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Organic geochemistry, depositional environment and hydrocarbon potential of the tertiary oil shale deposits in NW Anatolia, Turkey.

Introduction

With a reserve of 1.6 billion tonnes, Turkey has a significant oil shale potential. The most important oil shale deposits are located in Beypazari (Ankara), Seyitomer (Kutahya), Himmetoglu and Hatildag (Bolu), Golpazari (Bilecik) and Bahcecik (Izmit) (Fig. 1). The deposits are characterized by the following values for oil shale sequence thickness (m) and reserves (million tonnes): Himmetoglu--84 and 660, Hatildag--26-120 and 360, Golpazari--46 and 120, Bahcecik--38 and 40, Seyitomer--8.5-50 and 120 and Beypazari--57-111 and 330, respectively.

These oil shale deposits have been studied by numerous researchers. Geologic, geochemical and economic investigations of these fields have been conducted [1-9].

Geological and economic features of oil shale fields have been studied separately. In this study, organic geochemical characteristics (organic material content, maturity, type and biomarker distribution) of oil shale deposits in Western and Northwestern Anatolia, their depositional environments and hydrocarbon potentials are discussed. In addition, oil shale fields are also compared.

Geological setting

Listed above deposits in Tertiary basins are located in the western and northwestern regions of Turkey (Fig. 1). The Hatildag shale deposits are of Paleocene-Eocene [6], the Golpazari-Bahcecik deposits of Oligocene [1, 2] and the Beypazari-Seyitomer-Himmetoglu deposits of Miocene age [3-5, 9].

Considering their geological characteristics, oil shale fields are located in four different regions. The Seyitomer field is located north of the Kutahya city. Upper Cretaceous ophiolitic melange comprises the basement of the Seyitomer field. It is unconformably overlain by the Miocene clastic deposits. Oil shales within the Miocene deposits are alternated with lignite, claystone and limestones. The Miocene units in the basin are unconformably overlain by the Pliocene tuffs and claystones.

The Beypazari field is located in the northwestern part of Ankara city. The basement is represented by the Paleozoic metamorphites. Tertiary deposits are unconformably set above the basement. Paleocene-Eocene is represented by red-colored clastic deposits. These units are unconformably overlain by the Miocene lignite and oil shale. The older deposits in the region are unconformably covered by the Pliocene units.

The Himmetoglu, Hatildag and Golpazari oil shale fields are formed in the Bolu-Bilecik basin. The Golpazari field is located northwest of the Bilecik city and Himmetoglu and Hatildag fields southeast of the Bolu city. Paleozoic metamorphites are the oldest rock units in the basin. They are unconformably covered by the Jurassic--Lower Cretaceous carbonate deposits. These units are unconformably overlain by thick turbiditic deposits of Late Cretaceous age which are widely distributed in the region. Oil shales in the basin are found in the Tertiary clastic deposits. Himmetoglu oil shales are found in the Miocene deposits, Hatildag shales in the Paleocene-Eocene deposits and Golpazari shales are associated with the Oligocene deposits.

[FIGURE 1 OMITTED]

The Bahcecik field is located south of the Izmit city. Paleozoic rocks are the oldest rock units in the Bahcecik field. The basement is unconformably covered by Eocene sandstones and limestones. Oil shales alternate with Oligocene marl, shale and tuff. Pliocene loose pebbles and sands are unconformably set above these units.

Materials and methods

The oil shale samples from Beypazari, Seyitomer, Himmetoglu and Hatildag areas are taken from the well cores obtained from MTA (General Directorate of Mineral Research and Exploration). In the Golpazari and Bahcecik areas, a systematic sampling through the stratigraphic sections was carried out.

Pyrolysis/TOC, extraction, column chromatography, gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) were conducted on selected oil shale samples at TPAO (Turkish Petroleum Corp.) Research Center's Organic Geochemistry Laboratories.

Total organic carbon (TOC) and Rock-Eval pyrolysis were performed on 100 mg crushed rock samples which were heated to 600[degrees]C in the helium atmosphere, using a TOC module equipped Rock-Eval II type instrument.

