Grain Size in Sedimentary Rocks

1. GRAIN SIZE ANALYSIS
Topics Covered:
• Importance of grain size data
•Techniques for measuring grain
size.

2. Importance Of Grain Size Data
✓Grain size is a fundamental physical property of
sedimentary rocks and, as such, is a useful descriptive
property.
✓ Grain size affects the porosity and permeability and
porosity of the rocks and thus is of considerable interest
to petroleum geologists.
✓ Grain size data reflects sedimentation mechanisms,
transportation and depositional conditions.
Coarse grained, well sorted sandstone is a
better reservoir for oil and gas.
Grains become round on being transported to
longer distance
Depositional
environment

3. Techniques for measuring grain size
➢ Manual measurement of the clasts
➢ Sieve Analysis
➢Settling tube analysis
➢ Pipette analysis
➢ Sedigraph
➢ Laser diffractometer
➢Coulter Counter
➢Image Analysis
➢Photohydrometer
➢Thin section measurement
➢ Electron microscope

4. SieveAnalysis ✓This method is used to
measure unconsolidated
sediments by sieving
them through a set of
nested wire mesh
screens.
✓Particles having size
smaller than the mesh
pass through while the
smaller ones remain.
✓ Size of mesh is
standardized and is
identified using mesh
number.
✓ The mesh number
denotes the number of
squares present in 1sq.
inch of area.
4 Mesh
6 Mesh
16 Mesh

5. SettlingTubeAnalysis
❖ The settling velocity of
particles falling through a fluid
medium depends on the size,
shape, and density of the
particle, and the density and
viscosity of the fluid.
❖In this technique, grains are
allowed to settle through a
column of water at a specified
temperature in a settling tube.
❖It is based on Stoke’s law:
where;
V= settling velocity
C= constant
D= diameter of the particle
V=CD2
(ρs - ρf)g
18μ
= C
μ= dynamic viscosity
ρs = particle density
ρf = fluid density

6. Photohydrometer
A photohydrometer is a
settling tube that relates the
changes in the intensity of
the beam of light passed
through a column of
suspended sediment to
particles settling velocities
and thus to particle size.
LaserDiffractometer
A laser-diffractor size
analyzer operates on the
principle that particles of a
given size diffract light
through a given angle, which
increase with decreasing
particle size.
Image Analysis
This technique use T.V.,
cameras to capture and digitize
grain images from which, with
the aid of appropriate computer
software, grain size diameters
can be calculated.

7. GRAIN SIZE
• By grain size we normally
mean “Grain Diameter”.
GRAIN SIZE SCALES
1. Udden-Wentworth “Grade”
Scale-based on a centre of 1mm
And a multiplier, or divisor of 2.
2. “Phi” Scale-proposed by‘Krum-
-bein’.

8. GRAPHICAL PRESENTATION OF
GRAIN SIZE DATA
1.Histogram-a bar
graph
2.Frequency curves-
Show the
variation of wt.%
as a continuous
function of grain
size.
3.Cumulative
curves-an S-
shaped curve on
the arithmetic
ordinate scale

9. STATISTICAL PARAMETERS OF GRAIN SIZE
1.Mode-most frequency occ-
-uring particle diameter
2.Median-midpoint of grain
Size distribution
3.Mean-arithmatic mean can be
Arrived at by picking selected
Percentile value
And averaging them

10. STATISTICAL PARAMETERS OF GRAIN SIZE
4.Standard Deviation-measures the ‘sorting’
(uniformity of particle size distribution)
5. Skewness-measures of
The asymmetry of the
distribution

11. STATISTICAL PARAMETERS OF GRAIN SIZE
6.Kurtosis-degree of “peakedness”
- If the central portion is better sorted=excessively
Peaked or “leptokurtic”
- If the tails is better sorted
=flat peaked or “platy-kurtic”

12. Presentation By:
Neha Panwar (1409)
Mohd. Imran (1407)