Fundamental vibration frequencies of standard natural clays (I=illite, ISmML=illite-smectite mixed layer, Na-mont=Na-montmorillonite,C=chlorite, P=palygorskite).
Clays are naturally occurring, fine-grained minerals under surface conditions mostly as alteration products with distinct crystal structures. They may show plastic behavior when mixed with sufficient water and become stiff when dried or cooked. They, having different physical properties, occur in three modes: (I) surface clays that may be old or very young sedimentary formations, as suggested by the name, they occur near surface, (II) shales that have been subjected to pressures to become rigid and layered due to various reasons (e.g., tectonic or subsidence-related), and (III) fire clays that are mined out from deeper sites comparing to the other two and they behave refractory and include less impurities thus they are physically and chemically more uniform.
Some clay minerals having large surface area, high ion exchange capacity and molecular grid properties have been pioneered for the development of many new products (Falaras et al., 2000). Clay-organic complex structures have been a research subject particularly since 1930’s. Results of these researches made benefical contribution to the process of expanding their use in diverse areas (Smith, 1934). Besides, progresses in analytical techniques that made possible to determine mineralogical and chemical compositions of the clays appreciably expedited this process.
Clay minerals are formed as a result of changes in temperature, pressure, geochemical, and physical conditions (Murray, 1999). Although clay minerals could be resulted from weathering, sedimentation, burial, diagenesis and hydrothermal alteration processes in general, occurrence of monomineralic clay deposits is scant. Even in an ordinary clay sample, several clay species could occur together. Clay minerals can be categorized in 4 subgroups in natural environments: (I) kaolinite group, (II) smectite group, (III) illite group, and (IV) chlorite group (Murray, 1991).
The relationship between heat and pressure with the formation of clay minerals and its consequence on the formation of petroleum has been an imperative subject for research past 40 years (Perry and Hower, 1970; Dypvik, 1983). In the recent years one of the methods in the petroleum explorations is organic maturation and the other is clay mineral diagenesis. In both the clay mineral diagenesis and the organic maturation, clay minerals show structural changes with the changing temperature, which reveals the degree of metamorphism. The principal factors including pressure, temperature, depth and burial that are efficient during the conversion of the clay minerals and hydrocarbon formation and primary migration can be explained through clay mineral diagenesis and organic maturation (Dunoyer de Segonzag, 1970; Dypvik, 1983).
Clay mineral characterization could be carried out employing spectroscopic methods for various purposes in the geological sciences (Heroux et al., 1979). In the literature, there is a voluminous research on determination of clay mineral chemistry using diverse techniques. Today, one of the most preferred methods is the FTIR (Fourier Transform Infrared) Spectroscopy. There is a significant increase in number of studies using this method dealing with the clay characterization in Turkey (Akyuz and Akyuz, 2003; Davarcioglu et al., 2005; Davarcioglu and Kayali, 2007; Davarcioglu et al., 2007; Davarcioglu and Ciftci, 2009). One of such studies is on the quantitative and qualitative characterization of Central Anatolian clay deposits and diatomites by employing the spectroscopic methods (Kayali et al., 2005; Davarcioglu et al., 2008; Davarcioglu, 2009). The Central Anatolia is one of the richest in occurrence of clay deposits in the world. Therefore, investigation of these deposits, their quantitative and qualitative characterization is highly important.
In the XRD measurements, characteristic peak of glycol-saturated montmorillonite is 17 A˚ peak (Cradwick and Wilson, 1972). As it disappears, mineralogical conversion of montmorillonite to illite becomes evident. As a consequence of chemical reactions depending on increasing depth and temperature, increase of Al and decrease of Si in montmorillonite’s tetrahedral sheet results in changes in structure and by taking-up K ions available in the environment due to feldspar alteration, montmorillonite converts to illite (Weaver, 1960; Suchy et al., 2007). Expelled water as result of this change results in an increase in salinity of connate water already present in shale. Mg, Si and Ca ions, products of such conversion, could form minerals like calcite and dolomite facilitated by increasing temperature and these new minerals deposit within shale. This in turn results in a decrease in porosity and permeability and an increase in density (Bishop et al., 2002; Dunoyer de Segonzag, 1970).
