Lecture 2 con't part (2)

2.1.9 Structural geology

2.1.9.1 Introduction

At destructive plate margins, the sediments and the top part of the crust are compressed

and deformed by the process of collision. The rocks are bent and fractured. The study of

the structures that result and the processes that form them is called Structural Geology.

2.1.9.2 Earth movements

Most rocks are fractured during earth movement, resulting in cracks called joints. If the

rock layers on one side of a fracture have moved in relation to the other side, the fracture

is called a fault (Figure 2-4). Displacement - or how far apart the sides of the fault have

moved - may range from only a few inches to many miles, as along the San Andreas fault

in California.

2.1.9.3 Faults

A simple classification system outlines four kinds of faults: normal, reverse, thrust, and

lateral (Figure 2-4). The names are derived from the movement of adjacent blocks.

Movement is up or down in normal and reverse faults but is mainly horizontal in thrust

and lateral faults. A combination of vertical and horizontal movements is also possible in

all faults.

Rotational faults and upthrusts (Figure 2-5) are variations of normal and reverse faulting.

They are most important to the petroleum geologist because they affect the location of

oil and gas accumulations.

Earth movements often bury or prevent the depositing of part of a sediment series that is

present elsewhere. Such buried erosion surfaces are called unconformities. Two general

kinds of unconformities are the disconformity and the angular Figure 2-6). Earth

movements are most important to petroleum geology because they produce barriers that

cause a large proportion of petroleum accumulations.

2.1.9.4 Folds

Folds can be classified in many ways, one of the simplest is into anticlinal and synclinal

folds.

As compressional forces increase, the folds become tighter and the limbs drop more

steeply. Assymetric folds are ones in which one limb dips more steeply than the other.

These dips can eventually become greater than vertical and folds become overturned.

Axial plane cleavage can develop which is caused by alignment of platey minerals

parallel to the fold axis. With increasing deformation this cleavage can dominate the

structure of the rock, obliterating the original bedding. Fold axes need not be horizontal,

in which case they are said to plunge.

If more than one episode of the folding takes place, then the axial planes cleavage

developed by the first phase may itself be folded. This is then known as superimposed

folding and can often be recognized by statistical analysis of several fold axes in one

area.

Folding in sedimentary rocks is important as it creates the potential for oil traps on the

Crest of folds, and these are a major cause of hydrocarbon accumulations.

2.1.9.5 Joints

These are fractures in the rock which are not associated with any significant movement

of the rock. They typically occur in Limestones and Dolomites due to solution along

natural planes of weakness by percolating underground waters, or by removal of

overlying weight of rock by erosion which allows the rock to expand slightly from stress

release, and fracture. They normally develop in three planes, all at right angles, and often

have a strong control on the geomorphology of the area. Jointing in the rocks can lead to

large volumes of porosity and is an important reservoir type, particularly in carbonate

rocks. It can also give lost circulation problems when drilling a highly jointed or

cavernous area.

2.1.9.6 Unconformities

Although these are not strictly structural features, we will look briefly at unconformities.

An unconformity is any break in the geological sequence.

Lecture 2 con't

2.1.3 Sedimentary rock types

In order to differentiate between the various rock types, several classifications are

structured either on the basis of the grain size or on the fundamental mineralogy. The

sedimentary processes which have formed the rock can as well be invoked into the

classification.

When it comes to classifying rocks seen at the well site, we stick to a descriptive

classification and leave environmental factors alone. This simplifies matters a great deal.

We will now go on to look at the main rock types encountered in the drilling of oil wells

and how these rocks can induce the drilling process.

2.1.4 Terrigenous sediments (clastic)

These are land derived sediments and are represented by the clay minerals that coarser

material formed from the fragmentation of silicate rocks. They have been sub-divided

further on the basis of their grain size.

·  Group A - Clays (particles with a diameter less than 0.004 mm)

·  Group B - Silts (particle diameter 0.004 to 0.06 mm)

·  Group C - Sands (particle diameter 0.06 mm to 2 mm)

·  Group D - Rudites (coarser rock fragments).

2.1.4.1 Clays

Clay minerals are hydrous platy aluminosilicates. They form a complex and extensive

series due not only to variations in ordering of the sheet-like crystal lattices, but also to

the presence of different cations between the lattices.

Clay minerals can be subdivided into five important groups with different chemical and

physical characteristics; kaolinite, illites, smectites, chlorites and glauconites.

