PETROPHYSICS Lesson (1)

INTRODUCTION

Petrophysics is the study of rock properties and their interactions 
with fluids (gases, liquid hydrocarbons and aqueous solutions). Because 
petroleum reservoir rocks must have porosity and permeability, we are 
most interested in the properties of porous and permeable rocks. The 
purpose of this text is to provide a basic understanding of the physical 
properties of permeable geologic rocks and the interactions of the various 
fluids with their interstitial surfaces. Particular emphasis is placed on 
the transport properties of the rocks for single phase and multiphase 
flow. 
 The petrophysical properties that are discussed in this text 
include: 
• Porosity 
• Absolute permeability 
• Effective and relative permeabilities 

• Water saturation
Irreducible water saturation 
• Hydrocarbon saturation 
• Residual oil saturation 
• Capillary pressure 
• Wettability 
• Pore size 
• Pore size distribution 
• Pore structure 
• Net pay thickness 
• Isothermal coefficient of compressibility 
• Mineralogy 
• Specific pore surface area 

• Dispersivity

PETROLEUM RESERVOIR ROCKS

A petroleum reservoir rock is a porous medium that is sufficiently 
permeable to permit fluid flow through it. In the presence of 
interconnected fluid phases of different density and viscosity, such as 
water and hydrocarbons, the movement of the fluids is influenced by 
gravity and capillary forces. The fluids separate, therefore, in order of 
density when flow through a permeable stratum is arrested by a zone of 
low permeability, and, in time, a petroleum reservoir is formed in such a 
trap. Porosity and permeability are two fundamental petrophysical 
properties of petroleum reservoir rocks. 


Geologically, a petroleum reservoir is a complex of porous and 
permeable rock that contains an accumulation of hydrocarbons under a 
set of geological conditions that prevent escape by gravitational and 
capillary forces. The initial fluid distribution in the reservoir rock, which 
is determined by the balance of gravitational and capillary forces, is of 
significant interest at the time of discovery. 
 A rock capable of producing oil, gas and water is called a reservoir 
rock. In general, to be of commercial value, a reservoir rock must have 
sufficient thickness, areal extent and pore space to contain a large 
volume of hydrocarbons and must yield the contained fluids at a 
satisfactory rate when the reservoir is penetrated by a well. Any buried 
rock, be it sedimentary, igneous or metamorphic, that meets these 
conditions may be used as a reservoir rock by migrating hydrocarbons. 
However, most reservoir rocks are sedimentary rocks. 
 Sandstones and carbonates (limestones and dolomites) are the 
most common reservoir rocks. They contain most of the world’s 
petroleum reserves in about equal proportions even though carbonates 
make up only about 25% of sedimentary rocks. The reservoir character 
of a rock may be primary such as the intergranular porosity of a 
sandstone, or secondary, resulting from chemical or physical changes 
such as dolomitization, solution and fracturing. Shales frequently form 
the impermeable cap rocks for petroleum traps. 
 The distribution of reservoirs and the trend of pore space are the 
end product of numerous natural processes, some depositional and some 
post-depositional. Their prediction, and the explanation and prediction of 
their performance involve the recognition of the genesis of the ancient 
sediments, the interpretation of which depends upon an understanding 

of sedimentary and diagenetic processes. Diagenesis is the process ofphysical and chemical changes in sediments after deposition that convert 
them to consolidated rock such as compaction, cementation, 
recrystallization and perhaps replacement as in the development of 
dolomite.

 MINERAL CONSTITUENTS OF ROCKS - A REVIEW

The physical and chemical properties of rocks are the consequence 
of their mineral composition. A mineral is a naturally occurring 
crystalline inorganic material that has specific physical and chemical 
properties, which are either constant or vary within certain limits. Rock-
forming minerals of interest in petroleum engineering can be classified 
into the following families: silicates, carbonates, oxides, sulfates 
(sulphates), sulfides (sulphides) and chorides. These are summarized in 
Table 1.1. Silicates are the most abundant rock-forming minerals in the 

Earth’s crust. 

ROCKS

A rock is an aggregate of one or more minerals. There are three
classes of rocks: igneous, metamorphic and sedimentary rocks .

 Igneous Rocks

 These are rocks formed from molten material (called magma) that
solidified upon cooling either:
1. At the earth’s surface to form volcanic or extrusive rocks, e.g.,
basaltic lava flows, volcanic glass and volcanic ash.
or
2. Below the surface, usually at great depths, to form plutonic or
intrusive rocks, e.g., granites and gabbros.
 Igneous rocks are the most abundant rocks on the earth’s crust,
 making up about 64.7% of the Earth’s crust

Metamorphic Rocks

 These are rocks formed by transformation, generally in the solid
state, of pre-existing rocks beneath the surface by heat, pressure and
chemically active fluids, e.g., quartz is transformed to quartzite and
limestone plus quartz plus heat gives marble and carbon dioxide.
 Metamorphic rocks are the second most abundant rocks on the
earth’s crust, making up 27.4% of the Earth’s crust.

Sedimentary Rocks

 These are rocks formed at the surface of the earth either by
1. Accumulation and consolidation of minerals, rocks and/or
organisms and vegetation, e.g., sandstone and limestone.
or
2. Precipitation from solution such as sea water or surface water,
e.g., salt and limestone.
 Sedimentary rocks are the source of petroleum and provide the
reservoir rock and trap to hold the petroleum in the earth’s crust.
Sedimentary rocks are the least abundant rocks on the earth’s crust,
making up about 7.9% of the earth’s crust. Because most reservoir
rocks are sedimentary rocks, they are of particular interest to us in the
study of petrophysics. Therefore, we will examine sedimentary rocks in
more detail than igneous and metamorphic rocks.