The characterization of clay minerals in the Kashafrud Formation in a gas field in northeast Iran

The preparation of samples

The preparation of core samples from well #3 in the Khangiran Gas Field (Fig. 2A) in the Mineral Processing Laboratory of the School of Mining Engineering was accomplished by crushing the core samples into several centimeter-sized pieces with a geological hammer (Fig. 2B). A geological hammer was used with the aim of reducing the dimensions of the given core samples from a minimum of 10 cm to a maximum of 2 cm. Then, for the next tests, a powder with dimensions of 75 µm was ground into a powder device (polarizer) for 30 min (Fig. 2C). The powdered sample is shown in the watch glass in Fig. 2D. Also, a piece of lump was kept for conducting thin section studies from the selected core samples.

PXRD analysis

The X-ray method is an approach for general analysis and identification of minerals and specialized identification of clay minerals which provides useful information about their composition, type, and content³¹. Using the advanced diffractometer 2θ–θ, D8-Advance model, in the X-ray laboratory of the School of Mining Engineering, University of Tehran, the minerals that made up 10 core samples were identified. It is worth to mention that the purpose of this analysis is to identify the type of clay minerals and their weight percentage. In this test, the weight of the samples was at least 3 g in the form of powder with a size of 75 µm (under sieve 200 mesh). Constituent minerals in the studied samples, based on the PXRD analysis report, in order of abundance, were: clay minerals, alkali feldspars, plagioclase, ankerite, and pyrite. In Fig. 3, the average weight percentage of minerals of all samples is drawn separately.

According to the pie chart, quartz mineral had the highest percentage of constituent weight among the eight minerals identified in the samples. Also, illite and alkali feldspar minerals stood at the second and third ranks, respectively. According to Table 1, samples #10 and #4 had the highest and lowest amounts of clay with a total of 33.5% and 6.20%, respectively. Among these minerals, samples with high clay content were chosen for the SEM studies, while those with low to medium clay content were assigned for the thin section studies.

Specialized identification of clay minerals by the PXRD test

The preparation of samples for quantitative PXRD test involves eliminating potential interferences, especially when clay minerals are not the primary components. Different treatments were used to reduce non-clay mineral interference, enabling precise identification of clays for analysis. These methods involved separating minerals, removing impurities such as iron oxides, and employing settling techniques. The decantation method played a crucial role in isolating clay fractions for a successful PXRD identification.

The specialized identifications revealed the clay minerals in the sample in order of abundance were illite, chlorite, and kaolinite. Figures 4 and 5 present both the general XRD and comparative graphs for these tests, respectively, for two of the samples. The percentages of the most common clay minerals in all samples along with the average and total weight percentages values are presented in Table 1. The qualitative identification curves of the clay minerals from these two samples are drawn in Figs. 6 and 7. These samples had the highest and lowest amounts of clay compared to the other samples.

The results of the PXRD test are generally based on weight percentage. In order to have a comprehensive understanding from the results of the laboratory examinations, the weight percentage of each samples converted into volume percentage (Table 2).

Based on the results of the experiments, it can be concluded that sample #10 with a total of 12.8% at a depth of 4390 m had the highest content of clay minerals. Conversely, sample #2 with a total of 6.7% at a depth of 4397 m accounted for the lowest content of clay minerals. Figure 8 shows the volume percentages of these common minerals in all samples.

Polarizing light microscopy studies

According to 10 core samples taken from well #3 of the Khangiran Gas Field, samples with codes #1, #3, and #9 were selected for the preparation of thin section and light microscope studies. This selection was based on their clay content which means that these three samples had low to medium clay content compared to other remained samples. Thin sections were prepared from crushed core samples. Subsequently, Zeiss Axioplan 2 polarizing light microscope was used for detailed examination in the Optical Mineralogy and Mineralography Laboratory at the School of Mining Engineering. The sandstones in the samples were classified following the Folk (1980) classification system. The samples then were analyzed based on petrographic observations considering lithological, textural, structural criteria, sedimentary facies, diagenetic changes, and sedimentary environment of sandstones.

