geology & hydrocarbons

8/19/2019 Geology & Hydrocarbons

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GEOLOGY AND HYDROCARBONS

© 2006 ENSPM Formation Industrie — IFP Training

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© 2006 ENSPM Formation Industrie – IFP Training

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SUMMARY

CHAPTER 1INTRODUCTION TO GEOLOGY ................................... 7

1 THE EARTH’S STRUCTURE ......................................................... ............................................... 7 1.1 Vertical structure.............................................................. ........................................................... ........ 7 1.2 Plate tectonics ........................................................ ........................................................... .................. 10

1.2.1 Divergence of plates.. ............................................................ ............................................... 11 1.2.2 Convergence of plates ........................................................... ............................................... 11

1.2.3 Slide of plates (figure 1.7)..................................................... ............................................... 13 1.3 Vertical movements of the lithosphere....................................... ......................................................... 14

2. THE MAIN MINERALS MAKING UP THE LITHOSPHERE .................................................. 17 2.1 Silica and silicates.............................. ............................................................ ..................................... 17

2.1.1 Silica: Si O2 ....................................................... ........................................................... ........ 17 2.1.2 Silicates ................................................... ........................................................... .................. 18

2.2 Carbonates ................................................... ............................................................ ........................... 21 2.3 Evaporite minerals ........................................................... ........................................................... ........ 21 2.4 Other minerals.................................................................. ........................................................... ........ 22

3. ROCKS ....................................................... ............................................................ ........................... 23 3.1 Magmatic rocks............................................ ............................................................ ........................... 24

3.1.1 Identification and classification............................................. ............................................... 24 3.1.2 Origin of magmas................ ............................................................ ..................................... 25

3.2 Metamorphic rocks .......................................................... ........................................................... ........ 25 3.3 Sedimentary rocks....................................................... ........................................................... .................. 25

3.3.1 Nature and origin............................................... ........................................................... ........ 26 3.3.2 General characteristics of sedimentary rocks ..................................................... .................. 29

3.3.3 Classification of sedimentary rocks........................................................... ........................... 33

4. TECTONICS ........................................................ ........................................................... .................. 47 4.1 Brittle distortion.... ........................................................... ........................................................... ........ 47

4.1.1 Joints, tension fractures ......................................................... ............................................... 47 4.1.2 Faults....................................................... ........................................................... .................. 47

4.2 Ductile formation: folds ................................................... ........................................................... ........ 49 4.3 Measurements of distortions ...................................................... ......................................................... 52 4.4 Dating of tectonic distortions..................................................... ......................................................... 52


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5. SEDIMENTOLOGY AND SEDIMENTARY BASINS ......................................................... ........ 53 5.1 Different types of sedimentation.......................................................... ............................................... 53

5.1.1 Detrital sedimentation ........................................................... ............................................... 53

5.1.2 Chemical sedimentation ........................................................ ............................................... 53 5.1.3 Biochemical sedimentation ................................................... ............................................... 54

5.2 Variation of sea level ....................................................... ........................................................... ........ 55 5.2.1 Transgression .................................................... ........................................................... ........ 56 5.2.2 Regression......................................................... ........................................................... ........ 57 5.2.3 Sedimentary cycle ....................................................... ......................................................... 57

5.3 Sedimentation environments.............. ............................................................ ..................................... 58 5.3.1 Continental environment ....................................................... ............................................... 58 5.3.2 Lakeside environment ........................................................... ............................................... 58 5.3.3 Marine environment .................................................... ......................................................... 58

5.4 The requisite time for the deposit of a layer............................................................. ........................... 63 5.5 Sedimentary basins .......................................................... ........................................................... ........ 63

5.5.1 Basins linked to a thinning of the lithosphere............................................................... ........ 64 5.5.2 Basins linked to the flexion of the lithosphere ................................................... .................. 69

6. STRATIGRAPHY ......................................................... ........................................................... ........ 72 6.1 Stratification............................................................................... ......................................................... 72 6.2 Lithostratigraphic units .................................................... ........................................................... ........ 74

6.2.1 Formation .......................................................... ........................................................... ........ 74 6.2.2 Sequence ........................................................... ........................................................... ........ 74

6.3 Stratigraphical discontinuities: lacks and discontinuities............................................................ ........ 74 6.4 Dating of layers......................... ........................................................... ............................................... 75

6.4.1 Radiochronology – Absolute dating of a rock.................................................... .................. 76 6.4.2 Relative chronology .................................................... ......................................................... 76 6.4.3 Stratigraphical scale .................................................... ......................................................... 77 6.4.4 Practical methods of stratigraphy .................................................... ..................................... 79

CHAPITRE 2HYDROCARBONS ................................................ 81

1. GENERALITIES ........................................................... ........................................................... ........ 81 1.1 Chemical composition................................................................ ......................................................... 81

1.1.1 Satured hydrocarbons............................................................ ............................................... 81 1.1.2 Non-saturated hydrocarbons............................................................ ..................................... 81 1.1.3 Resins and asphaltenes .......................................................... ............................................... 82

1.2 Main hydrocarbon categories....................... ............................................................ ........................... 82

1.2.1 Natural gases ..................................................... ........................................................... ........ 82


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1.2.2 Crude oils .......................................................... ........................................................... ........ 83 1.2.3 Solid hydrocarbons...................................................... ......................................................... 83

2 HYDROCARBON GENESIS ........................................................... ............................................... 84

2.1 Organic matter at the origin of hydrocarbons .......................................................... ........................... 85 2.2 Organic matter environments and hydrocarbon formation....................................... ........................... 85

2.2.1 Shaly-sandy paralic environments........... ........................................................... .................. 87 2.2.2 Carbonate and evaporitic environments .................................................... ........................... 87 2.2.3 Lacustrine environments....................................................................................................... 87

2.3 Tansformations leading from the organic matter up to hydrocarbons............ ..................................... 88 2.3.1 Kerogen formation. Biochemical diagenesis...................................................... .................. 88 2.3.2 Hydrocarbon formation ......................................................... ............................................... 91 2.3.3 Influence of the time factor ................................................... ............................................... 92

2.3.4 Conclusion............................................................................. ............................................... 92

3. MIGRATION ....................................................... ........................................................... .................. 93 3.1 Primary migration .................................................. ........................................................... .................. 93 3.2 Secondary migration ........................................................ ........................................................... ........ 94 3.2.1 Mechanisms and criteria acting upon secondary migration................................................................. 96

3.2.2 Main directions of migration........................................................... ..................................... 98

4. FORMATION OF ACCUMULATIONS................................................... ..................................... 100 4.1 Hydrocarbon trapping - closure ........................................................... ............................................... 100

4.1.1 Geometrical standpoint.................................................................... ..................................... 100 4.1.2 Dynamic standpoint................................. ........................................................... .................. 100 4.1.3 Closure and hydrodynamics ............................................................ ..................................... 101 4.1.4 Concept of impregnated zone closure ....................................................... ........................... 101

4.2 Reservoirs .................................................... ............................................................ ........................... 101 4.2.1 Sandstone reservoirs...................... ............................................................ ........................... 102 4.2.2 Carbonate reservoirs................................ ........................................................... .................. 102 4.2.3 Fractured reservoirs............. ............................................................ ..................................... 102

4.3

Permeability barriers (cap rocks) ......................................................... ............................................... 102 4.3.1 Petrophysical characteristics ........................................................... ..................................... 102

4.3.2 Main permeability barrier rocks ...................................................... ..................................... 103 4.4 Relationship between reservoir and cap rock................... ........................................................... ........ 103 4.5 Different types of traps .................................................... ........................................................... ........ 104

4.5.1 Structural traps .................................................. ........................................................... ........ 104 4.5.2 Stratigraphic traps ....................................................... ......................................................... 106 4.5.3 Combination traps - salt domes ....................................................... ..................................... 108


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APPENDIX ....................................................... 111

Table of grain size classes.......................................................... ......................................................... 111

Terciary and quaternarty cenozoic eras................................................ ............................................... 112 Secondary Mesozoic era .................................................. ........................................................... ........ 113 Primary paleozoic era ...................................................... ........................................................... ........ 114 Representation of sediments ...................................................................................................... ........ 115

GLOSSARY OF GEOLOGY ........................................ 127


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CHAPTER 1

INTRODUCTION TO GEOLOGY

The aim of geology, or Earth’s science, is to describe the Earth’s composition andstructure, to try to regenerate its history and to use that knowledge in order to find depositssuch as hydrocarbons.