The term “shale", generally used to name those argillaceous sediments, mostly describes

the tendency of those materials to split, especially when they have been exposed to high

compactions and pressures. Soft clays which are encountered in the topmost sections of

wells usually drill fine unless their affinity to water causes them to form so-called gumbo

formations.

Gumbo is a term used to describe claystone formations which absorb water, hence,

hydrating shales tend to expand. Sticking mechanisms are associated with such

formations:

·  Contraction of the wellbore behind the BHA makes it difficult to trip out.

·  Large clumps of gumbo will fall into the wellbore and will eventually stick to the

BHA. Chemical inhibitors can be added to the drilling fluid in order to restrict or

avoid such phenomenon.

Kaolinite clays generally form by sub-aerial weathering of granites: rocks with a low

proportion of iron or magnesium rich minerals. The clay particles may be washed out as

colloids or formed in situ. When they come in contact with water rich in potassium ions

(for instance sea water) they slowly alter to illite.

Illites are the dominant clay mineral group. They are formed by the direct weathering of

feldspars or by alteration of kaolinite and montmorillonite under marine or later

post-depositional conditions.

Montmorillonites form by the alteration of minerals rich in iron and magnesium. For

instance, certain types of volcanic ash. They too gradually change to illite when

transported into sea water.

Clays may also be the subsidiary minerals of other sedimentary rocks. Their origin

(petrogenesis) may be primary in that they were deposited at the same time as other

major constituents. Alternatively, they may have formed as a later alteration product of

those less stable minerals in the original sediment, their origin in this case being

secondary or diagenetic.

2.1.4.2 Silts

Silts are clastic sediments, intermediate in size between clays and fine sands (particle

diameter 0.004 to 0.06 mm). They are derived from fragmented rocks or minerals and are

called clastic or detrital sediments. Silt size particles are generally the result of extreme

abrasion (mechanical wearing down), and therefore all the minerals found in that size

range may also be found as sand-grade particles. They consist of quartz, feldspar, heavy

minerals, iron ores and phosphates. While sand may be silt free, most clays and

claystones contain about 35% silt or more (thus named silty claystones or argilaceous

siltstones). Very abrasive siltstones can be encountered in the drilling processes and,

therefore, frequent bit changes become necessary.

2.1.4.3 Sands

Sands, like silts, are defined by their grain size (0.062mm) and not by their mineralogy.

Terrigenous or siliclastic sands are of prime, economic importance because they are

often of wide lateral extent and are frequently porous and permeable, thereby satisfying

three basic requirements for major aquifers and hydrocarbon reservoirs.

Quartz, feldspar, lithic fragments, micas and heavy minerals are the major mineralogical

groups found in detrital sands.

2.1.4.4 Rudites

These are sediments whose grain size exceed 2 mm in diameter. This coarse rock

fraction is not unique to terrigenous deposits, and rudite grade particles are common in

both the carbonate and pyroclastic groups.

The shape of the class is also important. Rounded rock fragments which have undergone

physical abrasion are called conglomerates. Angular ones, physically as well as

chemically immature, are termed breccias.

2.1.5 Pyroclastic sediments

These are derived by volcanic eruption into the air. They may be chemically weathered

or physically reworked to closely resemble terrigenous deposits. This is because they

may have similar mineralogies, and the range of grain sizes are comparable.

Volcanic ash or tuff can fall as clay, silt or sand grade particles whereas the still coarser

agglomerate is the direct equivalent of conglomerate.

While traces of volcanic ash are common in most deep sea sediments, pyroclastic

deposits are generally rare within sedimentary sequences.

2.1.6 Carbonates

This major group of sediments is fundamentally different to the terrigenous (or

siliclastic) and pyroclastic rocks just discussed. In these clastic groups, the mineralogy of

the deposits is largely controlled by the processes of weathering and erosion of the

bedrock in the area of the sediment source; namely outside the basin. In carbonate rocks,

however, it is the depositional environment within the basin which exerts the prime

control on the mineralogy and sediment type. In this respect carbonates have closer

affinities to the evaporite and carbonaceous rocks.

Biological activity around the area of deposition is of prime importance in generating the

basic particles of carbonate sediments.

In order to determine the depositional environment and the genesis of carbonate rocks, a

fairly precise description of the chemical and physical components is required.

