haya assignment


151PTE321 Engineering Geology

Lecture 4

Dr. Seyed Mehdi Alizadeh

- All of the sedimentary rocks are important to

the study of petroleum reservoir engineering

- It is possible to interpret them by considering

the processes of rock degradation

- The principal Sedimentary rocks may be

organized according to their origin

(mechanical, chemical, and biological) and

their composition.

Why study clastic sediment?

Sedimentary rocks make up only 7.9% of the Earth’s crust.

Relative abundance of rocks

in the earth’s crust


66% of the surface of the Earth is covered by sediment or

sedimentary rocks.

Humans interact with the Earth largely at or near its surface.

Relative abundance of rocks

in the earth’s crust

Relative abundance of rocks

in the earth’s surface

 Based on the recognition of the signature of changing

environments over time, as preserved in the rock record.

Environmental interpretation of rocks


Age of rocks

= Earth History

Sedimentary rocks record the history of changing

environments on Earth.

Mechanical Weathering

• Mechanical weathering is responsible for breaking large pre-existing rocks into small fragments.

• The most important mechanism is the expansion of water upon freezing, which results in a 9% increase of volume.

• Hot temperatures can give rise to thermal expansion in rocks and cool temperatures can cause rocks to contract.

• This continuous expansion of the rock during the day and contraction during the night exerts stress on the rock and cracks form eventually causing pieces of the rock to fall away.

• Mechanical weathering produces boulder-size rocks, gravel, sand grains, silt, and clay from igneous and metamorphic rocks.

• These fragments remain in the local area, or they may be transported by winds and water to other sites.

Mechanical Weathering


Chemical Weathering

• Chemical weathering occurs when rocks are broken down by a chemical change.

• Water is the principal contributor to chemical weathering, which occurs simultaneously with mechanical weathering.

• Chemicals dissolved in the water, such as carbonic acid, enter into the chemical reactions that are responsible for rock degradation.

Chemical Weathering (cont.) • Some minerals react directly with the water

molecules to form hydrates.

• Carbonic acid, formed from biogenic and atmospheric carbon dioxide dissolved in water, plays an important role in the chemical weathering process by reacting with the minerals to form carbonates and other minerals such as clays.

Biochemical and Chemical Sedimentary Rocks • Chemical sediments are primarily classified, of course, by

mineralogical composition.

1 5

Carbonate Rocks Most carbonate rocks are entirely biochemical sediment, made up of the body parts of calcite or aragonite-precipitating organisms

Deep-sea carbonate ooze is made of foram shells Reef carbonates are made of coral reefs (usually) Stromatolites are formed by carbonate precipitation by microorganisms

Examples of Chemical Reactions

• Conversion of Feldspar 2KAlSi3O8 + H2CO3 +H2O  Al2SiO5(OH)4 +4SiO2 +


• Dolomitization Process 2CaCO3 + Mg

2+  ACaMg(CO3)2 + Ca 2+

• Acid Rain Reaction CaCO3 + H2SO4  Ca

2+ +SO4 2- +H2O +CO2

Biological Weathering

 Biological weathering is weathering done by living things.

 It takes place when rocks are worn away by living organisms.

 It could really be called a special case of either physical or chemical weathering.

• Tree roots Tree roots grow into cracks and widen them, which helps physical weathering.

• Bacteria Some bacteria and other organisms secrete acidic solutions, which helps chemical weathering.

Differential Weathering

 Weathering that occurs at different rates, as a result of variations in composition and resistance of a rock or differences in intensity of weathering, and usually resulting in an uneven surface where more resistant material protrudes above softer or less resistant parts.

 When the more weather-resistant rock is left behind, this process is called differential weathering. A rock's exposure to the weathering elements and its surface area can affect its rate of weathering.

Siltstones (mud-rocks) generation steps

• Quartz grains (originating from weathering of igneous and metamorphic rocks) are very hard; they resist further breakdown, but are displaced by currents of winds and water and distributed according to size.

• Larger grains accumulate as sandstones, and grains having an average size of 15 µm mix with clays and organic materials, that are transported and later deposited in quiet, low energy, valleys from flooding rivers, lakes, and the continental shelves.

• Tidal currents on the continental shelves effectively sort the grains of sand, silt, and clay once more until they settle in quiet regions, forming very uniform thick beds.

• Bottom-dwelling organisms burrow through the mud, kneading and mixing it until the depth of burial is too great for this to happen.

Siltstones (mud-rocks) generation steps

• The material then undergoes compaction and diagenesis, with the clay minerals changing composition as they react with chemicals in the contacting water.