It gathers various disciplines such as mineralogy, petrology, geochemistry,sedimentology, stratigraphy, tectonics and paleontology.

Geologic phenomena, except seisms and volcanic eruptions, are very slow andimperceptible phenomena to human scale. In geology, time unit is the million of years.

The Earth approximately exists for 4 600 millions of years, whereas the beginning of theuniverse is estimated to 15 000 million years old. The Earth’s atmosphere was originallydifferent from todays’. It permitted the synthesis of important organic molecules, allowingthen the progressive development of life. Up to the beginning of primary era, about 570million years ago, living organisms have left visible fossilized tracks (see appendix: thestratospheric scale with the principal steps of the development of living organisms).

1 The Earth’s structure

1.1 Vertical structure

The study of seisms reveals a structure of concentric layers (figure 1.1). It exist strongvariations in the speed of the transmission of seismic waves between each layer which pointsout either chemical variations in the environment (a change into the mineralogicalcomposition of the components) or physical variations (fluid, viscous or solid material). From

the Earth’s surface to its centre, you can notice:• The oceanic crust with an average density of 2.9 making up the floor of the oceans and

the continental crust with an average density of 2.7 making up the continents. They canbe distinguished by their thickness and by their mineralogical composition. The junctionof both types of crust is located under the continental slope.

The oceanic crust is about 7 km thick. It is mostly composed of basaltic rocks.

The continental crust is about 30 km thick and up to 70 km under the mountain range. Itis mostly composed of granitic rocks.

The constituents of basalt and granite are mainly aluminosilicates with high calcium,

sodium and potassium contents.


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The study of seisms gives information about the Earth’s structure with its different concentric layers.

FIG. 1.1 Vertical structure

They can also be distinguished by their age: the oldest oceanic crust ever known is 200million years old at the most, whereas the continental crust is up to 3 700 million yearsold in some areas.

The oceanic and continental crusts are covered with sedimentary rocks of a variablethickness.


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• The mantle separated from the crust (continental and oceanic) by the Mohorovicicdiscontinuity or Moho (figure 1.2). This discontinuity is due to a mineralogical change,ferro-magnesian silicates are dominant. The average density of the mantle is 3.4. Youcan see:

The upper mantle composed of:- A rigid layer which is able to stand up to about 100-bar constraints without anynotable deformation, located between the Moho and an about 100 km-depth (70 kmunder the oceans and up to 150 km under the continents). This layer and the crust(oceanic and / or continental) build up the lithosphere.

- The asthenosphere, located between 100 and 300 km in depth, characterized by aslowing up of the seismic waves. This layer is not rigid but is able to flow because ofweak constraints, allowing then the lithosphere to move. It seems that this layer is theseat of movements of thermal convections.

- A transition area, located between 300 and 700 km in depth, characterized by a high

rise in the velocity of the seismic waves.

FIG. 1.2 Details of the lithosphere and the asthenosphere

The lower mantle called mesosphere, located between 700 and 2 900 km in depth,characterized by a lower rise in the speed of seismic waves.

• The external core (2 900 to 5 100 km in depth) separated from the mantle by thediscontinuity of Gutenberg. It is a physical and chemical discontinuity at the same time:the external core stands up as a fluid and would be essentially composed of iron andnickel. This layer would be responsible for the existence of the Earth’s magnetic field.


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• The seed or the internal core (5 100 to 6 370 km in depth) separated from the externalcore by the discontinuity of Lehman. This layer would have the same chemicalcomposition as the previous one except that the latter would be solid. The temperaturein the Earth’s centre is about 6 000°C.

1.2 Plate tectonics

The lithosphere, a rigid layer composed of the continental and/or the oceanic crust and ofa small part of the mantle, is divided in manyplates independent from each other (figure 1.3).These plates, which have a shape of a segment of a sphere, are moving on the asthenosphereand their limits are independent from the continent-ocean limit.

FIG 1.3 Layout of the different tectonic plates

The displacement of plates is due to cells of thermal convection between an ascendingand/or parting thermal spring responsible for the accretion of oceanic ridges and continental rifts and a descending and/or separating spring creating subduction and collision areas. Theresult of this would be a traction of the oceanic ground towards the areas of convergence ofplates. The seismic Earth’s activity is located at the junction of plates.

It exists three different types of contact between the lithospheric plates (figure 1.4). Theseare:

- The zones of divergence characterized by tectonics in extension (normal faults, graben).These are the medio-oceanic ridges and the continental rifts where there is an accretionof the oceanic crust.

- The zones of convergence characterized by tectonics of compression (inverted faults,thrust faults). These are the zones of subduction.

- The zones of slide characterized by tectonics of shearing off. These are transform faults.


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Plate tectonics gives a relatively simple explanation of volcanism, seisms and their layouts,the formation of the mountain range and the sedimentary basins.

FIG 1.4 Different types of contact between the plates

1.2.1 Divergence of plates

The divergence occurs on the ridges or medio-oceanic ridges and the continental rifts.There is a supply of basaltic magma from the deep layers of the mantle and a permanentcreation of oceanic crust. Geophysical measurements, based on periodical inversions of thepolarity of the Earth’s magnetic field, prove that the relative speed of displacement(expansion rate) is about 5 to 10 cm a year (up to 17 cm / year in the South Pacific).

Along the ridge, volcanism is intensive and basaltic (very fluid lavas). Seisms arenumerous but have a low magnitude; their centres are near the surface (10 to 20 km in depth).

1.2.2 Convergence of plates

Various cases are possible according to the nature of the crust (figure 1.5).

FIG. 1.5 Different types of convergence of plates


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1.2.2.1 Convergence of two plates of oceanic crust

When one plate sinks under another, there is subduction. At the Earth’s surface, it ischaracterized by a deep oceanic trench edged with an insular arc. Seismicity and volcanismare important (case of Japan and The Philippines).

1.2.2.2 Convergence of a plate of oceanic crust and a plate of continental crust

There is subduction of the oceanic crust, which is the most dense, under the continentalcrust. An oceanic crust is formed on the fringe of the continent as well as a mountain range ofhigh seismicity (case of the Andes Cordillera, the Rocky mountains).

1.2.2.3 Convergence of two plates of continental crust (figure 1.6)

There is a collision between continental crusts, which cannot sink into the mantle because

of the low density of the materials. The result of this is the surrection of a mountain range(case of The Himalayas). Compression is intense and provokes thrust faults with horizontaldisplacements of pieces of crust ten up to hundreds kilometres thick.

FIG. 1.6 Example of convergence of two plates of continental crust

Formation of the Himalayas

The zones of subduction are characterized by an andesitic volcanism (explosive eruptionsand viscous lavas) and by the presence of very deep oceanic trenches (case of the pacifictrenches that reach up to 11000 metres in depth). Seism cores are deeper than in the case ofoceanic ridges. Seismic activity can be found on an oblique surface that lies up to 700 km indepth (at this level the subducted plate is completely digested in the deep layers of themantle).

The speeds of subduction are lower than the speeds of expansion.


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At the beginning, India and Asia were 7000 km apart and separated by an oceanic ground.For the 30 million years that have passed before the collision, the speed of the joining wasabout 10 cm a year. It is now 5 cm a year. Since the collision of the two continents, the

joining of the two plates has been estimated at 2000 km.

1.2.3 Slide of plates (figure 1.7)

The slide occurs along transform faults when two plates in contact move along oppositedirections or at different speeds. These faults usually affect the whole thickness of thelithosphere. The case of California is the best known where there is a slide along San Andreasfault.