For extraction of the [C.sub.15+]-soluble organic matter (SOM), samples were treated with dichlorometane (C[H.sub.2][Cl.sub.2]) for 40 h in a Soxhlet apparatus. Extracts were seperated into saturated hydrocarbon, aromatic hydrocarbon and NSO-compound fractions by liquid chromatography. Fractions of saturated hydrocarbons were dissolved in hexane and analysed by a Varian 3400 gas chromatograph, equipped with a flame photometric dedector (FPD) and a flame ionisation dedector (FID). A fused capillary column (60 m 0.20 mm i.d.) coated with cross-linked dimethylpolysiloxane (J8N, 0.25 [micro]m film thickness) was used. Helium was used as a carrier gas. The oven temperature was programmed from 40 (initial hold-up time 8 min) to 270[degrees]C (initial hold-up time 60 min) at 4[degrees]C/min.

For GC-MS analysis, a MAT GCQ-Ion trap mass spectrometer was used that was coupled to a Trace-2000 gas chromatograph. The gas chromatograph was equipped with a DB-1 fused silica capillary column of 60-m length, 0.25-mm inner diameter and 0.25-[micro]m film thickness. Helium was used as a carrier gas. The oven temperature was programmed from 50[degrees]C (initial hold-up time 10 min) to 200[degrees]C (initial hold-up time 15 min.) at 10[degrees]C/min, to 250[degrees]C (initial hold-up time 24 min) at 5[degrees]C/min and then to 280[degrees]C (initial hold-up time 24 min) at 2[degrees]C/min. Finally, the oven temperature was increased to 290[degrees]C (initial hold-up time 37 min) at 1[degrees]C/min. The mass spectrometer was operated in EI mode at an ionization energy of 70 eV and a source temperature of 230[degrees]C. For the analysis of biomarkers, metastable ion transition for sterans (m/z 217) and triterpanes (m/z 191) was recorded at a dwell time of 25 ms per ion and a cycle time of 1 s. Compounds were identified through retention time. Triterpane and sterane distributions were quantified by measuring peak heights in the m/z 191 and m/z 217 chromatograms for saturated hydrocarbons, respectively. The peaks recorded in chromatograms are defined in Tables 1 and 2.

Results and discussion

Total organic carbon (TOC) and Rock-Eval pyrolysis

The total organic carbon (TOC) values (average) of the Beypazari, Seyitomer, Himmetoglu, Hatildag, Golpazari and Bahcecik oil shale samples are 8.91; 9.17; 15.81; 4.75; 5.17 and 7.15 wt.%, respectively (Table 3). The TOC values of the Beypazari, Seyitomer and Himmetoglu oil shale samples show a large variation. On the other hand, the samples from the other areas show a more or less homogenous TOC distribution.

Hydrogen index (HI) values obtained from Rock-Eval analysis are exceptionally high, but oxygen index (OI) values are low as it is expected in oil shale samples. The average HI values of Beypazari, Seyitomer, Himmetoglu, Hatildag, Golpazari and Bahcecik oil shales are 822, 648, 723, 720, 781, 812 mg HC/g TOC, and their average OI values are 30, 55, 39, 30, 43, 15 mg C[O.sub.2]/g TOC, respectively.

[S.sub.1], [S.sub.2] and [S.sub.3] values are also measured by using a Rock-Eval analyzer. The ratio of [S.sub.2]/[S.sub.3] gives an idea about the kerogen type. Potential yield (PY) ([S.sub.1]+[S.sub.2]) values are the reflection of hydrocarbon potential of a rock sample, and finally production index (PI) [[S.sub.1]/([S.sub.1]+[S.sub.2])] ratio is used for maturity evaluation. In the studied samples, generally, [S.sub.2]/[S.sub.3] ratios and PY values are relatively high and PI are low (Table 3).

The [T.sub.max] values, which are measured by the the Rock-Eval analyzer and show the maturity level of the organic matter are, on average, 429, 423, 432, 436, 431 and 442[degrees]C for the Beypazari, Seyitomer, Himmetoglu, Hatildag, Golpazari and Bahcecik oil shale samples, respectively.

Composition

The extraction analysis is performed on 11 samples selected from different areas. Total bitumen amounts are divided according to TOC values which indicate expulsion efficiency, and maturity of the samples is generally low (varies from 0.024 to 0.073). Only the value from the Beypazari oil shale sample (0.116) is relatively high compared to the other samples.

n-Alkanes and isoprenoids

Only [C.sub.17] and [C.sub.18] n-alkanes are recorded in the gas chromatograms of the Seyitomer and Beypazari oil shale samples (Fig. 2a-b). Isoprenoids are dominant in the gas chromatograms. High peak intensity is observed in the biomarker region.

