Introduction to

petrophysics

edited by P. Vass

for Petroleum Geoengineer MSc Students

Lecture 2

In general, rocks are composite materials produced by natural

processes. The primary source of these materials is the earth's

interior.

They are unchanging apparently, but the geological events going

together with transport of energy and material print their effects on

the rocks. The dynamism of these processes has been changing

both in time and space since the formation of the Earth.

Getting knowledge of the ancient events and environments which

contributed to the present arrangements and appearance of rocks

is based on the detailed investigation of rock properties.

The study of many different properties of rocks also provides

information to the assessment of potential mineral and petroleum

resources.

The properties of rocks can be studied by means of different tools

or apparatus and at different levels (macroscopic, microscopic).

Properties of rocks

A possible way of grouping the rock properties tries to emphasize

the hierarchical dependencies of properties.

By that classification the following groups can be distinguished

(Wayne M. Ahr 2008, Geology of carbonate reservoirs)

Primary (or fundamental) properties

The properties describing the rock texture, fabric, grain type,

mineralogical composition and sedimentary structures can be

listed here.

The rock properties of other groups are fundamentally dependent

on them.

The textural properties characterize the constituents of rocks

(grains, crystals) according to their size, shape, sorting (distribution

of different size fractions) and arrangement (packing).

Properties of rocks

The fabric properties of rocks include among the others the spatial

alignment, the orientation and the geometric configuration of the

constituents (depositional fabric, diagenetic fabric, biogenic fabric

and their combinations).

Secondary (or dependent or derived) properties

The petrophysical properties of rocks belong to this group:

porosity, permeability, saturation, wettability, capillary pressure.

They are dependent on the primary properties.

Third order or tertiary properties

The physical properties whose measurements are mainly

connected to the different geophysical methods form this group

(e.g. bulk density, electrical resistivity, acoustic wave velocity,

natural radioactivity, magnetic susceptibility).

They are dependent on both the primary and the secondary

properties of rocks.

Properties of rocks

The most important reservoir parameters whose values we

want to estimate from the evaluation of well logs with as

small uncertainties as possible are the following:

• porosity,

• water saturation,

• permeability,

• reservoir thickness.

The relationships which connect the measured (tertiary) rock

properties and the secondary rock properties are based on a

suitably constructed petrophysical model.

Reservoir parameters

A widely used model of reservoir rocks is divided into three

main components:

• rock matrix (the solid rock framework),

• clay or shale (because of their unfavourable effects on the

reservoir parameters they are generally separated),

• the pore space filled with fluid(s).

The ratios and properties of these components basically

influence the petrophysical properties of reservoir rocks.

Their effects also determine the well log responses of the

different methods (that is the shape of well log curves).

Petrophysical model of reservoir rocks

Petrophysical model of reservoir rocks

Pore space filled with

fluidsRock matrix

Volume of

rock matrix

Volume of

water

Volume of

hydrocarbons

Clay or

shale

Volume

of clay or

shale

Petrophysical model of clean reservoir rocks

Pore space filled

with fluidsRock matrix

Volume of

rock matrix

Volume of

water

Volume of

hydrocarbon

Bulk volume of the reservoir rock

For a petrophysicist, the rock matrix includes all the solid

constituents of a rock except the clay or shale component.

This conception of the rock matrix significantly differs from

that used in geology.

For a geologist, the matrix (or groundmass) of a rock may be

defined as follows:

“the smaller or finer-grained, continuous material enclosing,

or filling the interstices between the larger grains or particles

of a sediment or sedimentary rock; the natural material in

which a sedimentary particle is embedded” (Glossary of

Geology, A.G.I., 1977).

Rock matrix

So, the petrophysical term, rock matrix, covers a wider range

of solid components. It includes the grains, the “sedimentary”

matrix (allogenic), and the cement (authigenic minerals).

In the simplest case, the rock matrix is composed of a single

mineral (e.g. calcite or quartz).

In general, the rock matrix may contain a mixture of different

minerals: (e.g. quartz, feldspar and mica grains with calcite

cement).

Material balance equation of the rock model:

Vrock = Vma + Vpore + Vcl (or Vsh) for the total rock volume

1 = vma + vpore + vcl (or vsh) for the unit volume of rock

V: volume [m3],

v: volume fraction (dimensionless)

For a rock matrix of complex mineralogy:

vma = vma,1 + vma,2 + … + vma,n

Rock matrix

The ability of well logging methods to resolve the lithology is limited. It

means that only the mineral composition has significant effect on the

measured logs from the group of primary rock properties. Thus, textural

and fabric properties cannot be derived generally from the well logs.

The fundamental rock types which may be distinguished by means of well

logging methods are the following:

• sandstone - SiO2 (made up of mostly silica) : silt, conglomerate and

chert also belong to this group independently of the texture and fabric,

• limestone - CaCO3 (made up of mostly calcium carbonate): chalk also

belongs to this group,

• dolomite - CaCO3 + MgCO3 : due to its different physical properties, it

can be separated from the limestone by means of well log curves.