FIG. 1.7 Example of a slide of plates linked to a ridge

The speeds of displacement are about 5 cm a year. These areas are the centre of seisms ofhigh magnitude: the epicentres are near the surface. On the contrary, there is no volcanism.


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Slides also exist in areas of convergence, collision or divergence.

Mountain ranges also called slide ranges and sedimentary basins called “pull-apart” mayappear after a slide. That is the case in the Sinai - The Dead Sea - Lebanon area.

The present structure of the lithosphere in plates, which move independently from eachother, gives a temporary feature to the continents and the oceans (at the scale of geologicaltimes).

Along geological times, continents were on different places than they are today. TheAtlantic Ocean opened about 150 million years ago, creating the split between Africa andSouth America (figure 1.8). India separated from the Antarctic continent about 80 millionyears ago and collided with Asia giving birth to The Himalayas.

FIG. 1.8 Position of the continents on the Jurassic

1.3 Vertical movements of the lithosphere

As well as horizontal movements of plates on the asthenosphere, vertical movementsoccur. These movements have variable magnitudes and usually affect important surfaces(from some thousand to hundreds thousand square km). We notice that mountain ranges tendto rise while transforming and oceanic zones tend to sink while they get further from theoceanic ridge.


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Measuring the value of the acceleration of gravity on the Earth’s surface, we can noticethat it exists much more low amplitudes than expected. In order to explain this phenomenon,we have to suppose that a compensation of anomalies of gravity occurs in depth. It exists asurface of compensation, approximately corresponding to the lithosphere-asthenosphere limit,where the value of the acceleration of gravity would be constant on the whole Earth’s surface.

The rigid lithosphere floating on the asthenosphere (which performs like a fluid) rises to avertical position of balance after a longer or shorter lapse of time. The height, reached by thesurface of the lithosphere in relation to the surface of compensation, depends on the thicknessand the density of the different layers making up the lithosphere.

When the thickness (presence of mountain ranges coming from the asthenosphere) or thedensity of the lithosphere (due to variations of the materials temperatures) are modified bytectonic or thermal phenomena, an adjustment of the levels called isostatic adjustment occurs.

• In certain areas an upheaval of the lithosphere occurs: that is the case of Scandinavia,

which rose 400 m during 12 000 years because of the melting of the icecap whichcovered this area (figure 1.9). The rate of the rise has sometimes reached 1 cm/ year.

• In other areas, the lithosphere sinks in.

Lines of equal upheavals of Scandinavia since 12000 years bc(The discontinuous line shows the extension of the icecap around 15000 years bc).

FIG. 1.9 Example of vertical mobility of the Earth’s crust

Speed of upheaval of the median part of Scandinavia from 15000 years bc up to now.

We call subsidence, the phenomenon of tectonic and/or thermal origin that locally causesthe gradual sinking of the lithosphere. The subsidence allows the permanence of theconditions of sedimentation in the lithosphere. The speed of subsidence is about a few metresper million of years. Sedimentation does not entail the subsidence but it is the gradual sinkingof the crust that allows the deposit of important layers of sediments.


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Subsidence has numerous cases, the main ones are:

- A stretch (figure 1.10) of the lithosphere which leads to its thinning due to the risingof the asthenosphere because of tension stresses.

- The cooling of the lithosphere which increases its density (thermal subsidence). This

cooling causes the rupture of the lithosphere after a certain lapse of time (about 200million of years) and leads to the formation of a new zone of subduction.

- An overload due to sedimentary, volcanic deposits or to the presence of an icecap.

- A flexion of the lithosphere or a folding of syncline shape because of compressionstresses usually located next to zones of subduction.

FIG. 1.10 Example of subsidence due to a thinning in the continental crust The different sources of the subsidence may take over from one another (stretchening

followed by a thermal subsidence). The thinning of the lithosphere, its flexion and the thermalsubsidence are the origin of two types of sedimentary basins located in the interplate zone forone and in the border of plates for the other.

Note: It exists various cases of subsidence provoked by the extraction of hydrocarbons.That is the case for the Ekofisk field, with a sinking of about ten cm a year(about 50 cm/year now).


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2. The main minerals making up the lithosphere

Only a limited number of chemical elements take basically part in the making up of therocks of the lithosphere. Only eight elements constitute virtually 100% of its making up.

Oxygen O 46.60% Magnesium Mg 2.09%

Silicon Si 27.72% Hydrogen H 0.14%

Aluminium Al 8.13% Phosphor P 0.10%

Iron Fe 5.00% Manganese Mn 0.09%

Calcium Ca 3.63% Barium Ba 0.04%

Sodium Na 2.83% Carbon C 0.02%

Potassium K 2.59% Chlore Cl 0.01%

The percentages of this board are percentages of mass.

Silicon, aluminium, and oxygen combine to make up a large number of minerals.

2.1 Silica and silicates

These are the most common minerals. They also are the minerals that gather the greatestnumber of families.

2.1.1 Silica: Si O2

It mostly comes in an anhydrous and crystallized state.

Quartz is widespread in eruptive rocks and also in sedimentary rocks, since it is verystrong to weathering agents (it is only attacked by hydrofluoric acid) and it is hardly soluble inwater. Its forms are varied: transparent and clear when pure, milky white or more or lesscoloured by impurities.

Other varieties are:

- Chalcedony which is a fibrous, badly crystallized and disorganized silica. It iscommon in sedimentary rocks.

- Opal, which is an hydrated silica, made up of fine-grained crystals. It is mostlyassociated to chalcedony in sedimentary rocks (flint, millstone).


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2.1.2 Silicates

Silicates build up a mineral family of very varied chemical composition. They build upalmost all the rocks of the lithosphere. They have chain- and layer-shaped structures and makeup complex spatial constructions. They become altered much easier than quartz.

We can mainly distinguish:

- Light-coloured minerals or white elements (feldspars and feldpathoids),

- Dark-coloured or black minerals (amphiboles and pyroxenes),

- Phillitic minerals that cleave in very thin layers (micas and clayey minerals).

2.1.2.1 Feldspar group

These are anhydrous aluminosilicates, which contain variable proportions of sodium and

calcium depending on the types. Generally speaking, all these minerals are light-coloured,milky white and sometimes pinkish or bluish.

We can distinguish:

• Potassium feldspars: orthoclase and microline are the two representatives of this family.These minerals are mostly seen in acid and eruptive rocks (rocks rich in free silica suchas granites).

• Sodium and calcium feldspars or plagioclases: they gather a string of aluminosilicatesprogressively going from one to the other changing Si-Na into Al-Ca. The sodic pole iscalled albite whereas the calcium pole is called anorthite. Acidity decreases from one to

the other.• Feldpathoids: these are alkaline silico-aluminates poorer in silica than feldspars proper.

These minerals can be essentially seen in basic eruptive rocks.Feldspars are widespread in eruptive and metamorphic rocks. The classification of theserocks is mostly based on the nature of feldspars, which compose them. They rarely canbe found in sedimentary rocks.

Weathering of feldspars:

Feldspars are fairly quickly weathered at outcrops and mainly by hydrolysis (action ofwater). Some ions are released in solution with a part of silica. In this way, different clayeyminerals appear depending on the environmental conditions where the weathering takes place(slow or rapid leaching, hot or cold climate, presence or absence of vegetation, pH ofwater...). The very easy destruction of these minerals explains their absence in sedimentaryrocks.

2.1.2.2 Group of amphiboles and pyroxenes

These minerals are rich in iron and magnesium but poor in aluminium. They can be foundnumerous in eruptive rocks and metamorphic rocks. These minerals are rarely present insedimentary rocks because of their easy weathering, such as feldspars.


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2.1.2.3 Group of phyllites

These minerals are characterized by a structure of very thin piled up layers or by a fibrousstructure. Micas and clayey minerals are the main representatives of this group. Micas mayform very large-sized crystals, whereas clayey minerals are always small-sized minerals

(some microns).

