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.