[FIGURE 2 OMITTED]

A bimodal distribution, where the n-alkanes with low carbon number are dominant, is generally observed in the gas chromatograms of the extracts obtained from the Himmetoglu, Hatildag, Golpazari and Bahcecik oil shale samples (Fig. 2c-d-e-f). Gas chromatograms of the Golpazari and Himmetoglu oil shale extracts show a bimodal distribution where [C.sub.18] and [C.sub.23] are the center, the maximum peak belonging to [C.sub.18] n-alkane. [C.sub.17] is the maximum n-alkane peak in the gas chromatograms from the Hatildag and Bahcecik oil shales, and a bimodal distribution with [C.sub.17] and [C.sub.23] in the center is observed. The n-alkanes are the dominant peaks in the gas chromatograms from the Himmetoglu and Hatildag oil shale samples, and isoprenoids are widely observed (Fig. 2c-e). The n-alkanes are the dominant peaks in the gas chromatograms from the Golpazari and Bahcecik oil shale samples (Fig. 2d-f).

The Carbon Preference Index (CPI) values are calculated in [C.sub.24]-[C.sub.34] {[([C.sub.25]+[C.sub.27]+[C.sub.29]+[C.sub.31]+[C.sub.33]) / ([C.sub.26]+[C.sub.28]+[C.sub.30]+[C.sub.32]+[C.sub.34]) + ([C.sub.25]+[C.sub.27]+[C.sub.29]+[C.sub.31]+[C.sub.33]) / ([C.sub.24]+[C.sub.26]+[C.sub.28]+[C.sub.30]+[C.sub.32])]/2} and [C.sub.22]-[C.sub.30] {2([C.sub.23]+[C.sub.25]+[C.sub.27]+[C.sub.29])/[[C.sub.22]+2([C.sub.24]+ [C.sub.26]+[C.sub.28])+[C.sub.30]]} [20] ranges from the gas chromatograms of the Bahcecik, Golpazari, Hatildag and Himmetoglu oil shale samples and these CPI values are 1.36, 1.31; 1.76, 1.68; 2.29, 1.48 and 3.60, 1.55, respectively. CPI values are evidently higher than 1.0 for all oil shale samples, which means that the odd number n-alkanes are dominant.

The pristane/phytane (Pr/Ph) ratios calculated for the Beypazari, Seyitomer, Himmetoglu, Hatildag, Golpazari and Bahcecik oil shale samples are 0.14; 0.17; 1.47; 0.93; 1.54 and 1.52, respectively.

Biomarkers

The m/z 217 and m/z 191 mass chromatograms showing the sterane and triterpane distributions in the oil shale samples from different areas are presented in Figs. 3 and 4; the peaks recorded in chromatograms are defined in Tables 1 and 2.

Many biomarker parameters, which are used to find different characteristics of the organic matter, are obtained by using the sterane and triterpane distributions (Table 4). In general, when the sterane distributions belonging to different areas are studied, it is observed that [C.sub.29] is the dominant sterane in the Bahcecik, Beypazari, Golpazari and Seyiomer oil shales. As for the Himmetoglu and Hatildag oil shales, [C.sub.27] is observed as a dominant sterane, but [C.sub.29] sterane shows values close to [C.sub.27]. Considering that [C.sub.27] steranes are recorded together with diasteranes in m/z 217 mass chromatograms, it can be concluded that [C.sub.29] sterane is probably a little bit more dominant in these oil shales. Traces of [C.sub.30] sterane are recorded only for the Seyitomer oil shales (Fig. 3e), as for the m/z 217 mass chromatograms from the other shales, [C.sub.30] sterane is not recorded. While the isosteranes are dominant in comparison to the normal and rearranged steranes in the Beypazari and Seyitomer oil shales, very high ratios of the normal steranes and rather low ratios of the rearranged steranes characterize the Himmetoglu, Hatildag, Golpazari and Bahcecik oil shales. The diasterane/sterane ratios are generally very low (Table 4). Diasteranes are widely observed only in the Seyitomer oil shales.