As a general rule, silicates of aluminium, potassium and magnesium and more seldom insilicates of iron differ from the other silicates by the presence of hydroxyl OH - groups in thecrystalline network. The making up of each layer, the periodicity of the piling-up and thenature of interfoliar ions are characteristic of the mineral.

FIG. 1.11 Structure of the different clayey minerals

Each sheet is composed of at least 2 layers:

- A tetrahedral layer composed of SiO4 groups linked by covalent bonds.

- An octahedral layer where an Al³+ ion is present in the centre of the octahedron andoxygen atoms or OH - groups are on the summits.

a) Micas

The two main micas are the muscovite or “white mica” characterized by the absence ofmagnesium and iron and the biotic or “black mica” which contains iron and magnesium.These minerals are especially present in crystalline rocks and can be found in sedimentaryrocks (muscovite, in particular, holds out well against chemical agents). Micas become alteredto give clayey minerals.


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b) Clay minerals

These are minerals of a complex structure and chemical composition. They can only beidentified by diffraction to X-rays. They usually come from the alteration of other minerals

and build up the main part of sedimentary rocks (about 80%). Along the diagenesis, thesedifferent minerals progressively change into illites and then into micas along themetamorphism.

The main elements of this group are:

• Koalinite: the basic sheet is composed of two layers (a tetrahedral layer and anoctahedral one). It is the simplest clayey mineral.

• Illites are very common minerals, which have a very close structure from that of micas.The sheet is composed of three layers (an octahedral layer between two tetrahedrallayers). The substitutions of atoms of silicon by atoms of aluminium are numerous in

the tetrahedral layer (from one to four). A substitution of atoms of aluminium by atomsof iron and magnesium may occur in the octahedral layer. It leads to a high electriccharge deficiency on the external side of the tetrahedral leaves. Potassium ions take up aposition between the sheets in order to fill this imbalance. The structure is such thatwater molecules cannot penetrate the crystalline network. Illites are non-swelling clays.

• Smectites, whose montmorillonites are the most spread (main constituent of thebentonite), are minerals in sheets composed of three layers such as illites. Substitutionsin the octahedral and tetrahedral layers are less numerous than in the case of illites andthe electric charge deficiency is lower (if there is no substitution, sheets tend to repeleach other). The result is that the bond between sheets is low and water molecules and

cations can be incorporated between the sheets producing the swelling of the mineral.The importance of the swelling depends on the cations present in solution.

In presence of Na+ ions, the swelling can be so important that the sheets are completelyseparated (phenomena of dispersion and defloculation of clays). On the contrary, K+ andespecially Ca++ ions limit the entry of water molecules and maintain the sheets betweeneach other. These different cations can be easily exchanged and that leads to differentbehaviours according to the types of smectites.

We take advantage of these exchanges of cations with mud made of calcium (lime,gypsum) and of potassium to limit the swelling of clays and to reduce the bore holeproblem during drillings. On the contrary, we use sodium ions (caustic soda) to make

bentonite swell and rise the viscosity of the mud.During the diagenesis, smectites progressively change into illites.

• Vermiculites have sheets separated by various cations but mainly by magnesium. Thenumber of water layers between sheets is usually restricted to two layers.

• Attapulgites or sepiolites are fibrous minerals which have a structure in bands oftetrahedrons and not in sheets. The external band is neutral, electrically speaking, sothey are little reactive minerals.

• Chlorites and serpentines mainly come from the alteration of ferromagnesium silicates(mainly micas). They are usually fibrous minerals or set up of four-layer sheets.


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• The interstratified minerals are composed of sheets coming from different clayeyminerals, which come in between each other. Interstratified minerals of illite andmontmorillonite and those of chlorite and vermiculite are the most common. Thesequence is usually at random. These minerals easily spread in water.

c) Geological importance of phylites

Micas and feldspars are not numerous in sedimentary rocks because they easily becomealtered producing clayey minerals such as vermiculites, smectites and interstratified accordingto the environmental conditions where the alteration occurs.

Clayey minerals are the main constituents of sedimentary rocks. They can lose, gain orexchange ions with the solutions they are impregnated in, modifying the environment whilethey are altering. Moreover, they are usually rich in organic matter and therefore they activelytake part in the genesis and the trapping of hydrocarbons.

2.2 Carbonates

Carbonates are minerals mainly found in sedimentary rocks.

• Calcite: CaCO3

Calcite is soluble in hydrochloric acid and the reaction is accompanied by the release ofcarbon dioxide. This property is commonly used to identify calcite in rocks. Sincecalcite is soluble in water containing dissolved carbon dioxide, it is easily carried out inthe subsoil. It is sometimes the only component of some calcareous rocks and accounts

for a substantial proportion of the external skeleton (or test) of living organisms.• Aragonite: CaCO3

Aragonite is less spread than calcite, which is the stable form of calcium carbonate. Itslowly changes into calcite. It accounts for a substantial proportion of the shells ofmolluscs and gasteropoda.

• Dolomites: Mg Ca (CO3)2

Dolomite is less soluble than calcite and reacts very slowly to hydrochloric acid innormal temperature conditions. It can make up rocks on its own, called dolomites, but itusually coexists with calcite.

2.3 Evaporite minerals

This term covers the rock-forming minerals, which have precipitated from brines producedduring the evaporation of seawater. Chemically speaking, they mainly are sulphates andchlorides.


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• Sulphates:

- Calcium sulphate:

Calcium sulphate has two aspects: anhydrite CaSO4 and gypsum CaSO4, 2H2O.

Anhydrite usually occurs as an intergrowth of fine crystals. Generally colourless, it is

frequently bluish grey. It is rarely found in outcrops where it is changed into gypsum.Gypsum is transparent. When heated over 100°C, it is changed into plaster. During thediagenesis, it is changed into anhydrite at low depth (about 500m).

- Barium sulphate or barytine:

Because of its high specific gravity (4.2) and its low reactivity, barium sulphate isused to weigh drilling muds.

• Chlorides:

There are several minerals in evaporitic formations. The two most common are halite (NaCl) and sylvite (KCl). Because of their high solubilities in water, chlorides are onlyfound in outcrops in arid climates.

During the evaporation, calcium carbonate first precipitates, then calcium sulphate comesand finally sodium chloride comes.

Halite, anhydrite and gypsum are the most spread minerals of this category.

2.4 Other minerals

Among the sulphide minerals, pyrite is the most common in sedimentary rocks. Itsoccurrence indicates a reducing environment.

Among the oxide minerals, iron oxides are the most widespread. They are very oftendispersed throughout the sedimentary rock volume, giving it a reddish colour. The mostcommon are hematite and limonite, which are essential constituents of sedimentary iron ores.


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3. Rocks

The Earth’s crust is made of rocks and their area of study is called petrology. Theproperties of rocks can be considered as a “record” of their history. Rock properties can helpto reconstruct the conditions of formation and the type of environment that existed when theywere formed.

There are three main categories of rocks:

• Magmatic rocks: they are the result of the cooling and crystallization of moltenmagmas located at several dozen kilometres in depth. They are also called eruptive orcrystalline rocks. They are often separated in volcanic and plutonic rocks.

• Metamorphic rocks: they are the result of the transformation of sedimentary rocks

subjected to elevated pressures and temperatures.• Sedimentary rocks: they are the result of action of surface processes leading to the

deposition of loose sediments, usually full of water, which are changed into more or lessindurate rocks by gradual burial at moderate depths.

The volume of magmatic and metamorphic rocks represent 95% of the rocks making upthe lithosphere. Although sedimentary rocks only represent 5% of the Earth’s crust, theyset up 75% of the rocks in outcrops. As other rocks, they are mainly composed ofsilicates. Only a low proportion is composed of carbonates and evaporites.

These three categories of rocks build up a closed cycle in the Earth’s crust (figure 1.12).