• evaporates (e.g. sylvine KCl, halite NaCl, anhydrite CaSO4, gypsum

CaSO4)

• clay and shale (fine grained siliciclastic sedimentary rocks with

significant amount of clay mineral content).

The more detailed identification within the groups is not always possible.

Rock types

Sandstone group (SiO2)

quartzite

(metamorphic)

sandstone and sand

siltstone and silt

conglomerate and gravel

chert

Limestone group (CaCO3)

chalk

limestone

(in a wide variety)

marble (metamorphic)

Dolomite CaCO3 + MgCO3

dolomitic marble

(metamorphic)

dolomite crystals

dolostone

dolomite breccia

Evaporites

gypsum (CaSO4 + crystalline water)

halite (rock salt, NaCl)

anhydrite CaSO4

sylvine (or sylvite KCl)

Clay is an unconsolidated very fine-grained sediment, whose

clay mineral content is higher than 50 %.

As a result of the diagenesis, clays change into claystone.

Subsurface clays and claystones usually have a high water

content.

Most of the water is bound to the clay minerals (clay bound

water CBW), so the movable water content of these rocks is

very low or none. Accordingly, clay and claystone beds are

typically impermeable and play the role of seal in petroleum

systems.

Clay minerals with their bound water (wet clay) can be

regarded as electrical conductors because of the cations

weakly connected to their surface (clay minerals have much

higher specific surface area than other minerals and their

surface area usually negatively charged).

Clay and claystone

Shale is a fine-grained, indurated sedimentary rock with

laminated structure. It is a frequent member of sedimentary

sequences.

Similarly to claystone it belongs to the group of mudrocks.

It normally contains at least 50% silt with, typically, 35% clay or

fine-grained mica and 15% other chemical or authigenic

minerals.

Due to its high clay mineral content, shale beds and laminae

usually form an impermeable barrier (seal) for the fluids

migrating through porous and permeable rocks.

Some shales (called black shales) formed in reducing marine

environments (low oxygen environments) have significant

organic carbon content (it may contain up to 20 % organic

carbon).

Black shales are regarded as source rocks in petroleum geology.

Shale

Shale and clay

claystone

shale black shale with pyrite concretions

surface clay

Clay and shale may also be present in reservoir rocks

(mainly in sandstones) as a contamination (e.g. fine laminae,

aggregates, pore-filling particles).

A sandstone contaminated with shale or clay is called shaly

sandstone or clayey sandstone.

A reservoir rock formation is clean when it does not contain

appreciable amount of clay or shale.

Clay and shale content has significant effect on the well log

readings of most logging methods, and typically reduces the

effective porosity and permeability of reservoir rocks.

Shale and clay

Porosity

the ratio of the volume of pore space within a rock to the total bulk

volume of the rock.

It can be expressed as either a decimal fraction or a percentage.

The collective void space is referred to as pore volume, so the total

porosity () is calculated as follows:

𝜙 = 𝜙𝑡 =𝑉𝑝

𝑉𝑡∙ 100 %

where Vp is the pore volume and Vt is the total rock volume.

In practice, different types of the porosity were defined. The most

commonly used two of them are the total porosity and the effective

porosity.

The theoretical interval of the porosity ranges from 0 – 1 (0 –

100%), but it is practically less than 0.5 (50 %) in rocks.

Porosity

Effective porosity

means the ratio of the interconnected pore space to the total bulk

volume in theory

𝜙𝑒𝑓𝑓 =𝑉𝑝,𝑒𝑓𝑓

𝑉𝑡∙ 100 %

where Vp,eff is the effective pore space and Vt is the total rock

volume. The relation between the total and effective porosity:

0 ≤ 𝜙𝑒𝑓𝑓 ≤ 𝜙𝑡

Problem with the theoretical definition

Not the total volume of interconnected pores contributes to the

fluid flow under pressure drop.

Its interpretation is highly dependent of the speciality (core

analysis, production engineering, petrophysics)

Other types of the porosity such as secondary, water-filled, vuggy,

and fracture porosity are also used in the characterization and

description of rocks.

Porosity

Baker Hughes Inc., Introduction to Wireline Log Analysis

Illustration of the effective, noneffective,

and total porosity

Porosity types in a complex model of the

reservoir rocks

Pore classification

Megaporosity: >256 mm (caverns)

Macroporosity: 1mm – 256 mm (small, medium and large

vugs)

Mesoporosity: 1 (or 2) m – 1 mm

Microporosity: < 1 (or 2) m

The so-called Darcy flow (laminar flow) holds true within

mesopores.

For micropores, the interfacial forces (surface tension)

impede the fluid flow.

For larger pores, the stream of fluid is more complex

because of the increasing heterogeneity.

Some of the porosity types differentiated (1)

Connected porosity: the ratio of connected pore volume to total

rock volume.

Effective porosity: actually the same as the connected porosity.