[FIGURE 3 OMITTED]

The m/z 191 mass chromatograms of oil shales from all areas show low concentrations of tricyclic terpane. Tm ([C.sub.27] 17[alpha](H)-22,29,30-trisnorhopane) is more dominant than Ts ([C.sub.27] 18[alpha](H)-22,29,30-trisnorneophane) in all oil shale samples. The Seyitomer oil shale sample contains more [C.sub.29] norhopane than [C.sub.30] hopane, and [C.sub.30] hopane is more dominant in the samples of the other areas. Low amounts of the oleanane are recorded in the Hatildag, Golpazari and Bahcecik oil shales, and it is not found in the other areas. High ratios of gammacerane are detected in the Himmetoglu, Hatildag and Golpazari oil shale samples, and medium amounts in the Seyitomer, Beypazari and Bahcecik samples (Fig. 4). In general, a homohopane distribution where [C.sub.31] is dominant is observed in all oil shale samples (except Himmetoglu where [C.sub.32] dominates) (Fig. 5). Homohopane distribution typical of Seyitomer, Hatildag and Bahcecik oil shales decreases smoothly towards higher numbers. [C.sub.35] homohopane indexes for the Seyitomer, Himmetoglu, Hatildag, Golpazari and Bahcecik oil shale samples are 9, 12, 15, 15 and 12, respectively. [C.sub.35]/[C.sub.34] homohopane ratios for Seyitomer, Hatildag and Bahcecik are 0.83; 0.91 and 0.45, respectively.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Type of organic matter

The type of organic matter in a rock sample can be determined by using the results of pyrolysis, GC and GC/MS. Kerogen types of the oil shale samples are determined by using [S.sub.2]-TOC [10], HI-[T.sub.max] [11] and HI-OI [12] kerogen classification diagrams (Figs. 6 and 7).

In the [S.sub.2]-TOC diagram, the Beypazari, Golpazari, Bahcecik oil shales are plotted in the Type I area, Seyitomer in Type II, and the Himmetoglu, Hatildag oil shale samples in the Type I-II area (Fig. 6). In the HI-[T.sub.max] and HI-OI diagrams, the samples taken from all the areas are located generally in the Type I area, and only one sample from each of the Seyitomer and Hatildag areas in the Type II area (Fig. 7). Accordingly, it is found that the oil shales generally contain Type I kerogen, only the Seyitomer and Hatildag oil shales contain very low ratios of Type II kerogen. Type I kerogen characteristics of all the oil shales imply that they dominantly contain algal (lacustrine) organic matter. The presence of small amounts of Type II kerogen in the Hatildag and Seyitomer oil shales is probably caused by spores and pollen of land plants and their leaf and stem cuticles.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Bimodal distribution where the low number n-alkanes are dominant is observed in the gas chromatograms of the Himmetoglu, Hatildag, Golpazari and Bahcecik oil shale samples. Such a distribution shows the addition of very little amounts of terrestrial organic matter with algal dominance [13, 14]. Furthermore, the oleanane recorded in m/z 191 mass chromatograms indicates contribution of organic matter from angiosperms of Cretaceous and younger higher plants [15-19]. Oleanane is detected in the Hatildag, Golpazari and Bahcecik oil shales (Fig. 4b-c-e) and the oleanane indices calculated are 14, 7 and 8% respectively. These values show that, according to Hunt's [19] classification, the Hatildag oil shales contain a little input of terrestrial organic matter, whereas that in the case of the Golpazari and Bahcecik oil shales is unimportant. The fact that the oleanane is not recorded in the m/z 191 mass chromatograms of the other shales does not indicate that there is no terrestrial input. The sterane/hopane ratio gives an idea about the organic matter input. Steranes originate mainly from algae and higher plants, and hopanes originate from the cell material of bacteria [18]. The sterane/hopane ratio calculated for the Beypazari, Seyitomer, Himmetoglu, Hatildag, Golpazari and Bahcecik oil shales is 1.10; 3.80; 2.02; 2.93; 1.91 and 0.90, respectively. These values show the presence of algal organic matter and dominance of the sterane for the other fields except the Bahcecik oil shales. Abundance of hopane in the Bahcecik oil shales shows the dominance of bacterial organic matter compared to its content of the other oil shales. In general, Type I kerogen characteristic of these oil shales, bimodal n-alkane distribution where the low carbon number alkanes are more abundant, and the other biomarker data indicate the dominance of algal organic matter and low inputs of bacterial and terrestrial organic matter.