FIG. 1.12 The cycle of rocks in the Earth’s crust


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3.1 Magmatic rocks

Magmatic rocks result from the crystallization of molten magma, which comes from theEarth’s depths. Occasionally, when weathered and fissured, they may be oil-bearingreservoirs. However, the evidence of magmatic activity is usually a negative criterion for oil

prospects. They form the major part of the basement of sedimentary basins.

3.1.1 Identification and classification

Magmatic rocks are essentially made of interlocking crystals of silicates. Identificationcriteria are based on the texture of the rock, which reflects crystallization speed andenvironment, as well as the determination of the nature and the proportion of minerals whichindicate the chemical composition of the magma.

3.1.1.1 Texture

Different textures can be distinguished:

• A granular texture when all the crystals are equally developed. This texturecorresponds to a slow cooling in depth, which promotes the development of a smallnumber of large crystals.

• A microgranular texture when some large crystals appear to float in a mass of smallercrystals.

• A microlithic texture that exhibits some large isolated crystals floating in ahomogeneous mass of very small crystals surrounded by glassy material. The coolingoccurred in several stages.

• A glassy texture without any crystal: this corresponds to a sudden cooling (e.g. volcanicbombs).

3.1.1.2 Mineralogical composition of magmatic rocks and classification

Different mineralogical combinations reflect the silica content. They allow to distinguishgroups of rocks rich in silica called acidic rocks and groups of poorer rocks called basic rocks.

The classification of the magmatic rocks is given below.Groups Acid Intermediate Basic

Silica content Over 66% 66 to 52% 52 to 44%

Granular Granite Diorite Gabbro

Microgranular Microgranite Microdiorite Microgabbro/dolerite

Microlithic rhyolite andesite basalt


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3.1.2 Origin of magmas

Among rocks resulting from the consolidation of magma, two categories can bedistinguished: volcanic rocks that solidified on the Earth’s surface and plutonic rocks whichcrystallized in depth.

Basaltic magmas come from the depths whereas acid magmas come from the melting ofthe crust, thereby resembling the last stage of metamorphism.

The silica/oxygen connection is a magma fluidity gauge. Magmas rich in silica producevery viscous lavas and an explosive volcanism as prevailing characteristic. Magmas poor insilica produce very fluid lavas (Hawaiian type volcanism).

3.2 Metamorphic rocks

Metamorphic rocks result from the transformation of pre-existing rocks under highpressure and temperature conditions. The overall process of transformation is calledmetamorphism.

Different degrees or stages of metamorphism can be distinguished:

• Dynamic metamorphism deforms and crushes rocks, but few or no new minerals areformed. There are stages of gradual transition from the simple compaction of sedimentsunder their own weight to the effects of high stresses caused by tectonic movements.Both are reflected in a mechanical rearrangement of the minerals, particularly the sheetsilicates, which tend to lie flat and become oriented perpendicular to the axes of stress.In the extreme case, the minerals themselves are deformed. Dynamic metamorphismproduces shales and slates.

• Metamorphism (sensu stricto) involves a temperature increase in addition tomechanical stresses. It causes the appearance of new minerals called metamorphicminerals.

The following distinction has to be made:

• Regional metamorphism produced by burial of formations at great depths (8 000 to10 000 m) where high temperatures and pressures prevail.

• Contact metamorphism, which is more localized and related to the effects of magmaticmassifs on the surrounding rocks. There is mainly an increase in temperature.

3.3 Sedimentary rocks

Formation and trapping of hydrocarbons are strictly related to this rock type, that is thereason why they are interesting for oil prospecting.


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3.3.1 Nature and origin

Contrary to magmatic rocks, sedimentary rocks are formed by the accumulation of mineralsubstances on the Earth’s surface according to a number of processes. The main part resultsfrom deposition in an aqueous environment (lakeside or mainly marine) in sedimentary

basins.

They are characterized by their stratified arrangements in successive layers due to theirmode of deposition and by the frequent presence of animal and plant debris (i.e. fossils).

Physical, chemical and biochemical phenomena are at the origin of these rocks. Generallyspeaking, they result from the erosion of pre-existing rocks, the transport of weathered debris,the deposition of this material in a basin and finally the transformation of sediment into therock by diagenesis.

Sedimentary rocks are progressively transformed into metamorphic rocks at a depth of

8 000 to 10 000 m (the depth varies according to the value of the geothermal gradient).

The maximal noticed thickness of sedimentary rocks on the Earth’s surface is about25 000 m (Caspian Sea basin). The average thickness on the Earth’s surface is about 2 000 m.

3.3.1.1 Weathering and erosion

Generally, rocks in outcrops are neither homogeneous nor continuous. They presentdiscontinuities such as stratification joint, fissures of tension, fractures and faults.Atmospheric, chemical and mechanical agents attack the rocks in these areas and destroy

them. This process is called erosion.

a) Chemical weathering

Rainwater attacks minerals and much more easily when it is acid. This acidity depends, inparticular, on the dissolved carbon dioxide content.

Evaporites and carbonates are easily dissolved, silicates (micas and feldspars) are alsoeasily hydrolysed except silica. The most mobile ions (Na+, K+) go first into solution.

The results are:- Rich solutions in different minerals.

- Formation of new minerals (clay, iron and aluminium hydroxides, ...) insoluble or notmuch soluble in water, disintegrated and reduced to the state of grains of variablesizes which set up soils.

- On a more important scale, fissuring and crumbling of rocky massifs occur.


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b) Mechanical weathering

Mechanical agents, that also may carry out sediments, may break down rocks. These agentsinclude:

- Temperature variations, which cause periods of frost and thaw and make rocks burstand break up.

- Gravity which causes crumbling and sliding on the relief and produces a mass offallen rocks sensitive to other agents.

- Running water, sea currents, waves, ice and wind which wear rocks away by abrasionof transported particles, then remove the debris by carrying them off.

This mechanical weathering may produce a fragmentation of rocks and makes pieces ofrocks of variable sizes up.

3.3.1.2 Transport

Debris from chemical and mechanical weathering of rocks are transported from the site oferosion towards the site of sedimentation. Water is the main transporting agent. Erodedmaterial is carried away in solution and / or in the form of debris of various sizes.

In addition to running water, there are the effects of wind and ice.

On the continental slope, gravity is responsible for the sliding of important volumes ofsediments soaked up with water, which produce massive mudflows, which may reach high

speed. They have a great power of transport. They are called density or turbidity currents.Turbidites are detrital sediments carried by currents. Slidings are usually released by seisms.

Depending on the mean of transport of sediments, they have different shapes (roundedsediments by water, angular sediments by the wind).

3.3.1.3 Deposition and sedimentation

Debris is transported over greater or lesser distances depending on the energy of the meanof transport. In a watercourse, the energy varies according to the slope. It gradually decreases

downstream causing a sorting out of carried particles in decreasing sizes in the same way.When the watercourse opens out into the sea, sea currents may take sediments. They aresorted out in function of sea roughness, that is to say in function of the energy of thesedimentation environment.

Ions carried in solution are in balance in their mean of transport. If a rupture of this balanceoccurs (the arrival in seawater for instance), a precipitation may appear.

These different factors cause two types of deposition:

- Terrigenous detrital sediments deposition,

- Chemical sediments deposition.


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The activity of some living organisms that lead to the formation of mainly calcareousbiochemical sediments (i.e. reefs) should as well be taken into account.

As a result of these different processes, a sediment made of solid grains of variable sizes,nature and origin is formed. It is sometimes combined with much smaller grains that partly

come from chemical or biochemical precipitation phenomena.

3.3.1.4 Diagenesis

At the stage of deposition, sediments are loose and generally soaked up with water. Theyare gradually transformed into consolidated rocks by diagenesis.

This term covers several phenomena:

• Settling or compaction is particularly apparent in fine-grained sediments (clays,calcareous mud), that can contain up to 80% of water by volume at the stage of

deposition. Under the effect of their own weight and pressure of more recent overlyingsediments, they are gradually compacted. Water is expelled, density increases, porositydecreases and the contacts between minerals grains become more frequent. Thesephenomena are extremely important for oil industry, since compaction affects themovement of fluids in sedimentary basins.