Montmorillonite, being very sensitive clay mineral to temperature and depth, play an important role in both oil formation and its migration. As a result of reactions occurring in association with increasing temperature and depth, montmorillonite converts to illite losing its structural water (smectite converts to illite in alkaline environment). This conversion occurs at about 2500-4500 m burial depth and at 100 °C. In this process, K contents of pore waters show increase. Organic matter requires H ions to become hydrocarbon. H ions facilitate structural break-down (so-called cracking) of hydrocarbons with large organic molecules to smaller ones. During the loss of water from montmorillonite, H ion concentration of the environment raises. Water expulsion takes place at fairly steady and regular temperature. Some of the layers start losing water at 50-60 °C through 300 °C. About 70-80% of the water is expelled between 120-160 °C that corresponds to the formation of petroleum. This study suggests that results acquired from the organic maturation can be obtained through spectral analysis of structural changes occurring in clay minerals.
One of the geological studies in the Kolsuz area (Ulukisla-Nigde, Central Anatolian) deals with the clay profiles. In this study, using columnar sections and local observations, distribution of clay horizons and their lithostratigraphic relations were investigated (Oktay, 1982). On the other hand, sedimentological, mineralogical and chemical properties of the clays of the same area were studied in depth later by Gurel (1999). The Kizilbayir Formation outcropping in the north of Ulukisla basin is the key formation due to its association with the clay occurrences. The formation starts with gravel-bearing red-green clays at the base, progressing upward, large-scale cross-bedded conglomerate, sandstone with clay intercalations occur and conglomerate with mudstone interbeds dominate towards the top (Demirtasli et al., 1986). Thickness of the formation in the area ranged from 10 to 250 m.
However, no comprehensive study on the qualitative and the quantitative characteristics of the clays occurring in this area was available to date. Thus, this study aims to investigate clays of the area using the spectroscopic method and observed structural changes in the clay structures were interpreted in terms of petroleum formation and their possible use in petroleum exploration in the area.
2.1. Preparation of samples
A combined profile representing the constructed profiles chosen for this and nearby area was shown in Figure 1. The clay samples were taken from three different levels shown in the litostratigraphic columnar section of a selected locality in the Kolsuz-Ulukisla region (Figure 2). The samples taken from lower level, middle level and upper level labeled as (Kk1), (Kk2), and (Kk3), respectively.Initially about 1 kg of samples were collected, and 20-40 g splits were prepared for further analyses. Samples were heated at 110 °C for 24 hours and crushed to powder and screened using an 80 mesh sieve. Clay fractions were prepared following the procedure including removal of carbonates, sulfates and organic matter by dissolving, through washing to acquire stable suspension, and siphoning (to acquire the clay fraction - <2 micron) (Brown, 1961; Gundogdu, 1982).
In general, the clay minerals contain significant amount of water. Conversely, they contain less alkaline and alkaline earth elements. Absorption bands due to the water molecules occupy large spectral fields, the ones critical for identification of clay minerals. Thus in order to minimize this undesired overlap, samples for the FTIR measurements were prepared through clay concentration without employing a centrifuge. However for the chemical analyses, samples were analyzed as bulk sample without concentrating for clay fraction. Organic matter was removed through boiling in H2O2solution and then samples were dried in an oven at 110 °C for 24 hours.
2.2. FTIR measurements
Samples collected from the study area were prepared applying the disc technique (mixing ~1 mg clay sample with ~200 mg KBr) and put in molds. These intimate mixtures were then pressed at very high pressure (10 tons per cm2) to obtain the transparent discs, which were then placed in the sample compartment. Bruken Equinox 55 Fourier transform FTIR spectrophotometer (Department of Physics, METU, Ankara-Turkey) was used for the IR spectral measurements of these samples with standard natural clay and the spectra were recorded over the range of 5000-370 cm-1 (% transmission versus cm-1). Before taking the spectra measurements of the samples, spectrophotometer was calibrated with polystryrenesand silicate oxide of thickness 0.05 nm.