Maturity of organic matter

The maturity of the organic matter is determined by the creation process of hydrocarbon compounds through undergoing physical and chemical changes by different agents like heat, pressure, burial and time after deposition in a sedimantary basin [13]. Thermal history of the organic matter changes many of its physical and chemical characteristics, and the maturity of the organic matter can be measured by making use of these characteristics. In this work, the maturity of the kerogen is interpreted by using the results of pyrolysis, GC and GC/MS analysis.

It is found that the [T.sub.max] value measured during the pyrolysis and the calculated PI values are closely related to thermal history of the organic matter [13, 14]. The average [T.sub.max] values calculated for the Beypazari, Seyitomer, Himmetoglu, Hatildag, Golpazari and Bahcecik oil shales are 429, 423, 432, 436, 431 and 442[degrees]C, respectively. These values show that the oil shales, except Bahcecik, are characterized by low [T.sub.max] values and show immature characteristics. The average [T.sub.max] value of 442[degrees]C characterizing the Bahcecik oil shales generally reflects the character of early maturity, and it corresponds to the maturity at the entrance into oil generation window [20]. However, considering that these oil shales contain Type I kerogen, it becomes clear that this [T.sub.max] value is not sufficient for oil generation [13] and it reflects immaturity. The PI values of all oil shales are quite low (Table 3), corresponding to the immature character in the maturity classification made by the Production Index [21].

Average values of the bitumen/TOC ratio, calculated for the Beypazari, Seyitomer, Himmetoglu, Hatildag, Golpazari and Bahcecik oil shales, 0.071; 0.043; 0.032; 0.066; 0.034 and 0.073, respectively, also reflect their immaturity. Isoprenoids dominate in the gas chromatograms of the Beypazari and Seyitomer oil shale samples (Fig. 2a-b), and they are also widespread in the Himmetoglu and Hatildag oil shales (Fig. 2c-e). High concentrations observed in the biomarker region of the gas chromatograms of the Beypazari, Seyitomer and Hatildag oil shales (Fig. 2a-b-c) show also low maturity. Furthermore, CPI values which are evidently higher than 1.0 for the Himmetoglu, Hatildag, Golpazari and Bahcecik oil shales result from their low maturity degree [19, 20].

The 20S/(20S+20R) sterane ratio increases with increasing maturity degree [17, 20, 22-25]. This ratio could not be determined for the Beypazari area oil shale, as well as for the other areas, due to quite low values that have not reached equilibrium value yet (Table 4). Another maturity parameter calculated using the sterane data is the [beta][beta]/([beta][beta]+[alpha][alpha]) ratio, and this ratio increases with increasing maturity [14, 17, 22, 23]. This ratio could not be determined for the Beypazari area; middle values characterize the Seyitomer area and very low values for the other areas are calculated (Table 4). The transformation has not reached the equilibrium in any area. These two ratios calculated for steranes also reflect immaturity of all oil shales.

The 22S/(22S+22R) homohopane ratio increases with maturity [17, 19]. The equilibrium value for the homohopane ratio is measured to be 0.57-0.62 [25]. This ratio, which has not reached the equilibrium value for all the areas, reflects the immature characteristics of Turkish oil shales.

The Ts/(Ts+Tm) rate also increases in proportion with maturity [19, 26], but the moretane/hopane ratio decreases [27-29]. In general, all the oil shale samples are immature enough, according to their low Ts/(Ts+Tm) ratio and the moretane/hopane ratio exceeding 0.15 [17].

The maturity data obtained indicate that all the oil shales are immature.

