introduction to diagenesis

DIAGENESIS

• Presented to: Dr. Mirza Kamran

• Presented by: Muhammad Wajid Manzoor

• Roll No: 09Institute of Geology PU. Lahore

General overview of sedimentary rockAND

The processes before Diagenesis

IMPORTANCE OF SEDIMENTARY ROCKS:

i) 75% OF ROCKS EXPOSED AT THE EARTH’S SURFACE ii) SOURCE OF FOSSIL FUELS, IRON AND ALUMINUM ORES, AND GROUNDWATER iii) RECORD OF EARTH’S HISTORY •

How sediments form

A very basic classification of all sedimentary rocks is based on the type of material that is deposited and the modes of deposition.

HOW LAYERS ARE FORM

Look at the processes that are occurring in

this animation

Depositional Environments by figure

INTRODUCTION TO DIAGENESISDeposited sediments are characterized by loosely packed, uncemented fabrics, high porosities, and high interstitial water content. As sedimentation continues in subsiding basins, older sediments are progressively buried by younger sediments to depths that may reach tens of kilometers. Burial is accompanied by physical and chemical change take place in the sediments in response to increase in pressure from the weight of overlying sediments and increase in temperature downward in the subsurface . These changes act in concert to bring about compaction and of the sediments, ultimately converting it into consolidated sedimentary rock.

i.e Unconsolidated sand to sandstone rock, carbonate sediments change to limestone or dolomite, evaporite deposites become evaporite rock such as gypsum, and salt rock.

NOW

According to definition of DiagenesisThe process that brings about change in sediments during

burial and lithification is called diagenesis.Or

Physical and chemical processes that convert sediment into sedimentary rock. due to an increase in temperature, an

increase in pressure, and changes in pore-water composition

Diagenesis normally occurs at a temperature below ~200oC

When Does Diagenesis Occur?

Diagenesis can begin immediately , After the depositon has taken place.

Diagenesis occurs at temperature and pressure higher than those of weathering environment but below those that produce metamorphism, but there is no clear-cut boundary b/w the realm of diagenesis and metamorphism.

Factors control the diagenesis• Composition of the sediments.• Pressure applied due to overburden.• Geothermal temperature.• Grain size of the sediments. • Porosity of the sediments.• Permeability.• Amount of fluid flow.

Porosity

Definition:- Amount of void spaces within a rock. or

The ratio of the volume of space to the total volume of a rock.

i.e Porosity = volume of pore space/ volume of rock

Types of porosity

• Primary porisity

• Secondary porisity

• Primary porisity:- Primary porisity is due to the function of grain size.

Or Due to the properties of the rock sediments

• Secondary porisity:- Development of pore spaces in rock through diagenesis.

Factors effecting porosity

1. Shape - Well rounded particles have greater porosity than angular.

ROUND ANGULARPO

ROSI

TYROUNDNESS

2. PACKING- The more closely packed the particles the lower the

porosity.

UNPACKED PACKED

PORO

SITY

PACKING

3. SORTING-

- If all particles are the same size they are sorted. - If the particles are different sizes they are unsorted (poorly sorted)

• The more sorted, the higher the porosity.• Poorly sorted material have low porosity.

PORO

SITY

SORTING

Note:-Particle Size alone does NOT affect

porosity!!!!! (Shaping, Packing, Sorting does)

Permeability

Number of pores inter connected. or The ability to transmit fluids.

Factors effecting permeability

• Packing and size:- *Tighter packing and Smaller particles having less

permeability.

*Looser packing and large size particles is more permeable.

Rapid drainage Moderate drainage slow drainage

Permeability rate:- How fast a fluid can flow in a material is

called rate of permeability.

Note:-• Highly porous material may or may not be

good permeable. Because permeability is the pores inter

connected to each other.

Small pores, butinterconnected –high permeability

Large pores, butnot connected –low permeability

MAJOR DIAGENESIS PROCESSES

• The three principal processes that bring about diagenesis are,

1. Physical processes2. Chemical processes3. Biologic processes

PHYSICAL PROCESSES

Compaction:- Force exert on the sediments due to the overlying material. Resulting

* Reduce the volume of sedimentary body. * Decrease in water content inside the the sediments. * Decrease the porosity of sediments. * Thinning of beds.