The infrared spectra of the illite (IMt-1; Silver Hill, Montana, USA), illite-smectite mixed layer (ISMt-1; Mancos Shale, Ord.), montmorillonite (SCa-3; Otay, San Diego Country California, USA),Ca-montmorillonite (STx; Gonzales Country, Texas, USA), Na-montmorillonite (SWy-1; Crook Country, Wyoming, USA),kaolinite (KGa-1; Washington Country, Georgia, USA), chlorite (ripidolite, CCa-1; Flagstaff Hill, El Dorato Country, California, USA), and palygorskite (PFI; Gadsden Country, Florida, USA) known as standard natural clay minerals (“The World Source Clay Minerals”) were taken(Table 1),and then the spectra of anhydrite, gypsum, and mixtures of the illite+quartz+feldspar, quartz+feldspar have been taken for the analyses of subject samples.
Along with XRD (X-ray powder diffraction) analysis, the functional groups in the clay minerals structures could only be determined through the FTIR spectra. Therefore, qualitative and quantitative analysis of the minerals by employing the FTIR spectroscopy is very important and promising.
A second treatment was employed only to the clay sample taken from the lower level (Kk1) to see whether there is a change in the structure of the samples or not due to FTIR spectrum measurements. For this procedure, HCl, bicarbonate (Na2CO3), and sodiumdithonit (Na2S2O4), and sodiumstrate (Na3C6H5O7) liquids were added to the sample to remove carbonates (mainly calcite and dolomite), amorphous materials and manganese oxides, which were expected to be present. This mixture was treated in an oven at 120 °C for 24 hours and washed using ethyl alcohol until complete removal of those unwanted components was achieved. The FTIR spectrum of the precipitate was then taken.
3. Chemical and minerological analyses
The chemical analyses of the Kolsuz-Ulukisla clay samples (dried in an oven at 110 °C for 24 hours) were carried out at the ACME-Canada laboratories by means of XRF-ICP (X-ray fluorescence spectrometry-Inductively Coupled Plasma) technique.Major oxide composition of the samples representing the lower, middle and upper parts of the profile was given in Table 2. These data suggest that the clays of the region are essentially rich in SiO2, Al2O3, and CaO.Main cause of these enrichments was due to ascending briny and carbonated waters through capillary actions and precipitation due to transpiration in arid and semi-arid regions. On the other hand, aluminum enrichment could be due to presence of either other aluminum silicates (such as K-feldspars) or Al3+ being in the clay structures.
XRD measurements were employed to determine the mineral phases included in the same samples (Siemens D-5000 Diffract AT V 3.1 diffractometer, CuKα radiation λ=1.54056 A˚ and 0.03 steps; General Directorate of Mineral Research and Exploration laboratories-MTA, Ankara-Turkey). According to the XRD measurements (Figures 3 and 4), subject clays are found to be composed of abundant chlorite (45%), illite (32%), quartz (20%), smectite (3%), feldspar, calcite, and trace quantities of palygorskite and Fe-oxide minerals. In the Kk1 lower part of the clay profile (Figure 2), amount of quartz tend to decrease while smectite, illite and chlorite show significant increase. Whereas at the top of the Kk1, just opposite of this abundance trend was observed and palygorskite was totally absent.
DTA(Differantial Thermal Analysis) and TGA (Thermogravimetric Analysis)measurements have been carried out for the determinations of the thermal behaviour of the clay samples (Figure 5).Measurement were carried out in the MTA Labs (Ankara-Turkey) using a Rigaku Thermal Analyzer Ver. 2.22EZ (SN#39421). Here smectite peak falls in the same field with the one of chlorite. Minute endothermic peak of smectite occurs in between ~100-250 °C, and second endothermic peak appears at ~700 °C and shallow endothermic/exothermic peak is observed at ~800-900 °C. Best observed endothermic peak of chlorite in the DTA-grams is the one observed between 500-600 °C. This peak may shift toward ~700 °C due to the iron content. Subsequently this peak may fall in the same interval with smectite’s peak at ~700 °C. Chlorite’s exothermic peak occurs at 750 °C (Kok, 2006; Kok and Smykatz-Kloss, 2009; Yener et al., 2007).