• Cementation. Pore water always contains a certain amount of dissolved salts. Under theinfluence of pressure, temperature and ionic concentration variations, they mayprecipitate and form cement which binds grains together.

• Recrystallization and cementation are sometimes difficult to distinguish. Dissolutions

and ion exchange occur between minerals and pore water, what tends to create anotherchemical balance.

• Metasomatism. When water of a different chemical composition from the onecontained in rock’s pores enters a sedimentary or a consolidated rock, ion exchange maybe considerable. These phenomena bring about drastic mineralogical transformations(e.g. dolomization).

• Segregation. On certain points of the sediment or the rock, segregation producesmineralogical accidents called concretion (flint, cherts). This phenomenon may occur ondifferent stages (in an early way, in a still loose sediment or in a belated way in analready hardened rock).

A distinction is often made between diagenesis and catagenesis.

• Diagenesis includes transformations that occur below 60°C (this temperature beingreached at variable depths depending on the value of the geothermal gradient, which isnormally 3°C/100 m).

• Catagenesis is characterized by the prevailing action of temperature and includestransformations occurring above 60°C. This is the step before metamorphism.


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The main agents of diagenesis are:

• Geostatic pressure and tectonic constraints have a main role during compaction.Pressure increase brings grains closer and causes the expulsion and circulation ofsedimentation water. It also intervenes on the cementation of sediments.

On the contact points between grains, the pressure is at its highest level. On thesepoints, grains are soaked into a solution and a precipitation in pores occurs.

• Temperature rises in function of the burial. It intervenes on ion solubility and modifieschemical equilibria between minerals and solutions they are soaked into. It acts onorganic matter contained in sediments.

• Water circulations, which convey ions, cause phenomena such as weathering ofminerals, precipitation of cement or metasomatose.

• Biological factors, which may have an important role, modifying the environmental pH.

Diagenesis starts rapidly after the deposition of sediments. It then goes in relation to thespeed of sedimentation. Transformations are more numerous near the surface. Without anyinterruption catagenesis intervenes and the first stages of metamorphism.

3.3.2 General characteristics of sedimentary rocks

A sedimentary rock is usually formed of grains, a matrix and "voids" (of solid matter):pores, which contain interstitial fluids (figure 1.13).

FIG. 1.13 Structure of a porous sedimentary rock


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3.3.2.1 Grains

The Wentworth scale is the scale generally adopted in oil industry. It is the scale of grainsizes and it is based according to the mesh sizes in screens used to separate loose rock

particles.

3 granularmetrical classes can be distinguished (figure 1.14)

- Rudites, coarser than 2 mm.

- Arenites or "sands", elements ranging from 2 mm down to 62.5 µm. They aresubdivided in 5 groups (from very thin to very coarse).

- Lutites or pelites, elements mainly less than 62.5 µm. They are subdivided in 2groups: silts (between 62.5 µm and 4 µm) and clays (finer than 4 µm).

FIG. 1.14 Table of classification of grains in relation to their granulometry

Note: The rudites, arenites and lutites terms are mainly used for the classification and thenomenclature of sedimentary rocks of detrital origin (see classification ofsedimentary rocks).

3.3.2.2 Matrix

The matrix binds grains together. It looks amorphous and more or less hard and compact.When it exists, it appears under two different forms according to its nature and origin:

- A contemporaneous binder of sedimentation represented by the lutite fraction of thedeposit.

- A cement, which is often well crystallized, resulting from a precipitation fromsolutions or a transformation of a part of the original sediment by diagenesisphenomena.


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These two different forms of matrix may coexist. When there is no matrix, the rock isloose. The abundance of the matrix and its nature control induration of rocks, which shouldnot be mixed up with the hardness of the most represented mineral:

- The abrasive character of the rock depends on hardness. It conditions the lifetime of adrilling bit.

- The mechanical resistance to fragmentation depends on induration. It mainlyconditions the penetration rate of the drilling bit.

Note: The matrix term is often used to indicate the solid part of the rock.

3.3.2.3 Pores and porosity

Fluids (water, hydrocarbons, hydrogen sulphide, carbon dioxide, ...) contained in a rock arelocated in the pores of the rock.

The porous volume of a rock is usually very heterogeneous. Pores have irregular forms andare more or less linked to each other by thin twisting cavities.

Porosity is the fraction of the rock volume that is not occupied by solid material.

Different types of porosity can be distinguished:

a) Absolute porosity

The absolute porosity øt is equal to the total volume of pores existing in the rock divided

by the total volume of the rock:

t

p

V

V t ====φφφφ

or t

st

V

V - V =t φ

Vp = volume of pores

Vs = volume of solid particles

Vt = total volume of the rock

Porosity is expressed in percentage.

Absolute porosity includes:

• On one hand, intergranular or crystalline porosity, making up primary porosity ø1, whichdepends on the shape, size and grading of the solid material.

• On the other hand, vugular porosity, resulting from dissolution, fissure and fractureporosity acquired mechanically, making up secondary porosity ø2, often found inchemical or biochemical rocks.

Absolute porosity øt is given by: øt = ø1 + ø2


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b) Effective porosity

The porosity accessible to mobile fluids is called effective (or useful) porosity. It is usuallylower than 20 to 25%, even 50% lower than absolute porosity. Whatever the total porosity of

a rock is like, reservoir engineers are interested in the useful porosity.

Porosity, which is one of the basic characteristics of a reservoir, is the result of a wholeseries of geological events.

• From a sedimentology standpoint, porosity increases with the deposition environmentenergy and the deposit cleanness (best classification and homogeneity of the sediment).

• From a geological history standpoint, it decreases with increasing burial, extent andduration of subsidence, temperature and tectonic events.

Porosity is a physical characteristic of a rock, easy to determine thanks to wire line logging

(neutron, density and sonic). The porosity of a rock is said to be weak below 5%, mediocrebetween 5 and 10%, medium between 10 and 20%, good between 20 and 30% and excellentabove 30%.

3.3.2.4 Permeability

A porous environment allows fluids to move on to the extent that pores are interconnected.Such an environment is said to be permeable. Permeability indicates how easily a fluid of agiven viscosity can move through a formation. Darcy’s Law defines it:

L .

P . S . k Q∆∆∆∆µµµµ

∆∆∆∆====

Q = flow rate of a fluid moving through the rock in cm3 /s

S = area in cm2

µ = viscosity of a fluid in centipoises (dynamic viscosity)

∆P = pressure gradient between the two faces of the rock sample in bar/cm

∆L = distance between the two faces of the rock sample in cm

k = permeability of the sample expressed in darcy (d)

The darcy is dimensionally equivalent to a surface and is equal to 1 micron2

(10- 6

mm2

).In spite of appearances, it is a very large unit, so the millidarcy (md) is generally used.

If a fluid is homogeneous, the only one in the rock and does not react with the porousenvironment, permeability defined by Darcy’s Law is said to be absolute. But usually, an oil-bearing reservoir contains several fluids (water, gas and oil). In this case, their flow ratesinterfere and have an influence on the rock permeability.

The effective permeability towards a fluid expresses the property of a rock to be crossed bythis fluid in presence of one or more other fluids. It depends on the rock itself, but also on thepercentage of the different present fluids. Each effective permeability is lower than the

absolute porosity defined for each fluid.


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Relative permeability towards a fluid is the ratio of effective permeability divided by theabsolute permeability to this fluid.

In a reservoir, horizontal and vertical permeabilities have to be distinguished. In fact,permeability often varies according to the direction because of rock heterogeneity.

Permeability is said to be weak between 1 and 10 md, mediocre between 10 and 50 md,medium between 50 and 200 md, good between 200 and 500 md and excellent above 500 md.Permeabilities of the best oil-bearing reservoirs are about some darcies. For a gas reservoir,the minimal permeability to allow the flow, is about 0.1 md and about 10 md for an oilreservoir.