2.1.6.1 The Chemical Components of Carbonates

The chemical components are:

·  Aragonite

·  Magnesian

·  Calcite

·  Dolomite

All these minerals, know as polymorphs of calcium carbonate (CaC03), present different

degrees of chemical stability depending on the environmental characteristics of the

depositional basin.

2.1.6.2 The physical components of carbonates

Four basic physical components are taken into account for the description of carbonates:

The grain types either mineral or biological.

The matrix which consists of the fine material.

The cement which grows in the pore spaces of the sediment after it deposition.

The pore space remaining after cement has taken place.

2.1.6.3 The Classification of Limestone Rocks

In an essentially monomineralogical calcium carbonate system, there is apparently as

wide a range of particle type as there is in the multi-mineral terrigenous group. The most

successful attempts to solve the nomenclature problem are those of Folk, in which the

basic components of the rock are described, and of Dunham where the basic fabric is

described.

Four more terms are frequently used to describe grain size in carbonate rocks.

·  Group A - Calcilutite up to 0.004 mm grain diameter

·  Group B - Calcisiltite 0.004 to 0.065 mm

·  Group C - Calcarenite 0.065 to 2 mm

·  Group D - Calcirudite above 2 mm grain diameter

Certain specific types of limestones like chalk, marl, bituminous limestones and dolomite

can be as well mentioned at this stage.

·  Chalk This is a soft white limestone composed of the tests (or skeletons) of once

floating micro-organisms.

·  Marl This is a calcareous clay - generally an intermediate mixture of terrigenous

clay and micrite.

·  Bituminous Limestones These are micrites which contain much organic or

carbonaceous matter, mostly in the form of tarry hydrocarbons which are usually

described as bitumen.

·  Dolomite This term is applied to limestones where the calcium carbonate has

been completely replaced by the mineral dolomite.

2.1.7 Evaporites

These sediments, which include mineral salts such as anhydrite. gypsum and rock salt

(halite), are believed to form by precipitation from brines (waters concentrated in salt by

evaporation processes).

They are important as mineral deposits sometimes occurring in thick, relatively pure

mono-mineralogic sequences. They play an important role in petroleum geology, being

excellent cap rocks for oil or gas reservoirs. They are also very plastic and thick salt

sequences deform and flow to produce salt domes. Salt movements frequently produce

hydrocarbon traps.

The more frequently encountered evaporate minerals are listed Table 2-2.

2.1.8 Carbonaceous rocks

Small traces of organic material are present in most sedimentary deposits with the

notable exception of desert red-beds where it has been entirely destroyed by oxidation.

However, in certain very reducing anaerobic (oxygen free) environments it may form an

appreciable proportion of the sediment.

2.1.8.1 Coal

Coals are formed by the action of fungi and anaerobic (oxygen hating) bacteria on

decaying vegetal or “humic" matter in a reducing environment. Compaction by deep

burial is an important agent in reducing the volatile content of the rock. The series

PEAT, LIGNITE, HUMIC COAL, ANTHRACITE expresses the increase in the carbon

content as oxygen and hydrogen are progressively driven off.

2.1.8.2 Oil Shale

This is more an economic rather than geological term, and refers to argillaceous

sediments with an organic content of at least 5%, but generally meaning considerably

higher (20 to 50%). They must be sufficiently rich in organic matter to yield free oil on

heating.

They generally form in lakes where algae matter decays in a strongly reducing (or

anaerobic) environment, thereby preserving the organic material. This is referred to as

"sapropelic" matter and is a good source for oil. Marine equivalents are also known.

2.1.8.3 Bituminous Limestone

This is again an economic rather than geological concept being the direct carbonate

equivalent of terrigenous oil shales. Such sediments may form in lagoons behind a reef.