The sample, labelled #1 (Fig. 9A and B) corresponds to the classification of arkose according to the Folk’s classification. The studied sample falls within the clastic sedimentary rock group, specifically in the subgroup of sandstones. The main grains comprising the stone’s framework consist of quartz and feldspar, while minerals with low abundance (3–5% by volume) include chlorite, biotite, and muscovite. The size of grains varies between 80 and 300 µm, with an average exceeding 125 µm. Therefore, the sample is categorized as fine to medium-grained sandstones with moderate sorting. The grains were semi-rounded to semi-angular shape. Some quartz fragments show stretch and extension, indicating a locally semi-oriented orientation and angular—stylolite bordersas a result of diagenesis processes and bearing compressive stress. Also, the effects of recrystallization can be seen in some quartz crystals in the form of polycrystalline fragments. Feldspar grains were mostly displayed in the form of argillite and sericite, their complete pieces were rarely found. Autogenous minerals resulting from feldspars alteration were mainly in situ and had a little movement. Small pieces of rock fragments were of the type of chert with interlayers of mica. The matrix was less than 5% by volume and it was considered as semi-mature to mature. The matrix includes clay minerals, phyllosilicate minerals e.g., chlorite, sericite, muscovite, and microcrystalline silica. The distribution of matrix in the rock was heterogeneous and was locally more concentrated in some parts. Furthermore, the cementation was small (less than 1%) and included anhydrite, iron oxide-hydroxides, clay minerals, sericite, muscovite and chlorite.

Scanning electron microscope analysis (SEM/EDAX)

Among 10 available samples, according to the percentage of clay minerals, three samples were selected for SEM/EDAX analysis. These three samples coded #5, #7 (1 and 2), and #10 were studied after preparation with Camscan MV2300 scanning electron microscope equipped with XFlash 6l10 elemental analysis at the laboratory of School of Metallurgical Engineering, Faculty of Engineering, University of Tehran. Their images were recorded and are present in Fig. 10.

The interpretation of SEM images of clay lam sample extracted from sample #5

The magnification of the studied image (Fig. 10A) is 2500 times and the depth of field is 20.4 mm. In the studied field, clay minerals of kaolinite, illite, and chlorite had stuck together in a coated form due to their adhesive properties and had created concretion structures (circle-like, spherical structures). It can be assumed that all kinds of clay minerals were placed inside these structures with a coated form. In addition, plate-shaped clay minerals were also seen in the background. Kaolinite clay minerals were visible in locations J-7, I-6, K-7, N-8, and O-8. Additionally, kaolinite clay mineral in the background was spread in the plates form in the area from M-3 to M-7. The maximum size of kaolinite clay pieces in the studied sample is 40 µm, with an average size of 4.7 µm and a minimum size of 2 µm. Regarding the sizes of illite and chlorite fragments, they cannot be measured because of being coating and inter-layered shape, forming spherical shapes. The distribution pattern of the clays in this sample was pore-filling type, and in a few cases, it can also be classified as covering. The pore-filling pattern can be clearly seen in different parts of the studied field.

The interpretation of SEM images of clay lam sample extracted from the first image of sample #7-1

The magnification of the studied image (Fig. 10B) is 5000 and the depth of field is 20 mm. In this sample, clays were better separated from each other and clay plates were completely visible. However, similar to the previous sample, there was a very different dimensional distribution. That is, from very small pieces of clay, e.g., kaolinite clay in C-7 and D-5 locations, to larger types in D-6 and C-5 locations. Clays were strongly intertwined and stuck together. The advantage of the image being studied was that the plates can be seen independently of each other with a high resolution. The distribution pattern of the particles was also similar to the previous example, i.e., as a pore-filling and in some cases as a covering pattern. The minimum, average, and maximum particle sizes are 1.4 μm, 2.7 μm, and 13.7 μm, respectively. Moreover, a highly heterogeneous distribution of clay minerals types e.g., illite, chlorite, and kaolinite with various sizes was available in the studied field. In locations G-4 and B-2, one or more illites can be seen scattered in a small number. In the H-2 location, illite can be observed in the form of fibrous. In the range from F-7 to H-4, there were areas where a mixture of chlorite and kaolinite which can be seen in an intertwined form.