Decreasing volume

• Decreasing water content

Decrease the porosity

• Thinning of beds

CHEMICAL PROCESSES

Chemical processes contain mainly, * Cementation. * Authigenesis. * Recrystallization. * Inversion. * Replacement. * Dissolution.

• Cementation:- Chemical precipitation of mineral material from the pore

fluids onto the surface of grains. Or

Precipitation of dissolved ions in the pore space.Commonly precipitating materials are

CarbonatesSilica

Iron compound

• Cementing material

• Cementation of sandstones

Cementation in sandstones can be examined quite easily with an ordinary petrographic microscope, i.e In thin section Quartz ( sandstone mineral) Calcite (cementing material)

Cementation of carbonate rocks• In carbonate rocks the cement is mostly aragonite, calcite,

dolomites.• Here cementation is some time occurs by the recrystallization,

such is calcium replace by magnesium forming dolomite from limestone i.e called dolomitization.

• Authigenesis:- Alteration of one mineral to form a new mineral. Or In broad sense, all processes that cause new minerals to

form sediment or sedimentary rock.

Note:- The new mineral may or may not act as a cement. Examples:- Iron bearing mineral change to pyrite(reduction). Iron bearing minerals change to hematite(oxidation). Feldspars change to clay mineral. i.e kaolinite.

• Recrystallization:- The process in which grains of a crystal structure are come

in new structure or new crystal shape, without changing the chemistry .

Examples:-• Lime mud change to coarse crystalline limestone.• Oolites change to mosaic of crystals(idealized model of an

imperfect crystal).

• Inversion:- Replacement of one mineral by it polymorph( Minerals having same chemistry but different structure or crystal form) is called inversion. It is same thing as recrystallization but just in narrowest sense. Example:- A very common example of inversion occur during diagenesis is the alteration of aragonite( orthorhombic ) to calcite (rhombohedral) while both having same chemical composition i.e calcium carbonate.

• Dissolution:- Solution of less stable mineral in the assemblage of minerals, leaving a cavity is called solution, or dissolution. Dissolution is very important diagenetic process,

Because it lead to increase in porosity and thinning of the beds.

Dissolution occur mostly in easily dissolved rocks such as carbonates and evaporites. Petroleum geologist and Hydrologists are very interested in

solution process of diagenesis.

BIOLOGICAL PROCESSES

• Bioturbation:- Boring, burrowing, and sediment ingestion activities of organisms such as molluscs, shrimps causing sediment degradation(organic substances are broken down by living organisms) and mixing , resulting, alteration of primary sedimentary texture and structures. For example :- Weathered and transported carbonates, By boring molluscs, causing degradation and formation of cavities.

Boring

MAJOR EFFECTS OF DIAGENESIS

The major deagenetic effects can be grouped under the general heading,

i.e

Physical change Mineralogical change chemical change

Physical Changes:- The principal physical changes that occur in sedimentary rocks during diagenesis are textural, shape changes, including bioturbation, compaction cementation, and solution.• Compaction produce changes in the arrangement of grains

, i.e packing of sediments.• Compaction thus cause an increase in the numbers of

contact b/w grains.• cementation plugs pore spaces with mineral, increasing

the out look and size.• Cementation decrease porosity , resulting increase the

bulk density, and that effect other properties of the rocks.

Mineralogical changes:-

• Mineralogical change occur during diagenesis as a result of cementation, authigenesis, inversion , and solution.

Chemical changes:-

During cementation , dissolution and minerals replacement, the composition is also change, i.e change of gypsum to anhydrite change of feldspar to clay minerals.

SUMMARY • Important of sedimentary rock• Classification of sedimentary rock• Introduction to DIAGENESIS• Limit of DIAGENESIS • Factors control DIAGENESIS• Porosity, factors effecting porosity porosity and diagenesis• permeability and factors effecting permeability • Porosity and permeability• Major processes of diagenesis chemical , physical , biological• Compaction , cementation , dissolution , recrystallization , inversion , bioturbation.• Major effects due to diagenesis physical, textural , mineralogical , and chemical changes .

ANY QUESTION

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