4. Petroleum formation
In the recent years, one of the methods used in petroleum explorations is organic maturation and the other diagenesis of clay minerals. Results acquired through the first approach could be attained using the second method. Fundamental nature of the second approach can be explained as follow: clays that immature or recently deposited may contain smectite (montmorillonite), illite and kaolinite depending on the source area. With subsidence, these minerals lose their water content and are subjected to mineralogical transformations. Smectite converts to illite within the range of oil formation temperature (60-150 °C) (Weaver, 1960; Dunoyer de Segonzag, 1970). When the upper limit of this range was approached, kaolinite and illite convert to mica, if ferromagnesian minerals are available in the environment; these minerals transform to chlorite instead (Figure 6). Consequently, clays can give an idea about be the degree of maturation of a sample of interest. For the fields where oil explorations are carried out, clays can be used to answer following questions: (I) which layers has source rock potential, their regional coverage and relationship with the paleogeography, (II) source rocks occurring what part and depth of the basin and which time interval have enough maturation, (III) when and how the oil migration occurred, (IV) the relationship with oil formation and oil migration (Hunt, 1995). However, time and duration plays an important role in this process accompanying the mineralogical transformations.
Mineralogical changes of the clay minerals are closely related to the temperature and water chemistry of the environment. Thus, they reflect better and more precise transformation temperature (paleotemperature) of the clay minerals. Reflectance degree of vitrinite, a major organic component of coals, shows increase with increasing degree of metamorphism (Teichmuller, 1987). In diagenesis stage, reflectance degree of vitrinite is 5%. In this stage, organic matter is not mature enough to produce oil and the second stage is catagenesis (boundary is 2%). While oil formation takes place between 0.7% and 1.3%,wet gas occurs between 1.3% and 2%. Lower boundary of metagenesis is 4%, under which metamorphism starts (Bozkaya and Yalcin, 1996).
During diagenesis and metamorphism, mineralogical changes occurring in the clay structures give extent of such events of the sedimentary rocks. Structural characteristics and parameters of the clay minerals are not unique for all depositional environments. However, depositional environments are characteristic to formation of certain clay minerals. Major parameters of sedimentary environment including pressure, temperature, subsidence, time, proton-electron concentration and metal-ion concentration greatly affect the clay mineral structures(Bozkaya and Yalcin, 1996). Most of the petroleum source rocks contain various clay minerals. Source rock properties like porosity and permeability vary depending on variety, abundance and distribution of the clay minerals (Bayar et al., 1987).
In the study area, the Kolsuz clays are loosely cemented with calcite, silica and Fe-oxides. Gravels are of various origins including sandstone, greywacke, claystone, limestone, marl, volcanics, granodiorite, gabbro, quartzite, chert, and serpentinite(Kayali et al., 2005). Petrographic investigations on thin-sections of 12 samples from the Kizilbayir Formation indicated prevalent presence of feldspars, quartz, calcite, lithic fragments of volcanic, magmatic and metamorphic rocks, of carbonates (e.g., limestone and marl) and clastic sedimentary rocks (e.g., sandstone, claystone and greywacke) cemented chiefly by calcium carbonate(Gurel 1999; Kayali et al., 2005).
5. Results and discussions
The clay samples were taken from three different levels shown in the litostratigraphic columnar section of a selected locality in the Kolsuz-Ulukisla region (Figure 2). The FTIR spectra of the samples taken from lower level (Kk1), middle level (Kk2), and upper level (Kk3) of Kolsuz-Ulukisla (Turkey) clay profile are given in Figures 7 through 9, respectively. Interpretation of the observed vibrational bands in these samples were carried out by comparing with those found in the world clay standards with known fundamental vibration frequencies for illite, illite-smectite mixed layer, Na-montmorillonite, chlorite (ripidolite), palygorskite (Table 1)and the other clay standards. Results are listed in Table 3.