• The compacted mud forms the siltstones and beds of shale making up two-thirds of the sedimentary deposits.

Siltstones (mud-rocks) generation steps

environments, such as swamps, form siltstones and shale that are gray to black in color.

• Many of these are the source rocks of petroleum hydrocarbons

• Beds of mud containing organic materials

that are deposited in anaerobic

Siltstones (mud-rocks) generation steps

Sandstones • The quartz grains and mixed rock fragments

resulting from mechanical and chemical degradation of igneous, metamorphic, and sedimentary rocks may be transported to other areas and later transformed into sandstones.

• After the loose sediments of sand, clay, carbonates, etc., are accumulated in a basin area they undergo burial by other sediments forming on top. The vertical stress of the overlying sediments causes compaction of the grains.

Sandstones (cont.)

• Transformation into sedimentary rocks occurs by lithification, or cementation. The main cementing materials are silica, calcite, oxides of iron, and clay.

• The composition of sandstones is dependent on the source of the minerals (igneous, metamorphic, sedimentary) and the nature of the depositional environment.

• A distinctive feature of sandstones is the as darkbedding planes, which are visible

horizontal lines. The bedding planes are the consequence of layered deposition occurring during changing environmental conditions over long periods of deposition in the region.

• Layering introduces a considerable difference between the vertical (cross-bedding plane direction) and horizontal (parallel to the bedding planes) flow of fluids.

Sandstones (cont.)

The major classifications of sandstones, based on composition

• QFL (ternary) diagram is used to interpret the ingredients of a


• Q is quartz, F is feldspar and L is lithics, or rock fragments

that are not broken down into single-mineral grains.

• A ternary diagram is a triangle,

with each of the three apexes

representing a composition, such

as sandstone, shale, and

limestone . For the moment they

are labeled A, B, and C

• Point A is at the top of the heavy

vertical red line (arrow). Along

this line is indicated percent

of A. A point plotted at the top of

the vertical line nearest A

indicates 100% A.

• A horizontal bar at the bottom of

the line (farthest from A)

represents 0% of A. Any other

percentage can be indicated by a

line appropriately located along

the line between 0% and 100%,

as shown by the numbers off to

the right.

• Point B is at the lower left apex of

the triangle. We construct a percent

abundance scale for B by rotating

the heavy red scale line 120

degrees counter clock wise so that

it runs from the right side of the

triangle to the lower left corner.

• The right side of the triangle now

becomes the base line for the

percent scale for B, and a series of

red lines have been drawn parallel

to the triangle's right side to mark

off the percentages.

• These lines are projected out to the

left and bottom sides of the

triangle, and the percent scale for B

laid out along the left side.

• Point C is at the lower right apex

of the triangle. We construct the

percent abundance scale for C by

rotating the heavy red scale line

another 120 degrees so that it runs

from the left side of the triangle to

the lower right corner, and the

percent scale lines and percent

abundance numbers rotate with it.

• The sum result is the ternary

diagram to the right with all the

scales present. Note that the heavy

red lines are not included in this

final triangle. Also observe that the

ternary diagram is read counter


• Note the numbers 1 - 4 on the

diagram. The composition for each

of these points is shown below. See

if you agree.

1. 60% A | 20% B | 20% C = 100%

2. 25% A | 40% B | 35% C = 100%

3. 10% A | 70% B | 20% C = 100%

4. 0.0% A | 25% B | 75% C = 100%

Carbonates Carbonate rocks form in shallow marine environments.

• Many small lime (CaO) secreting animals, plants, and bacteria live in the shallow water.

• Their secretions and shells form many of the carbonate rocks

• There are three major classifications of limestone (which is generally biogenic in origin):

1. Oolitic limestone is composed of small spherical grains of calcite (encapsulated fossils and shell fragments);

2. Chalk is composed of accumulated deposits of skeletal or shell remains of microscopic animals; and

3. Coquina is fossiliferous limestone composed almost entirely of fossil fragments cemented by a calcareous mud.

Evaporates • Evaporates are salts that are deposited in isolated

marine basins by evaporation of the water and subsequent precipitation of salts from the concentrated solutions.

• Salt Lake in Utah, United States, and the Dead Sea in the Middle East, are examples of lakes that are gradually forming beds of evaporates as the water evaporates

• Anhydrite (CaSO4), sodium halite (NaCl), sylvite (KCl), are typical examples of salts which are associated with evaporates.


• Three models for deposition of marine evaporites in basins of restricted water circulation.

• Depth of the water or the basis determines the model type.

Gypsum Anhydrite

Halite Syvite