XRD Evaluation of Clay Minerals in Shaley Formation and Its Comparison With Cross Plotting of Log Data

The aim of this research is to determine the percentage of clay minerals by X-Ray Diffraction (XRD) technique and compare with the range of CGR log data. The random orientation of the samples is more suitable for analysis of clay minerals using the X-Ray diffraction technique. Evaluation of clay minerals in shaley zones has long been a difficult task. Presence of shale and shaley zones in some of the reservoir formations can introduce uncertainties in determination and evaluation of petro physical parameters. Percentage of clay minerals, shale types and shale volume has to be considered exactly in this case. Existence of shale reduces porosity and permeability of the reservoir to some extent. Shale distributed in sand and carbonates in many different types. Each of these shale types has different effects on porosity, permeability and water saturation of a reservoir formation. Dispersed shale reduces porosity and permeability to a great extent, but laminar and structural shale have less effect on petro physical parameters. In this investigation shale types, shale volume and effective porosity of shaley reservoir formations have been determined from well log data based on cross plotting of the CGR log. In other words, a triangle Density-Neutron porosity cross plot is used to determine and correct the above parameters. The area of study lies in central oil fields of Iran, where well log data and core samples are used (Tabnak Field, Well-C). Well-C selected to study the Kangan Formation (Upper Khuff) at different depths interval. A comparison between the clay minerals determined by XRD and by cross plotting of log data is satisfactory. This study illustrates that distribution of shale types in Well-C is mainly dispersed shale with few of laminar shale and the percentage of effective porosity (φe) decreases with increasing depth along well-C in Kangan Formation. The great match between the shale type determined by the cross plotting of neutron vs. density porosities after correction for gas effects and by the result of Scanning Electron Microscopy (SEM) for the same core sample is interesting. Finally, we found that the cross-plotting techniques used in this investigation can determine shale types, shale volume, clay minerals and effective porosity with acceptable accuracy.

Flushed zone saturation (S xo ) is given by Archie's equation [3] ) mf mf xo m xo xo The residual hydrocarbon saturation is S hr = 1-S xo .
Where φ is porosity of the formation m is cementation factor a is formula constant R mf is resistivity of mud filtrate R xo is resistivity of flushed zone (micro resistivity tools response) The following equations have been used to construct the cross plot in shaley formation [4] φNe = φNcorr-(φNSh . Vsh), φDe = φDcorr-(φDSh .VSh) (12)

Methodology of Cross Plotting
We used the above algorithm to correct reservoir parameters and used the cross plotting techniques to evaluate the clay minerals [5]. The first cross plotting must be done ( Figure 1) based on Thorium vs. Potassium from CGR log data [6]. If shaley formation contains hydrocarbons, Neutron and Density porosities have be corrected before the second cross plotting is done [7]. The Kangan Formation is one of the largest carbonate reservoirs located in the South Pars Field, which is one of the major gas fields in the central basin of the Persian Gulf. Facies analyses indicate that the Kangan carbonates are formed in four micro facies belts: tidal flat, lagoon, shoal, and shallow open marine, in a platform ramp environment. The isotopic compositions of the Kangan carbonates indicate that they were deposited in shallow tropical waters. Location of well-C in Kangan Formation is shown in Figure 2. Data parameters for well-C are given in Table 1.

Analysis of core sample (2) of Kangan 2 formation (Table 1) at depth 2645.7m:
For core sample-2, kangan-2, well-C, the dominate mineral based on the XRD spectrum is illite ( Table 2). The log data is shown Table1: CEC Results and the XRD sample locations along Well-C, Tabnak Field.   Core sample-2 in the depth 2645.7m in range of Log data in Figure 3 located in the field of kangan-2 in the well-C. The shale amount is low and geologically the dominate composition are calcite and dolomite respectively. The green color points show the limits of the evaluating core sample data considered in the cross plot of Figure 4. The domain limits shows the shale type in Figure  5. Shale value in this domain ranges between 0 to 15 percents and porosity of 1 to 7 percents. Considering the fact that the domain of the study was gaseous and verification of gas effect over shale porosity was corrected. Cross plot in Figure 5 shows shale type is the dispersed shale.  Table  1)  For core sample-3, kangan-2, well-C, the dominate mineral based on the XRD spectrum is illite ( Table 3). The log data is shown in Figure 6. Using cross plot of Thorium vs. Potassium in Figure  7, the mineralogy according to cross plot is mica as prevailing mineral, because mica was used in the drilling fluid and detected by the tools. Core sample-3 in depth 2666.7m for the range of log data in Figure 5 located in the field of kangan 2 in well-C. The shale amount is low and geologically the dominate composition is calcite. The green color points show the limits of the evaluating core sample data considered the cross plot of figure 7, the domain limits show the shale type in Figure 8. Shale value in this domain ranges between 0 to 30 percents and porosity of 1 to 14 percents. Considering the fact that the domain of the study was gaseous and verification of gas effect over shale porosity was corrected. Cross plot in Figure 8 shows the shale type is the dispersed shale.