The interpretation of SEM images of clay lam sample extracted from the second image of sample #7-2

The magnification of the studied image (Fig. 10C) is 5000 and the depth of field is 20.1 mm. In another field of the same sample (code #7), the coated clay minerals were significantly crushed, forming a concretion. It should be noted that concretions were a mixture of clay minerals e.g., chlorite, kaolinite, and illite.

Some parts of the image that appeared very bright in color, (e.g., position N-11), seemed to contain iron-bearing clay fragments, possibly of the ferric chlorite type. This brightness could be attributed to the presence of elements with high atomic numbers (e.g., iron), found in chlorite. In this image, there was a mixture of clays e.g., kaolinite, chlorite, and illite coated and inter-layered. The only kaolinite piece that existed in an open form is the O-15 piece, and measuring its dimensions was not possible due to the pieces being chipped.

The interpretation of SEM images of clay lam sample extracted from sample #10

The magnification of the studied image (Fig. 10D) is 5000 and the depth of field is 1.20 mm. In sample #10, which was related to a shallower depth than the other samples, there was a lower degree of scaling and kaolinite pieces were mainly in the form of rounded plates. Some of these places are: B-10, C-10, D-11, E-11, E-12. The clay minerals in this sample appeared separately, although they were actually in a location where the background was a mixture of different types of clay minerals, e.g., kaolinite in the C-13 location, which was on a matrix. Kaolinite was the dominant mineral observed in the study field, but illite and chlorite can also be identified in the matrix in an interlock form. The maximum, average, and minimum sizes of kaolinite particle are 12.3 μm, 5.5 μm, and 2 μm, respectively. An important issue was the rotation of the kaolinite plates in this sample, potentially attributed to the shallow depth of this sample or other diagenetic reasons, e.g., alteration because of changes in temperature and pressure. The dominant pattern of clay distribution was of the covering type and the sub-pattern in this field was the pore-filling pattern. Moreover, a very long linear illite fragment with kaolinite fragments was observed from locations F-11 to D-12. In general, the SEM studies confirm the identification of clays types from the PXRD test. Furthermore, the distribution pattern of clay minerals and their changes in particle size were also investigated.

The field seen in the SEM diagram of Fig. 11 was subjected to general elemental analysis and the results are as follows. According to the diagram, the main peaks for three elements namely silicon, aluminum, and oxygen are clearly noticeable (the gold peak appears due to the coating of the sample with a gold–palladium alloy), which are the constituents of aluminosilicates or clay minerals in the sample. The origin of silicon and oxygen elements can also be related to the presence of quartz in the sample. Because the SEM analysis was conducted by performing the test on the slide of the clay section extracted from the sample, the amount of quartz was minimized and not completely removed due to the resistant nature of this mineral. Therefore, its peaks can be seen in the form of Si and O elements.

The fourth abundant element is sodium, which is related to plagioclase or some alkaline feldspars. Therefore, given the simultaneous presence of plagioclase and alkali feldspar in the PXRD test, sodium might be associated with these two minerals either separately or together. Calcium ranks as the fifth most abundant element in Fig. 11. Considering the presence of carbonate phases, e.g., ankerite and calcite, as indicated by the PXRD results, hence calcium is associated with these carbonate phases. Thus, calcium can be attributed to ankerite minerals or various types of classical plagioclase.

The sixth most abundant element is sulfur. The presence of this element can be justified due to the appearance of the cubic pyrite mineral in the PXRD test. The next long peak is related to the Fe element, which is probably due to the presence of ancrete, pyrite, and chlorite-type clay minerals. The eighth abundant element is Mg, undoubtedly present in chlorite. In total, the main phases of the sample are: oxygen, silicon, to some extent sodium, aluminum and calcium. The general elemental analysis prepared from the sample in the form of a map (Fig. 12) indicates that the dominant constituent phases of the sample were made of oxygen, silicon and aluminum. This means that the main producers were clay and quartz minerals. The results of the elemental analysis of sample #10 are listed in Table 3.

It can be seen that the majority of samples in the studied field were composed of silicon, oxygen, and aluminum elements, which are the main producers of clay minerals (aluminosilicates). Figure 13 shows the general view of the sample, presenting the elements separately.