3.4 Wireline log analysis
3.4.2 Wireline log-based mudstone discrimination
Definitions of mudstone like that in 1.2.1 are not practical when using the wireline logs as mudstone discriminator. A more practical, operational definition is needed for this purpose. The most commonly applied method uses the gamma-ray log as a standalone mudstone-sandstone discriminator. This method estimates the rocks shale volume from the gamma-ray log by using either a linear or a non-linear model for shale volume as a function of the gamma-ray log signal. Thus, an arbitrarily chosen cut-off value can be assigned to discriminate mudstones and sandstones (Heslop, 1974).
The gamma-ray log detects natural gamma rays emitted from radioactive isotopes of uranium, thorium and potassium. Since these elements (especially Th and K) tend to concentrate in clay minerals this is used as an indicator of clay mineral content. Therefore, the above method defines mudstones as a function of the clay mineral content and not of the clay size fraction (Heslop, 1974; Katahara, 2008).
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Figure 3.4. (a) Schematic diagram of variation of sand vs clay mineral content (increasing left to right) (b) Associated response on a plot of density versus the difference between neutron porosity and density porosity (based on Katahara, 2008).
Heslop (1974) showed that several log properties exhibit a change in slope when plotted against clay content (Figure 3.4b). The author proposed that with increasing clay content, eventually the clay matrix becomes load-bearing and that the change in slope is at the boundary between grain-supported sands and clay-matrix supported mudstones, thus it is a lithological boundary (Figure 3.4a). Katahara (2008) concluded that the clay content (and associated log response) at this slope discontinuity is the only reliable reference to discriminate mudstones from sandstones.
Another commonly applied method for estimating the clay mineral content uses the difference between neutron porosity (NPHI) and density porosity (DPHI).
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Figure 3.5. Schematic crossplot of density porosity vs neutron porosity illustrating a simple quartz- clay-water model. The clay mineral volume (red arrow) is proportional to the neutron porosity minus density porosity, if both are on a sandstone matrix (From Katahara, 2006b)
The density tool measures electron density, by emitting and detecting gamma rays. The electron density for most minerals and fluids found in natural sediments and rocks is directly proportional to their bulk density. Therefore, the average electron density correlates with average bulk density. Density porosity is calculated by assuming a density for the pore fluid and for the rock matrix. The neutron tool emits high-energy neutrons and measures how effectively the formations scatter and absorb these. This measurement relates to hydrogen concentration. Analysts use lithology dependent transform to convert this to neutron porosity. Since clay minerals have hydrogen in the form of hydroxyls in their crystal structure, neutron porosity in shales depends directly on clay mineral content (e.g. “Schlumberger Oilfield Glossary,” 2016).
As it is shown in Figure 3.5 the clay mineral volume (red arrow) is proportional to neutron porosity minus density porosity. If we assume very simple quartz-clay-water model of mudstone composition, then mudstones will plot within the triangle bounded by those three points (Figure 3.5). Clean sandstone points are on the line connecting quartz and water. Clay minerals are below the clean sandstone line and for a given clay mineral, lines of constant clay content are parallel to it. This will be true for all clay minerals, although the proportionality will vary for different clay mineral types (Katahara, 2006, 2008).
86 According to Katahara (2006, 2008) the difference between neutron porosity and density porosity is a better measure of clay mineral content than the gamma-ray log because mudstones in a limited range of neutron-density difference show a much tighter trend on a sonic-density than mudstones in a limited gamma-ray range.
In most of the studied wells (excluding the Haltenbanken) in this thesis a two-step process involving the gamma ray, density and neutron logs had been applied to discriminate mudstone data points from non-mudstone data in the wireline logs, which was followed by a moving median filter. This method is broadly similar to that of Katahara (2006).
It should be noted that in most of the wells’ log data no clear boundary could be recognised between the grain supported (sandstones) and mud supported (mudstones) lithologies. Cut off values for the gamma-ray log and NPHI-DPHI were chosen individually based on local lithological characteristics for all three case studies. The lack of a clear slope discontinuity in the wireline log data with increasing clay content can most likely be attributed to the overall fine-grained and clay-rich nature of the studied sections.
Mudstone discrimination in the Malaysian wells was done in two steps. The first step involved the gamma-ray log and calculating the mean of all the gamma log samples across the working intervals in all three wells. Samples with gamma log values less than 85% or greater than 130% of the calculated mean were discarded. The second step involved calculating the mean of the difference between the neutron porosity and density porosity (NPHI-DPHI) across the same intervals. The density log porosity was calculated as described in 3.4.3 using a fluid density of 1.015 g/cm3 and mudstone grain density of 2.75
g/cm3. Sample values of NPHI-DPHI less than 45% or greater than 250% of the calculated
mean were discarded. A moving median filter was applied to remove any remaining spikes from the density and sonic logs. The window size was set to 41 samples and the threshold level to 0.05 g/cm3 for the density log and to 21 samples and to 4 µs for the
sonic log. If the difference between the central and the median value in the window exceeds the threshold, the central value is removed from the log. Additional smoothing of the sonic and density logs was done using a Hanning window of length 100 m.
87 Mudstone discrimination in the Norwegian wells was done using the natural gamma-ray log. The mean values of all the gamma log samples across the working intervals were calculated in all of the wells. Samples with gamma log values less than 85% of the calculated mean were discarded. In addition, a moving median filter was used to remove remaining spikes and smoothing was done using a Hanning window of length 100 m. Density porosity was calculated according to 3.4.3 using a fluid density of 1.05 g/cm3 and
mudstone grain density of 2.75 g/cm3.
Mudstone discrimination in the Brazilian wells was done in two steps. The first step involved the gamma-ray log and calculating the mean of all the gamma log samples across the working intervals in all four wells. Samples with gamma log values less than 78% of the calculated mean were discarded. The second step involved calculating the mean of the difference between the neutron porosity and density porosity (NPHI-DPHI) across the same intervals. The density log porosity was calculated as described in 3.4.3 using a fluid density of 1.05 g/cm3 and mudstone grain density of 2.75 g/cm3. Sample values of NPHI-
DPHI less than 95% of the calculated mean were discarded. A moving median filter was applied to remove any remaining spikes from the density and sonic logs. The window size was set to 21 samples and the threshold level to 0.05 g/cm3 for the density log and to 4 µs
for the sonic log. If the difference between the central and the median value in the window exceeds the threshold, the central value is removed from the log. Additional smoothing of the sonic and density logs was done using a Hanning window of length 100 m.
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