200 ft
Q2: Write short notes on the following:
(12 Marks)
-Rules of contouring.
-Reasons behind focusing on shale sections rather than sand
sections in electric log correlations.
-Benefits of studying
subsurface geology, and mention source of data.
-Information provided by log correlation.
Model Answer
Rules of
contouring
·
A contour line connects
points of equal value, and cannot cross a line of the same value or lines of
different values.
·
Must pass between points
whose values are lower and higher than its own value.
·
The line is repeated to
indicate reversal of slope direction.
·
Chose the sea as a
reference.
·
The contour interval on a
map should be constant. The distance between successive contour lines has a direct relationship to
the steepness.
·
All maps should include a
graphic scale.
·
Every fifth contour line
should be wider than the other contours and it should be labeled with the value
of the contour. The fifth contour is referred as index contour.
·
Start contouring in areas
with the maximum number of control points.
·
Construct the contour in
groups of several lines rather than one single contour at a time.
·
Use smooth rather than
undulating style of contouring.
Reasons behind
focusing on shale sections rather than sand sections in electric log
correlations.
- Clay and mud particles which make up
shales are deposited in low-energy regimes.
- These low-energy environments are responsible for
shale deposition commonly cover large geographic areas.
- Therefore, the log curves in shales are highly
correlatable from well to well and
can be recognized over long distances.
- Sand beds are often not good correlation markers
because they frequently exhibit significant variation in the thickness and
character from well to well and are often laterally discontinuous.
- The resistivity curves for the same sand
on two well logs being correlated may be different.
Benefits of studying subsurface
petroleum geology.
·
To find and develop oil & gas reserves.
This is achieved by representing geological information via cross-sections and
maps.
·
Data are gathered from electric logs and
seismic sections.
Information
provided by log correlation.
·
Logs are used for
preparation of subsurface maps and cross sections for the purpose of
exploration and exploitation of hydrocarbon reserves.
·
A correlated well log
provides information on the subsurface, such as formation tops and bases, depth
and size of faults, lithology, depth to and thickness of hydrocarbon bearing
zones, porosity and permeability of productive zones, and depth to
unconformities.