3.3.2.5 Porosity / permeability relationship

There is usually no direct relationship between these two characteristics. In most cases,

permeability is seen to be an increasing function of the pore radius and porosity.Clays have high porosities but low permeabilities. On the contrary, fractured rocks which

are generally compact rocks have high permeabilities and low porosities.

Many empirical correlation laws used in logging exist to determine permeability infunction of porosity. The only way to determine the permeability of a formation is to runformation tests (DST, formation fluid sampling) or to study core samples.

3.3.2.6 Saturation in fluids

Since a rock may contain several fluids, saturation in a fluid (water, gas or oil) is definedby the fluid volume / pore volume ratio.

For example, the saturation in water Sw is defined by:

poresof Volume

waterof Volume Sw ====

The sum of all saturations is equal to 1.

In most hydrocarbon reservoirs, a certain quantity of water is wetting the pore walls. Thiswater is called connate water. The saturation in connate water depends on the size of thepores in the rock. It is in between 10 and 50% and the average value is about 20%.

The saturation can be easily defined by wire line logging (resistivity and induction survey).

3.3.3 Classification of sedimentary rocks

It exists different ways of classifying sedimentary rocks. The two most used systems are

based on their genesis and their chemical composition.


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According to their genesis, we can distinguish:

• Terrigenous detrital rocks (or clastic rocks) made of elements of pre-existing rocks. Inthis category, the size of grains is specified: rudites, arenites and lutites aredistinguished.

• Chemical rocks made by the precipitation of substances in solution in waters.• Biochemical or organic rocks made by the accumulation of dead organisms or by the

activity of living organisms.

According to their chemical composition, we can distinguish:

- Siliceous rocks (sands and sandstones),

- Alumino-siliceous rocks (clayey rocks),

- Carbonate rocks (limestones and dolomites),

- Saline or evaporite rocks (salts, gypsum and anhydrites),

- Carbon-rich rocks (coal and kerogene), ferrous and phosphated rocks.

The classification based on the chemical composition will be used. To classify asedimentary rock by considering its origin may be complex. Sediments of chemical and / orbiological origin, for example, may have been transported or reorganized, they then form adetrital rock.

It is difficult to distinguish both types of classifications; the origin, which may be various,of the different components will be specified for each type of rock in what follows.

Clayey rocks, on their own, represent 80% of sedimentary rocks. This percentage shows

the average composition of the lithosphere. Siliceous and carbonate rocks represent about 5 to10%. The remaining rocks (evaporites, carbonaceous rocks...) only represent some % ofsedimentary rocks.

Siliceous and alumino-siliceous rocks are almost terrigenous detrital rocks composed ofallochtonous sediments (the original place of the sediment is different from the place ofsedimentation) whereas carbonate and saline rocks are mainly composed of autochthonoussediments. The denomination of rocks is based on:

- The chemical nature of the main element (usually the grain): limestone contains morethan 50% of calcium carbonate, sandstone contains more than 50% of silica.

- The granulometry of grains in the case of terrigenous detrital rocks.- The nature of the matrix (cement or binder) and the size of the crystals making it.

For example, we talk about sandstone of calcareous cement, calcarenite...

3.3.3. Siliceous rocks

These are rocks essentially made of silica (quartz, chalcedony or opal). They are usuallyhard (they scratch glass and steel) and acid-resistant (except to hydrofluoric acid). Adistinction between siliceous rocks of detrital origin and non-detrital rocks (chemical and

organic) has to be made.


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a) Terrigenous detrital rocks

They form the main part of siliceous rocks. They are all rocks formed by the accumulationof debris extracted by erosion, carried away on variable distance and left in sedimentationareas. They may be loose or consolidated.

They mainly belong to the arenites’family: when the rock is loose, they are sands; when itis consolidated, they are sandstones. It also exist rocks made of elements smaller than62.5µm: these are silts (loose rock) and siltites (consolidated rock).

Arenites: sands and sandstones

The grains mainly include quartz, which is the most resistant mineral to erosion. Feldsparsand micas may also be included.

In a sandy formation, quartz become more abundant, grain size decreases, grading

improves and the degree of roundness increases the farther away from the origin area. Derivedsediments and rocks are said to have an increasing maturity.

In sandstone where grains larger than 62.5 µm are dominant, the proportion of lutitesdecreases as the degree of maturity increases. The nature and proportion of the matrix (cementor binder) are a function of the intensity and duration of diagenesis.

The binder is composed by the “lutite” phase mainly composed of clayey minerals. Itsinfluence on petrophysical characteristics is very marked, especially on permeability.

The cement is usually well-crystallized and is formed:

• Either from solutions that impregnate the sediment during its deposition or startcirculating later on at the beginning of diagenesis, e.g. carbonate and evaporitic cements.

• Or by exchange between the detrital fragments resulting from changes in solutionequilibrium or from pressure dissolution phenomena (formation of styloliths).

Carbonate and evaporitic cements often form large crystals that envelop grains (e.g. desertroses formed by gypsum crystals surrounding quartz grains).

Siliceous cements are mainly composed of quartz. The rock is very dense, very indurateand abrasive at the same time, with closely interlocked crystals and is called quartzite.

Evaporitic and carbonate cements are, on the whole, most common in recent formations(Tertiary, Cretaceous) and siliceous cements in older ones (Palaeozoic). The cement fillingthe voids between grains tends to decrease porosity and permeability.

Lutites: silts and siltites

They are composed of extremely fine grains (smaller than 62.5 µm) non-discernible to thenaked eye. Silts can be distinguished from shales by their rougher texture due to numerousquartz grains.


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b) Non detrital siliceous rocks

The main mineralogical elements are opal, chalcedony and cryptocristallin quartz. Silica isslightly soluble in water and can be transported in this shape. It is found in variable

proportions in all waters. It can also be fixed by some organisms that use it to build theirskeletons.

It exists rocks of chemical and biochemical origins.

Siliceous rocks of chemical origin

The precipitation of silica under any mineralogical form creates very hard and often finelybedded rocks, which may be several metres thick and form silexites or cherts.

In limestones, it exists local concentrations of silica:

- Flints form nodules, clearly separated from the calcareous rock.

- Cherts progressively change into limestone.

- A silicate network in some lakeside limestones forms millstones.

Hardness is a characteristic these rocks have in common.

Siliceous rocks of biochemical origin

Diatomites, spongoliths and radiolarites are siliceous rocks of organic origin made of the

accumulation of elements from skeletons of sea organisms: diatoma, sponges and radiolaria.

It seems that a volcanic supply in silica increases the appearance of organisms of siliceousshell in the neighbourhood of oceanic ridges.

c) Diagenesis of siliceous rocks

Alterations that occurred along the diagenesis of detrital siliceous rocks are not veryimportant. Compaction, which is one of the dominating factors of diagenesis, brings aboutminor consequences because quartz grains are not much compressible (the absence ofcrushing and deformation of the present fossils in these rocks confirms that fact). In the caseof a clean sand, compaction makes the volume of the rock diminish of about 10 to 15%. In thecase of a clayey sand, porosity diminishes more rapidly in function of the depth and thereduction of the rock’s volume is more important.

Chemical phenomena are also not much marked on account of the low reactivity of silica.Non or not much consolidated sand may be found at great depths.

Sands and sandstones represent about 60% of the known hydrocarbon reservoirs. These arerelatively homogeneous rocks which have a relatively constant porosity (almost primary onaccount of the scarcity of dissolution and fissuring phenomena) and permeability in the same

reservoir.


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3.3.3.2 Alumino-siliceous rocks

The composition of terrigenous rocks is close from the average composition of thelithosphere. They are also the most abundant.