Lecture 2

2 Fundamentals
About this chapter
A solid foundation in geology as well as algebra and trigonometry is essential to
understanding directional drilling techniques and procedures. Geology, a science based
on the history and structure of the earth, combined with a thorough knowledge of
mathematics, forms the core discipline necessary for proficiency in this complex
business. Although it is far beyond the scope of this manual to completely describe the
total essence of geology and mathematics, this chapter is designed to refresh any
knowledge that should already be a part of the directional drilling trainee.
Objectives of this Chapter
On completing this chapter the directional driller should be able to do the following
exercises:
Part A/Petroleum Geology
1. Describe the phases that occurred during the formation of the earth that resulted in the
various features of its surface and inner structures.
2. Be prepared to explain the several terrigenous sediments (clastic) that can found
under the Earth’s surface.
3. Be prepared to describe the four general classes of sedimentary materials and how
they are categorized by relative size.
4. Be prepared to define pyroclastic sediments and explain the various of sub-groups.
5. Describe the basic concepts of structural geology and how the various elements are
related to exploration and production of petroleum.
Part B/Algebra and Trigonometry
1. Observe and explain miscellaneous equations of algebraic and trigonometric
mathematics.
2. Be prepared to describe geometric descriptions of a circle.
3. Understand and solve equations applicable to various straight line problems.
4. Demonstrate an understanding of angles resulting from intersecting lines.
5. Demonstrate an understanding of circles and parts of circles resulting from
intersecting lines.
6. Demonstrate an understanding of the trigonometric quadrants identified by sine,
cosine and tangent; solve various equations using these quadrants.
2.1 Petroleum Geology
Rock is a natural substance composed of a mineral or group of minerals which lead to the
formation of rocks and to accumulations of hydrocarbons. A study of this phenomenon is
aimed to help directional drillers in understanding of geological environments. Geology
is so essential to the petroleum industry that a knowledge of the basic principles of this
science is desirable for anyone associated with oil or gas.

Geological information is acquired by observing rocks and their relationship to each
other as they were formed in the layers of the earth. Chronological events can then be
reconstructed in order to understand rock formations and, in the particular field of
petroleum geology, to be able to predict where oil accumulations might occur.
2.1.1 Basic concepts of geology
2.1.1.1 Early History of the Earth
The earth is thought to have originated some 4 to 5 billion years ago out of a condensing
cloud of cosmic dust. During its early life, the earth passed through a molten or partially
molten stage induced by gravitational compression or the release of energy by
radioactive elements. During this stage, the components of the earth separated to produce
a heavy core 4,400 miles in diameter, a mantle of lighter material some 1,800 miles thick
and a crust of the lightest materials some 10 to 30 miles thick (Figure 2-1). At the same
time, large amounts of water vapor and gases erupted to form the primeval atmosphere.

A second stage can be considered as the atmosphere developed and the earth cooled, rain
began to fall. Water coming down from the heights and flowing to the valleys eroded the
surface of the earth by removing particles of rocks whereas sedimentary accumulations
deposited in the lower reliefs.
The tectonic activity resulting from the internal adjustments led to the formation of
horsts and grabens filled with sedimentary deposits (Figure 2-2), as it is attested by the
Red Sea between Asia and Africa.

Life began in the oceans sometime in the Precambrian period and eventually spread onto
the land during the Devonian period (about 350 million years ago).
Fossils preserved in deposits attest to the progressive evolution of the fauna and flora
and enabled the succession of rocks to be subdivided into eras and smaller subdivisions.
The more important ones are shown in Table 2-1.

2.1.1.2 How is the duration of eras and subdivisions measured?
Absolute ages for these subdivisions were determined from studies of radioactive
minerals.
2.1.2 The Sedimentary Cycle
The classic cycle is one of uplift, weathering, erosion, transportation, deposition,
lithification and renewed uplift.

2.1.2.1 Weathering
Weathering can be either a physical process, a biological process or a chemical process
which all result in the breakup and the decay of rocks at the earth’s surface. Erosion is the
process or removing newly formed sediments and is caused by four agents which are also
responsible for the subsequent transportation of the sediment. This sediment
transportation can be achieved through the action of gravity, water, glaciers or wind.
2.1.2.2 Erosion and deposition
Erosion results from the action of many agents such as wind, freezing water, waves and
moving ice, which remove particles from the surface of the earth (Figure 2-3).
Unconsolidated deposits (A) resulting from the accumulation of those particles were
compacted by the weight of the overlying sediments (B) and, under the action of pressure
and heat, were transformed into metamorphic rocks. Most oil and gas accumulations
occur in sedimentary rocks.

2.1.2.3 Diagenesis
A name given to the processes which change sediments to a rock. As the energy of the
transporting agent slows down, removed sediments will deposit; the coarsest part would
drop first whereas the fine fraction (such as clay) might then be slowly deposited further
away. This natural segregation of coarse materials from fine, and the soluble from
insoluble, form the basis for the classification of sedimentary rocks.
Diagenesis occurs under the effects of compaction, dewatering (water squeezed out of
the sediments) or cementation (chemical processes). All these mechanisms can be
combined over a certain period of time.