As seen from Table 3, all of the samples belonging to the lower, middle, and upper levels of Kolsuz-Ulukisla (Turkey) clay profiles Kk1, Kk2, and Kk3, respectively, include illite, illite-smectite mixed layer, Na-montmorillonite, chlorite, palygorskite, calcite, feldspar and quartz minerals.
The FTIR spectrum of the sample representing (as summarized in FTIR measurements) lower level (Kk1) of Kolsuz-Ulukisla profile has been taken to see effects of the heat treatments on the structure of the subject samples (Figure 10).The assignments of the vibration frequencies of this spectrum were carried out following the same procedure applied to the spectra of the other samples and the results are given in Table 4. When results listed in Table 4 interpreted with the one in Table 3, it can be seen that the sample, Kk1, is composed of illite, calcite, chlorite, feldspar and quartz.Band assigned as vibrational frequencies for Na-montmorillonite and palygorskite were not observed after the thermal treatment. However, for illite at 916 and 833 cm-1(Al-Mg-OH) deformation and at 525 and 470 cm-1(O-Si-O) bending frequency bands were observed after the thermal treatment. This bands are resulted due to re-arrangement of Al and Mg atoms within the crystal structure during the thermal treatment (Bishop et al., 2002).
Only (Si-O) strecthing at 988 cm-1among the vibrational frequencies of chlorite at 3660, 988, 819, 543, and 441 cm-1was not observed after the thermal treatment. This is because that the organic molecules are forced into silicate layers during the thermal treatment. XRD data acquired from preheated sample (Kk1) between 350-550 °C, indicated that chlorite and illite peaks were not shifted but smectite peak was shifted towards 10 A˚. In the same sample, kaolinite should also be present, beacuse the 7 A˚ peak dissappears at 550 °C.Based on FTIR and XRD (even at slow scan) measurements, kaolinite was never observed. Thus we conclude that Kk1 does not include any kaolinite. The(OH) stretching vibrations of quartz at 798, 788 and 697 cm-1 and (OH) stretching vibration of calcite at 2515 cm-1 remain the same after the heat treatment. As a result from FTIR analysis, we can say that the framework of silicate (T-O-T) structures of Kolsuz-Ulukisla-Nigde (Turkey) clay minerals samples has not been destroyed.
Before the thermal treatment, in Kk1 and Kk3 samples, palygorskite’s only bands including (water-OH) scissoring vibrational bands at 1731 and 1673 cm-1 (Frost et al., 2001), OH deformation bands at 905 cm-1 and (O-Si-O) bending vibration band at 468 cm-1 were observed.While illite’s (OH) stretching vibrational band appears as shoulder at 3680 cm-1 in Kk3 clay samples, despite (O-Si-O) bending vibration band at 468 cm-1 observed in Kk1 and Kk3 samples was assigned as belonged to palygorskite, it could belong to Na-montmorillonite’s (O-Si-O) bending vibration band. Similarly, vibration band observed at 3680 cm-1 in Kk3 sample was assigned as (OH) stretching vibration band, it belongs to inner-layer OH, (Al-O...H) stretching band (Farmer and Russell, 1964). In samples Kk1 and Kk3, when bands assigned at 3622, 920, 805, 620, and 468 cm-1 were evaluated altogether; they appear well-matched with fundamental vibration bands of Na-montmorillonite standard. But vibration band at 920 cm-1 of the Na-montmorillonite standard clay was observed at 917 cm-1 in Kk3 sample.
Clay minerals of selected profiles in Nigde-Ulukisla area were determined both by the XRD and the FTIR.
Structural evaluation of these clay varieties was carried using the FTIR spectra.
Structural changes could be determined effeciently by the FTIR.
Findings are evaluated with the known data for depth-temperature-clay mineral transformations during burial processies in sedimentary basins.
Study area has potential to produce hydrocarbons as indicated by the presence of certain clay species.
We would like to thank Turkish Scientific and Technological Research Council (TUBITAK-Turkey) for the financial support (project code: CAYDAG 2005-101Y067). Professor Dr. Cigdem Ercelebi (Department of Physics, Middle East Technical University, Turkey) is also gratefully appreciated for the FTIR.
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