WELL LOG
CORRELATION
One of the first uses of well logs was correlation of
equivalent strata from one well to the next.
This is still one of their most important uses.
Intervals of logs from different wells are matched for
similarity or for characteristic log responses to lithological
markers.
Well logs have the advantage (for correlation) of providing a
continuous record over the entire well.
There are no missing sections as can be the case with core
samples.
Most important, because sonde depth is recorded, there is
no ambiguity as to the depth of the various markers.
Well-to-well correlation studies thus permit accurate
subsurface mapping and the determination of:
The elevations of formations present in the well relative to other
wells, outside, or geophysical projections.
Whether or not the well is within a given major geological structure.
Whether well depth has reached a known productive horizon, and,
if not, approximately how much remains to be drilled.
The presence or absence of faults.
The existence of dips, folds, unconformities; the thickening and
thinning of lithologic sections; or lateral changes of sedimentation or
lithology.
• The logs most frequently used for correlation are the
resistivity, SP, and gamma ray.
• The basic logs used in the majority of log-correlation
studies are listed in table with their various uses and
advantages.
• The dipmeter is a valuable supplement.
Some computed logs have good correlation features.
Examples are :
the apparent-matrix-density (ρma) curve,
the shale-fraction (Vsh) curve,
the silt-fraction (VSi,) curve,
lithology analysis, and Geocolumn* display (Fig. ).
Fig. Cross section
showing detailed
correlation with
computed Geocolumn
display.
The structural-dip angle from dipmeter logs is a valuable
additional piece of information for well-to-well
correlation, since it can be used to predict expected
changes in elevations of strata from one well to the next
(Figs. ).
This facilitates identification of corresponding beds and
helps in the analysis of structural and stratigraphic
anomalies such as faults and unconformities.
Cross section
showing faulting.
The dipmeter in
Well 1 confirmed
the correlations
in the upper
parts of the two
wells.
Cross section in which correlation was not readily apparent from electric logs
alone. Correlation was resolved by the dipmeter run in Well 3.
A useful presentation of dipmeter data for correlation use is the
“stick plot.”
In a stick plot, the wellbore is represented by a (usually) vertical
line; apparent dips in the plane of a given vertical cross section
are shown by short line segments intersecting the wellbore line
at the appropriate angles.
Fig. shows stick plots of three wells, with correlation lines
drawn in. (These three wells are not in-a line;the direction of
the cross section from Well 1 to Well 2 is 360”-180°, whereas
from Well 2 to Well 3 the direction is 3170-1370.)
The particular stick plots of Fig. along with stick plots of other
nearby wells, were used to study geological structure in beds
adjacent to a diapiric shale dome.
Cross sections
derived from
dipmeter stick plots.
The vertical lines
represent the
wellbores. The short
line segments
intersecting the
wellbore indicate
apparent dip in
the indicated cross
sections. Dipmeter
results are usually
presented in “arrow”
plots
Synthetic seismogram:
Well to Seismic tie
[ 15 ]
- Well to seismic tie is very important to know which seismic event is
pertaining to which geologic bed in the subsurface.
•Accuracy in well to seismic tie helps for better model building which is
very vital for obtaining better quality inversion, and when the quality of
inversion is good then realistic geologic information of subsurface
could be obtained.
•In the interpretation also, the well to seismic tie is very vital in order to
mark accurately the seismic events pertaining to different geologic
formations. The accuracy in Time to Depth relation is very important to
make correct well to seismic tie.
• Normally, Sonic data is used for Time –Depth relation and for making
synthetic seismogram. But Sonic data has some difficulties; the first
problem is - Sonic log get affected by bore-hole washouts (larger hole-
size) and for such case, the interval velocity obtained may not be the true
formation velocity of rock layers; the second problem is - the bandwidth
of sonic data is higher than the seismic data, and therefore, it gives some
problems while correlating the synthetic seismogram to observed seismic
data.
• Unlike the sonic log-data, the bandwidth of VSP is quite similar to
seismic data (as the ray-path is quite similar). VSP data do not get affected
by the bore-hole washout. These are the reasons for which VSP data give
better results in well to seismic tie. Therefore, for well to seismic tie, VSP
data is preferable than the Sonic data.
Vertical seismic profiling (VSP)
