Visual evaluation of
Saturation
Identifying potential hydrocarbon zones is not as easy as
identifying porosity, but a small number of rules of thumb
and the time-honoured resistivity-porosity overlay technique
will find many of them. The basic rule is "clean, porous,
and resistive". Hydrocarbon bearing shaly sands are harder
to find and fresh water sands might look like hydrocarbons.
Low resistivity pay zones are relatively common and the
rules on this page will not identify these zones. So in many
situations, visual identification is ambiguous, but it is
always worth the effort.
Here's how to start. First,
annotate the resistivity and porosity logs to identify the clean and
shale lines , then draw horizontal lines to represent the bed
boundaries, as shown below.
For zones of interest, draw bed boundaries (horizontal lines).
Then review the porosity logs: sonic, density, and neutron. All
porosity logs deflect to the left for increased porosity.
Hydrocarbons increase resistivity compared to water zones, causing
the resistivity to deflect to the right. When the porosity and
resistivity deflect in opposite directions, the zone is probably
hydrocarbon bearing. This is easier to see by using the density log
curve, although sonic and neutron logs can be used.
When the porosity and resistivity both deflect in
the same direction, they are said to be "tracking" each other.
When they deflect in opposite directions, they are "Not
tracking". The easiest way to see this artifact is to trace the
porosity log onto the resistivity log, as shown in the
illustration below:
Raw logs showing resistivity porosity overlay. Red
shading indicates porosity-resistivity crossover and possible hydrocarbon zones. The density or
density porosity (solid red curve) is placed on top of the deep
resistivity curve (dashed red curve). Line up the two curves so
that they lie on top of each other in obvious water zones. If
there are no obvious water zones, line them up in the shale
zones. If the porosity curve falls to the LEFT of the
resistivity curve, as in Layers A and B, hydrocarbons are
probably present. Layer B is an obvious oil zone based on it's
high resistivity and high porosity. Layer A is less obvious
because both the resistivity and the porosity are lower, but
they deflect in opposite directions, so hydrocarbons are very
likely. Layer C is an obvious water zone.
Crain’s Rule #3:
Tracking of porosity with resistivity on an overlay usually
indicates water or shale.
OR
Low
resistivity with moderate to high porosity usually indicates
water or shale.
Crain’s Rule #4:
Crossover of porosity on a resistivity log overlay usually
indicates hydrocarbons.
OR
High
resistivity with moderate to high porosity usually indicates
hydrocarbons.
The average of density and neutron porosity in
Layers B is 24 %; Layer C is 19%. This is close to the final answer
because there is not much shale in these zones. The average in
Layer A is 16 % - much higher than the truth due to the
influence of the shale in the zone. The density porosity is
about 11%, pretty close to the core data. Therefore all our
analysis must make use of shale correction methods.
Low resistivity and high porosity usually means
water, as in Layer C. Known DST, production, or mud log
indications of oil or gas are helpful indicators.
Layer B and Layer A show crossover when the
porosity is traced on the resistivity log, so these zones remain
interesting. In fresher water formations, it is often difficult
or impossible to spot hydrocarbons visually. If it was easy, log
analysts would be out of work!
Crossover on the density neutron log sometimes
means gas (not seen on the above example). Watch for rough hole problems, sandstone recorded on
a limestone scale, or limestone recorded on a dolomite scale,
which can also show crossover – not caused by gas.
Water zones with high porosity and low
resistivity are called “obvious water zones”. Fresh water may
look like hydrocarbons, particularly in shallow zones. The lack
of SP development will often help distinguish fresh water zones.
Low porosity water zones may not be obvious.
Low resistivity pay zones are usually found by
observation of oil in the mud system, oil or gas shows on the gas
log, oil staining in samples or cores, and some by accident. Low
resistivity pay is caused by one or more of the following:
conductive minerals (clay or pyrite), laminated shaly sands,
laminated porosity, very fine grained rock (silt) with high
irreducible water saturation, often associated with high water
salinity.
Water saturation is usually calculated from the Archie equation
or a shale corrected version of it. This is not easy to do with
mental arithmetic. An easier estimate of water saturation can be
made in obvious hydrocarbon zones by using a method attributed
to Buckle, and it is commonly used by reservoir engineers in a
hurry.
Crain’s Rule #5:
Approximate Water Saturation (SWa) in an obvious hydrocarbon
zone is estimated from: SWa = Constant / PHIe / (1 -
Vsh)
where
Constant is in the range from 0.0100 to 0.1200.
Use 0.0400
as a first try in sands, and 0.0250 in intercrystalline carbonates.
The Buckle's Number approach is
very quick and quite powerful, especially when the constant has
been calibrated with core data or a competent quantitative log
analysis.
Density Neutron Crossover
In gas bearing zones, another indicator of hydrocarbon may be
crossover, or at least close proximity of the density and neutron
porosity curves. The example below is sandstone, and
logs are recorded on a sandstone scale. The crossover is very
large because the zone is quite porous (30% porosity), and the
gas has not been flushed back from the borehole wall. The sonic
log also reads too high (equivalent to cross-over) in this case.
Density Neutron Crossover may show gas
CAUTION:
If the logs had been recorded on a limestone scale, there would
always be some crossover in a clean sandstone, whether there was
gas or not. Conversely, a gas filled limestone will infrequently
show crossover if it is recorded on a sandstone scale, but it
will (usually) if recorded on a limestone scale. Similarly, a
dolomite logged on a limestone scale (a common occurrence) will
show no crossover because the lithology effect is larger than
the gas effect. The same well
logged on a dolomite scale (not so common) will show crossover
if the zone is dolomite and filled with gas, but also if the zone
is limestone or sandstone without gas. Care should be taken to
account for logging scales and lithology when using the crossover
technique.
Because
of the large matrix effect due to dolomite, crossover can seldom
be seen in dolomitic limestone, dolomitic sandstone, or pure dolomite
unless an appropriate scale shift is made to the density and neutron
logs. In the absence of a computer, this may be accomplished by
overlaying the density and neutron curves in known oil or water
zones, and looking for crossover that might indicate gas or limestone
above or below the water or oil zones.
Other Indicators of Hydrocarbons
Even with such aids, it is obvious that hydrocarbon zones can
be missed by a visual interpretation of the logs. Only the most
obvious hydrocarbon zones will stand out and it is often necessary
to compute log analyses for all the zones in a well, and possibly
from other adjacent wells, in order to sort out those zones which
are likely to be hydrocarbon bearing and those that are not.
Other
indicators of hydrocarbons such as gas or oil in the mud, a gas
or mud log at the well site with shows of oil and gas, drill stem
test results, production from offset wells, and sample or core
staining or fluorescence, are often relied upon to narrow down
the possible zones which may contain hydrocarbons. All the data
in the well history is therefore very important to the log analyst.
Sonic
log skipping may be an indicator of gas in the formation or in
the mud, or a fractured formation, but may be due only to poor
logging instruments or poor quality control. It is hoped that
logs are run to minimize skipping and the log analyst should not
rely on the presence of skips to indicate gas.
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