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 and Strain:
The concepts of stress, strain and material behavior are fundamental to the
understanding of geological structures
including faults and folds.
Stress is the force applied to each unit area in particular direction.Measured in
Type of Stresses:
1-Normal stress:Perpendicular to plane
2-Shear stress:Parallel to plane
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
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
Brittle rocks: The fracturing of a rock in response to stress with little or no
permanent deformation prior to its rupture.
1. Tectonic history
2. Mineral exploration
3. Gas and oil exploration
4. Geotechnical engineering
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.
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.
Anticline: linear, strata normally dip away from axial center, oldest strata in
Syncline: linear, strata normally dip toward axial center, youngest strata in
Dome: nonlinear, strata dip away from center in all directions, oldest strata in
Basin: nonlinear, strata dip toward center in all directions, youngest strata in
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:
Syncline and Anticline
This diagram depicts an adjacent ANTICLINE and SYNCLINE with their
representative FOLD AXIS and AXIAL PLANES.
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".
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.
3-Cliff at aparticular region.
4-Discontinuity of the strata by displacement.
5-Repetition&omissionof strata due to an unconformity or fault.
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
Types with respect to formation:
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
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
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
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
Image provided by FCIT. Original image from Textbook of Geology by Sir Archibald
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
What does the Grand Canyon tell us about unconformities and the base level
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:
b- Residual or weathered chert nodulus (chert in chalky Lst.)
c-Burried soil profile.
Sedimentary criteria in the non continental(marine) environments:
a-Glauconite:Green colour mineral(Fe.KSio2).
b-Phosphatized pebbles:Shell&bones of animals &fishes.
a-Different in the dip angle of the bed a bove &under the unconformity(eg.angular
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 ,
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
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.
There are three hypothesis or theory for tectonic movements of the earth:
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 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
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)
There are dozens of smaller plates,
the seven largest of which are:
Juan de Fuca Plate
Philippine Sea Plate
Asthenosphere is a partially molten part of the mantle, below the
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
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.
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!
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
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:
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
1. Divergent plate boundaries where plates move apart from one
2. Convergent plate boundaries where plates move toward one
3. Transform plate boundaries whe re plates slide past one
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
Example of a convergent plate boundary. In this case, oceanto-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, oceanto-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.
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
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
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 presentday, 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
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 =
If you could push the continents back together, the ancient
climatic zones, as indicated by the rock types, would match
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
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
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 RidgeEmperor 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
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 seashores,such as Tsunamis travel rapidly & are very destructive when they reach
land with 750 Km|hour.
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
(eg:the foundation of building,dam should rest on asolid rock bed,best materials
should be used in the petroleum engineering projects.)
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
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,
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
2-Volcanic theory:Eruption of hydrocarbon gases from volcanic activity.
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
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
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
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.
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
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
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.
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 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
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.
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
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).
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
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
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.
Combination of structral elements (fold, fault)&stratigraphic elements(unconformity).
The Two Types of 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 nonpermeable, the movement stops. The impenetrable layer is called a “cap rock.”
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
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.
There are three basic forms of a structural trap in petroleum geology:
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.
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
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
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.
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
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
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
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
(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
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
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:
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
Classification of petroleum wells:
1-Exploratory well .
5-Pressure release 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
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 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:
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 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
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 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
There are five types of sedimentary rocks that are important
in the production of hydrocarbons:
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.
Shale is a type of detrital sedimentary rock formed by the consolidation of finegrained 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
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
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.
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
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.
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.
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.
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 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
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
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
Raw data for cross-sections come from stratigraphic sections, structural data, well sample
logs, cores, wireline logs, and structural, stratigraphic, and topographic 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.
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
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.
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
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
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
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
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 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, 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.
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
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