stem tests by their ability to sample many

different horizons in the well and produce not only fluid samples but also detailed

formation pressure data that are almost impossible to obtain from a DST alone.

Wireline Well-Logging Techniques Wireline logging involves the measurement of various properties of a formation including

electrical resistivity, bulk density, natural and induced radioactivity, hydrogen content and

elastic modulae. These measurements may then be used to evaluate not only the physical and

chemical properties of the formation itself, but also the properties of the fluids that the

formation contains. There are open hole logs and cased hole logs. The open hole logs are

recorded in the uncased portion of the wellbore. Cased hole logs are recorded in the

completed or cased well. There are measurements that can be made in both the open and

cased holes and some that can only be made in open holes. Resistivity and density porosity

are two examples of measurements that can be made in an open hole but not in a cased hole.

Perforation is the wireline procedure of introducing holes through the casing (inner wall)

and/or the cement sheath into a formation so that the fluids can flow from the formation into

the casing. Perforating is generally performed to bring a well into production, although it

could also be performed to establish circulation within the wellbore to free a stuck tool string.

Borehole Environment Reservoir properties are measured by lowering a tool attached to a wireline or cable into

a borehole. The borehole may be filled with water-based drilling mud, oil-based mud, or

air. During the drilling process, the drilling mud invades the rock surrounding the

borehole, which affects logging measurements and the movement of fluids into and out of

the formation. All of these factors must be taken into account while logging and during

log analysis. It is important to understand the wellbore environment and the following

characteristics: hole diameter, drilling mud, mudcake, mud filtrate, flushed zone, invaded

zone and the univaded zone.

Hole diameter (dh)— The size of the borehole determined by the diameter of the

drill bit.

References:

Raymond Siever, Sand, Scientific American Library, New York (1988), ISBN 0-7167-5021-X.

1. P.E. Potter, J.B. Maynard, and P.J. Depetris, Mud and Mudstones: Introduction and Overview Springer, Berlin (2005) ISBN 3-540-22157-3.

2. Georges Millot, translated [from the French] by W.R. Farrand, Helene Paquet, Geology Of Clays - Weathering, Sedimentology, Geochemistry Springer Verlag, Berlin (1970), ISBN 0-412-10050-9.

3. Gary Nichols, Sedimentology & Stratigraphy, Wiley-Blackwell, Malden, MA (1999), ISBN 0-632-03578-1.

4. Donald R. Prothero and Fred Schwab, Sedimentary Geology: An Introduction to Sedimentary Rocks and Stratigraphy, W. H. Freeman (1996), ISBN 0-7167-2726-9.

5. Edward J. Tarbuck, Frederick K. Lutgens, Cameron J. Tsujita, Earth, An Introduction to Physical Geology, National Library of Canada Cataloguing in Publication, 2005, ISBN 0-13-121724-0

6. Juergen Schieber, John Southard, and Kevin Thaisen, "Accretion of Mudstone Beds from Migrating Floccule Ripples," Science, 14 December 2007: 1760-1763. See also "As waters clear, scientists seek to end a muddy debate," at PhysOrg.com (accessed 27 December 2007).

7. Joe H. S. Macquaker and Kevin M. Bohacs, "Geology: On the Accumulation of Mud," Science, 14 December 2007: 1734-1735.

8. Robert G. Loucks, Robert M. Reed, Stephen C. Ruppel,and Daniel M. Jarvie "Morphology, Genesis, and Distribution of Nanometer-Scale Pores in Siliceous Mudstones of the Mississippian Barnett Shale", Journal of Sedimentary Research, 2009, v. 79, 848-861.

9 "Geology of Oil," Steven Cooperman, Ph.D.

"Understanding Petroleum Exploration and Production,"

National Energy Foundation, Student Activity Guide

10."The Upstream: A Guide to Petroleum Exploration and Production," Exxon Corporation Informational Brochure

11.NORTH, F. K., 1985, Petroleum Geology: Allen & Unwin,

Inc., Winchester, MA.