Primary porosity: the porosity of a rock coming from the original

deposition.

Secondary porosity: the volume fraction of pore space coming

from post-depositional processes (e.g. dissolution, dolomitization

and fracturing).

Microporosity: the volume fraction of small pores filled with

capillary bound water. (Pore tunnels having less diameter than 1

(or 2) m are regarded as small pores)

Intergranular porosity: the volume fraction of voids among the rock

grains.

Intragranular porosity: the volume fraction of voids within the rock

grains.

Dissolution porosity: a type of secondary porosity which is formed

by the dissolution of some rock minerals.

Some of the porosity types differentiated (2)

Fracture porosity: a type of secondary porosity which is formed by the

fractures in a rock.

Intercrystal porosity: the volume fraction of very small pores among

the crystals.

Vuggy porosity: a type of secondary porosity derived from the vugs

mainly in carbonate rocks. (Vugs are small to medium-sized cavities

inside rock bodies which may be partially filled with secondary

minerals. Vugs are mostly formed by tectonic activity, collapse of rock

bodies, erosion and dissolution)

Moldic porosity: a type of dissolution porosity which appears mainly in

carbonate rocks. The pore spaces preserve the original shapes of

dissolved minerals, fossils and other constituents.

Fenestral porosity: the porosity connected to the presence of

fenestrae (irregular cavities found in muddy intertidal to supratidal

carbonate sediments).

Rocks with fenestral porosity usually do not form good reservoir

rocks, because the cavities are usually isolated.

Typical ranges of total porosity in rocks

We must not forget that the effective porosity may be far less than

the total porosity in some cases (e.g. shales, clays, chalk etc.).

Main factors affecting the porosity

The amount of porosity is principally influenced by the

following factors:

• the shape of rock grains,

• the arrangement of rock grains (grain packing),

• the grain size and grain shape distributions,

• and the amount of cementing material.

In reality, porosity is rarely greater than 40%.

The highest values of the porosity occur in surface sands,

which are neither compacted nor consolidated.

Influence of the arrangement of grains on the

porosity

Baker Hughes Inc., Introduction to Wireline Log Analysis

Types of regular grain packing arrangements

with their total porosity

Paul Glover: Petrophysics MSc Course Notes

Exercise

Prove the correctness of total porosity values for the following

grain packing arrangements: cubic, hexagonal, rhombohedral

orthorhombic and tetragonal.

The volume of a spherical grain with a radius r : 𝑉𝑠𝑝ℎ𝑒𝑟𝑒 =4

3𝑟3𝜋

Influence of the grain size distribution on the

porosity

A wider range of grain size distribution reduces the porosity (not

well sorted grains), because the smaller grains are able to

partially fill the spaces among the larger grains.

In addition, non-spherical grains can fit better.

Influence of the cementation on the porosity

Baker Hughes Inc., Introduction to Wireline Log Analysis

In granular systems, the porosity normally changes between10%

and 35%, and the complete interval ranges from 3% to 40%.

Increasing cementation results in the reduction of porosity.

According to the type of rock matrix, hydrocarbon reservoirs

can be classified as follows:

• siliciclastic reservoirs, the rock matrix is mosty silica, the

primary (intergranular) porosity is typical,

• carbonate reservoirs, the rock matrix is made up of

limestone and/or dolomite, both primary and secondary

porosity can be present in the rock,

• igneous and metamorphic reservoirs, the porosity is

connected to the natural fractures (secondary porosity).

Lithological types of reservoirs

The sources of photosSandstone group:https://museumvictoria.com.au/melbournemuseum/discoverycentre/dynamic-

earth/overview/sedimentary-environment/turning-sediments-into-rock/

http://fionasrockproject.weebly.com/sedimentary-rocks.html

https://www.dwa.gov.za/groundwater/Groundwater_Dictionary/index.html?siltstone.htm

https://flexiblelearning.auckland.ac.nz/rocks_minerals/rocks/limestone.html

https://www.blinn.edu/STEM/Geology/faculty/Meta_Web_Page/pages/Quartzite.htm

Limestone group:https://flexiblelearning.auckland.ac.nz/rocks_minerals/rocks/limestone.html

http://www.vietnamlime.com/dolomite.html

https://www.dwa.gov.za/groundwater/Groundwater_Dictionary/index.html?siltstone.htm

Dolomite group:http://geology.com/minerals/dolomite.shtml

http://luirig.altervista.org/pics/display.php?pos=255556

Evaporites:https://www.flickr.com/photos/jsjgeology/8514005044

http://earthphysicsteaching.homestead.com/Anhydrite_Display.html

https://a2ua.com/gypsum.html

https://en.wikipedia.org/wiki/Sylvite

Shale and clayhttp://www.sandatlas.org/shale/

https://www.flickr.com/photos/jsjgeology/18859496909

http://growcreatively.blogspot.hu/2015_09_01_archive.html

http://paloaopalmining.weebly.com/what-rocks-are-opals-found-in.html