PET 303 LECTURE SLIDES 2
INFLUENCE OF GEOLOGIC FACTORS
ON ROCK AND ON PETROLEUM AND
MINING ACTIVITIES
Influence of Geological Factors on Rocks
Here we are considering the Rock and geological forces applied
on it
Imagine a Rock sample
• Such rock sample already is divided by discontinuities (tiny
fractures or pore spaces) and therefore subjected to inside
stress.
• superimposed on this fundamental mechanics circumstance
are influences of pore fluids/water flow and time.
• Therefore, geological factors has altered the rock and the
applied forces and it is necessary that the Engineers be aware
that this will have significantly affected the rock properties.
Intact Rock
• This simply put are rocks containing no significant fractures, but
could contain micro discontinuities.
• Subsequent geological factors may affect its properties and its
susceptibility to water penetration and weathering effects.
• Best description for this rock is as given in next slide where it is
subjected to uniaxial compression;
In a complete stress-strain curve showing
• Stiffness (or Modulus, E)
• Strength 𝜎𝑐
• brittleness
Complete stress-strain curve
For a high modulus material,
the initial part of the curve will
be steep; for low modulus
material it will be gentle
Compressive strength is the
maximum stress that can be
sustained, illustrated by the
dotted lines
Steepness of the descending
portion is a measure of the
brittleness (see next slide for
further explanation of this
property)
Difference between Brittle and Ductile Material
Ductile material will show
Continuing strain at the same
Stress level
A brittle material will show a
Drop in stress level to zero at
The same strain value
Therefore, brittleness is
Indicated by the steepness of
The curve between these two
Limits.
Stress-strain curve depends on microstructure
• A high grain strength, fine grain basalt will have high
stiffness, high strength and is very brittle
• Lime stone rock with variation in grain geometry has
medium stiffness, medium strength and more gentle
descending part by gradual deterioration of the
microstructure
Basalt and Limestone
High stiffness
High strength
Very brittle
Medium stiffness
Medium strength
Medium brittleness
Chalk and Rock salt
Low stiffness
Low strength
Quite brittle
Low stiffness
Low strength
ductile
Discontinuities and Rock structure
• Now we know that one characteristic of intact rock is stiffness
(young Modulus, E) and occurs before peak portion of the curve
• If rocks were to always behave elastically, then they wont absorb
energy but react instantaneously to it and can sustain any stress
level
• However rocks do break and they have post-peak mechanical
characteristics; this therefore means that
• The rock might have failed due to natural processes and formed
faults and joints
• These faults and joints may be the weak links in the rock structure
Discontinuities, geometrical configuration and rock structure
• Discontinuities comes in variety of shapes, certain sizes and
oriented in certain directions
• This variation in shape, size and orientation of discontinuities
is referred to as geometrical configuration of discontinuities.
• The overall geometrical configuration of discontinuities in the
rock mass is termed rock structure.
Formation of Fractures
Fractures form in three ways;
Low shear resistance to additional shear stress
• If the two sides of the fracture have been
pushed over one another, the discontinuities
are likely to have low resistance to any
additional shear stresses induced by
engineering activities.
• Hence the Engineer should know about
structural geology and rock structure.
How Discontinuity leads to differing mechanical properties
• An open joint which will allow free flow of
water
• Occurs in limestone and dolomitic rocks, where connecting
material is allowed across the discontinuity, it has high
resistance to shear. Yet still low compared to the intact rock
and also has permeability higher than the intact rock.
• Discontinuity plus slip movement under stress causing discontinuity
surface to be altered. Resistance to shear is lower here than the
intact rock