They are divided in three categories corresponding to the three granulometric classespreviously defined, with an obvious domination of shales.

a) Alumino-siliceous rudites

Most of detrital rocks of big elements are included in this group, pebbles of silicate rocksbeing more common than calcareous or quartzite pebbles. Pebbles are mostly pieces of rocksseeing that isolated minerals are rare. Loose and especially consolidated rudites formconglomerates. According to the pebbles’forms, we can distinguish:

- Breccia with angular elements,

- Pudding stones with rounded elements.

The observation of the pebbles’forms and their states of alteration, their arrangements andtheir orientations in space, give information about their deposit environment. Thedetermination of rocks that compose conglomerates, sometimes allows to determine theirorigins.

b) Alumino-siliceous arenites

These are mainly arkoses and grauwackes.

Arkose is the term for rocks directly resulting from the alteration of granites and gneiss(metamorphic rock of a mineralogical composition close to the granite) and almost withoutany transport. They often contain more than 30% of feldspars and not much weathered debrisof rocks. Its equivalent for a loose rock is the granitic sand.

Grauwackes are immature and badly sorted out rocks. An important clayey fraction (morethan 20%) cements minerals of a great variety (quartz, feldspars, micas...), small fragments ofrocks and sometimes volcanic ejections. They usually are angular debris. It is a rock whichmainly results from the alteration of basic volcanic rocks. Grauwackes deposits may beseveral thousands metres thick. These deposits are said to be due to turbidity currents or occur

in oceanic rifts (subduction areas) where seismicity and volcanism are important.

Psammites are rocks rich in micas and in organic matter too. Micas tend to lie in parallellayers.

Note: The term flysch is often used for a succession of sequences where sandstones,grauwackes, shales, conglomerates, limestones and calcareous clays are alternated.Its significance is much more structural than lithological. It is a rock laid by turbiditycurrents in oceanic rifts before the orogenesis phase.


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The term molasse is, as flysch, a sedimentary complex made by a succession ofsequences in which the sizes of grains diminish from the bottom up. They usually aresandstones with calcareous cement and calcareous clays.Unlike flysch, molasses are post-orogenic formations. In some places, they can beseveral thousands metres thick (between 5 000 and 8 000 m next to the Alps).

c) Alumino-siliceous lutites: clays

This group includes clays proper, which are loose rocks, and argilites which areconsolidated clays. The numerous and varied clayey minerals confer particular properties onthese rocks (rocks which are usually easy to hydrate and which become plastic andsloughing).

When bedded, the term shale is often used. Dehydration and action of pressure changeclays into schists, foliated and cleavable rocks.

Marls are clayey rocks, which hold about 35 up to 65% of calcium carbonate. They haveproperties close from clays’ones, but unlike the latter they are effervescent to acid.

Loess is a pulverulent deposition accumulated by the wind. It is a mixed rock made of fineparticles (10 to 50 µm) of quartz, clays and limestones.

Diagenesis of clays

Diagenesis of clays is a complex phenomenon. Compaction and mineralogicaltransformations are the most marked phases. Pressure and mineralogical transformations by

temperature especially influence compaction, the dominant phenomenon.

During the deposition, clayey sediments contain a large quantity of water (up to 90%). Thiswater is mainly located between layers. It can be easily evacuated during a certain periodwhere clay still possesses a non-negligible permeability.

The most important variations of density and porosity occur between 0 to 1 000 m at depth.Temperature and the presence of potassium ions mainly control mineralogicaltransformations, during which smectites change into illites.

The total volume of water evacuated during the diagenesis is about 70 to 80% of the initial

volume of the deposit. As important volumes of interstitial water are evacuated from notmuch permeable materials (about 1/1000 md), risks to encounter under-compacted formationsthat have unusually high pore pressures are numerous.

Clays are usually rich in organic matter and are tightly linked to the genesis ofhydrocarbons. They contain the major part of the source rocks of hydrocarbons and are thewitnesses of a reducing sedimentation environment.

Because of their very low permeability, plasticity and abundance, they form the cover ofnumerous hydrocarbon reservoirs.


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To drill clays with a water-based mud is problematic because of their high reactivity to thistype of fluid.

3.3.3.3 Carbonate rocks

Apart from some sedimentary minerals mostly of terrigenous origin, these rocks are almostonly composed of calcite and dolomites. Therefore, two main groups can be distinguished:

- Limestones made of more than 50% of calcium carbonate,

- Dolomites made of more than 50% of dolomite.

The conditions of formation of carbonate sediments, linked to the dominating influence ofchemical and biochemical factors, are very varied. Moreover, the role of diageneticphenomena on carbonates, relatively soluble in water, is very marked.

They hold an important quantity of carbon dioxide, which existed in a free way in theEarth’s primitive atmosphere.

a) Mode of formation and diagenesis of carbonate rocks

Most of carbonate sediments result from the chemical precipitation and the fixation ofcalcium carbonate in solution in waters by living organisms. Supplies of terrigenous origin aresometimes present in this type of rock.

In all waters, Ca++ and CO3--ions are found, the balance of the solution depends on the

dissolved CO2 content. A CaCO3 precipitation occurs when the CO2 content diminishes. The

balance reaction is:Ca++ + 2 HCO3

- ⇒ CaCO3 + H2O + CO2

The decreasing of the CO2 content in water may be linked to:

- A decrease of the CO2 content in the atmosphere mainly due to the photosyntheticactivity of plants,

- An increase of water temperature,

- Agitation of water.

Calcium carbonate precipitates into thin needle-shaped aragonites, which rapidly changeinto more stable calcites after their deposits. The setting up of an internal or external skeletonmainly composed of calcium carbonate by numerous living organisms is by far the mostimportant mode of fixation of carbonates.

Once organisms are dead, skeletons may be fragmented by mechanical actions (currents,waves) and by organic agents (predators, perforating algae).

The result of these different actions is a sediment, chemically composed of CaCO3, sharedout in a granulometric point of view among:

- Fragments of varied origins (composing the grain of the rock) of the sizes of arenites

and rudites.


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- Finer particles of the same sizes of lutites. They are the result of chemicalprecipitation CaCO3, intense grinding of visible fragments and accumulation ofparticles of microplankton skeletons. They form the matrix: these are oozes that aredeposited and soaked up with water.

By its precocity and its intensity, diagenesis plays an important role in the formation andtransformation of carbonate rocks. The relative instability of solutions rich in Ca++ ions whichsoak carbonate sediments, make them sensitive to diagenesis. Compaction of fine sedimentsevacuates highly charged in Ca++ ions, water that tends to precipitate CaCO3 in coarsersediments forming a cement.

Carbonate formations are sensitive to meteoric waters charged in carbon dioxide, whichproduce karstic relief (formation of canyons, caves, and underground lakes…).

Case of dolomites

Dolomites are made of crystals of dolomites: Ca Mg (CO3)2. All the terms about thechange from the limestone to the dolomite may exist. The term dolomite is used when theproportion of dolomites is over 50%.

Dolomites have two different aspects:

• Very fine grained dolomites with a bedded structure: insertions of clayey beds arefrequent. They are often associated to evaporites. Therefore, they have a chemicalorigin. They are called primary dolomites because they are deposited by directprecipitation of dolomite crystals.

• Coarse-grained dolomites (up to 2 mm grains). They are arranged in fairly welldeveloped layers between limestones beds. They may appear in irregular shapes (e.g.chimney or mushroom structures) which cut the stratification and grade laterally intolimestones. They are called secondary dolomites because they are formed up tolimestones under the effect of circulations of waters rich in magnesium ions duringdiagenesis (metasomatose phenomenon) or later on.

Dolomites are usually secondary dolomites. Dolomitisation of limestones are almostalways a diagenetic phenomenon, which usually occurs in a lagoon environment whereMg++ /Ca++ ratio is high. The substitution of calcium by magnesium tends to increase theporosity and the permeability of the rock.

Dolomites, which often look like limestones, may be distinguished by their absence ofreaction or their slow reaction to hydrochloric acid in normal temperature conditions.

b) Constitution of carbonate rocks

In this type of rocks, grains and matrix have the same mineralogical composition.


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Grains

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