Analysis of core sample (4) of Kangan 3 formation (Table 1) at depth of 2672.4m
For core sample-4, kangan-3, well-C, the dominate mineral  The core sample-4 (depth 2672.4m) in the range of log data in Figure 9 located in the kangan-3 Formation in well-C. The shale amount is low and geologically the dominate composition is calcite. The green color point's show the limits of the evaluating core sample data considering the cross plot of Figure 10, the domain limits show the shale type in Figure 11. Shale value in this domain ranges between 0 to 20percents and porosity of 1 to 15 percents.
Considering the fact that the domain of the study was gaseous and verification of gas effect over shale porosity was corrected. Cross plot in Figure 11 shows shale type is dispersed shale.      Table  1) at depth of 2680.2m  For the core sample-5 of kangan-3, well-C, the dominate mineral based on the XRD spectrum is illite (about 60% in Table  5). The log data is shown in Figure 12. Using cross plot of Thorium vs. Potassium in Figure 13, the dominate mineralogy according to cross plot is illite. For this example there is no effect of mica on the tools. Therefore, there is complete match between the results of the XRD in Table 5 and the cross plot shown in Figure 13. The core sample-5 in depth 2680.2m is in the range of log data shown in Figure 12. The shale amount is high and geologically the dominate compositions are calcite, anhydrite and shale respectively. The green points shows the limits of the evaluating sample considering the cross plot of Figure 13, the domain limits show the shale type in Figure 14. Shale volume for the sample in this domain ranges between 20 to 50 percent and porosity of 1 to 10 percent. Green points in Figure 14 relate to location of sample points which indicate that the shale type is dispersed shale.

Analysis of core sample (5) of Kangan-3 formation (
Considering the cross plot for the core sample-5 in Figure 14 and the Scanning Electron Microscopy (SEM) for the same core sample in Figure 15 indicate that the shale type is a dispersed

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Copyright © Ghassem Alaskari MK shale. The scanning electron microscope (SEM) uses a focused beam of high-energy electrons to generate a variety of signals at the surface of solid specimens. SEM can achieve resolution better than 1nanometer. The result of comparison between the cross plot in Figure 14 and the SEM in Figure 15 is satisfactory.

Similar analysis is done in many depth intervals in Kagan
Formation, Well-C, Tabnak field. Using cross-plotting methods, shale types, shale volume and effective porosity for each depth interval have been estimated after corrections for the hydrocarbon effects. The following results can be listed as: The cross-plotting methods can be used to determine shale types, shale volume, clay minerals and effective porosity with acceptable accuracy. ii.
These techniques illustrate the distribution of shale types in Kangan Formation as dispersed shale with few of laminar shale.
iii. Based on the effective porosity estimated from crossplotting in shaley formation, reservoir quality decreases with increasing depth in a long well-Cof Kangan Formation.

Conclusion
In case of structural shale, corrections for effective porosity and gas saturation are not necessary. In case of laminated and dispersed shale's, corrections for the effective porosity are essential. In case of gas saturated shale of any type, correction is needed. The clay minerals and the percentage of clay minerals determined by the cross plotting of Thorium vs. Potassium and by the analysis of XRD spectrums are satisfactory. The great match between the shale type determined by the cross plotting of neutron-density porosities after correction for gas effects and by the SEM result for the same core sample is interesting.