Depositional environment

Depositional environments of oil shales are interpreted by using the data of the organic geochemical analyses. Biomarkers provide information about the type of the environment where the organic matter deposits, the conditions existing in the depositional environment during the deposition and the litology of the rock in which the organic matter is present. The 17[alpha](H)-homohopane distributions change extensively from sample to sample and provide information about the paleoenvironment [17, 30]. A homohopane distribution with the dominance of low-number homologues is observed in the m/z 191 mass chromatograms of the oil shale samples (Fig. 4, 9), and homohopane index is very low for all the oil shale samples (Table 4). Such a homohopane distribution with low [C.sub.35] homohopane index values generally corresponds to a suboxic depositional environment [19]. Furthermore, the homohopane distributions, which have a regular decrease in the peak height from [C.sub.31] to [C.sub.35], especially in the Bahcecik, Hatildag and Seyitomer oil shales, indicate clastic facieses [17]. The Pr/Ph ratio reflects the redox potential of the depositional environment [20, 31]. The Pr/Ph ratio calculated for the Beypazari, Seyitomer, Himmetoglu, Hatildag, Golpazari and Bahcecik oil shales is 0.14; 0.17; 1.47; 0.93; 1.54 and 1.52, respectively. It is evident that the Beypazari, Seyitomer and Hatildag oil shales having a Pr/Ph ratio lower than 1.0 have been deposited in an anoxic environment. The homohopane and Pr/Ph data evaluated together indicate that the Himmetoglu, Golpazari and Bahcecik oil shales are the products of a suboxic depositional environment.

The [C.sub.29]/[C.sub.30] hopane ratio is used to distinguish the carbonate and clastic lithology [17, 32], The ratios measured for the Beypazari, Seyitomer, Himmetoglu, Hatildag, Golpazari and Bahcecik oil shales are 0.55; 1.58; 0.61; 0.34; 0.24 and 0.98, respectively, and these values indicate clastic rocks, except Seyitomer. The [C.sub.29]/[C.sub.30] ratio higher than 1.0 characterizing the Seyitomer oil shales shows that they contain much carbonate components; this conclusion concurs with the fact that the Seyitomer oil shales contain actually marl as has been indicated in literature many times.

Gammacerane, which is a typical biomarker for the lake and sea sediments of high salinity [17, 19, 20, 33], is recorded in higher abundances for the Himmetoglu, Hatildag, Golpazari and Seyitomer oil shales, in medium abundances for the Beypazari and Bahcecik oil shales. The gammacerane index characterizing these shales is 0.32; 0.48; 0.31; 0.54; 0.13 and 0.08, respectively. Furthermore, the pregnanes that are another indicator of salinity are present in low quantities in the Bahcecik and Hatildag oil shales, in medium amounts in the Seyitomer, Himmetoglu and Golpazari oil shales, and in abundance in the Beypazari oil shales (Fig. 3). The presence of both gammacerane and pregnane shows that all the oil shales have been deposited in a saline environment.

The [C.sub.25]/[C.sub.26] tricyclic terpane ratio is used to distinguish the marine and non-marine environments [34-36]. The values higher than 1.0 indicate a marine environment, whereas the low values a non-marine environment. This ratio calculated for the Hatildag oil shale sample is 0.82. As the [C.sub.25] tricyclic terpane is not recorded in the oil shales from the other areas, the ratio could not be calculated. The ratio lower than 1.0 calculated for the Hatildag oil shales and the existence of [C.sub.26] tricyclic terpane without recording [C.sub.25] tricyclic terpane for the other areas indicate that all the oil shales are the products of a non-marine depositional environment. Basing on the data about lithology and fossils of the oil shales in the former studies performed in these areas, it was suggested that they have been deposited in a lacustrine environment [1-3, 6, 8, 37, 38]. The biomarker and the other data obtained in this study also indicate a lacustrine depositional environment.

Potential of hydrocarbon generation

The potential yield (PY) parameter calculated by adding [S.sub.1] and [S.sub.2] values gives an idea about the hydrocarbon generation potential of a rock [13]. PY values of the Beypazari, Seyitomer, Himmetoglu, Hatildag, Golpazari and Bahcecik oil shale samples range between 16.53-153.38, 16.86-114.77, 20.94-301.51, 13.48-64.02, 14.53-95.6 and 40.71-78.86 mgHC/g rock, respectively, and the average values are 78.99; 50.21; 124.07; 37.49; 43.98 and 59.54 mgHC/g rock. In general, the oil shales of all the areas are characterized by a rather high PY values. However, Himmetoglu oil shales show rather high values in comparison with the others. The high PY values indicate that hydrocarbon generation potential of the oil shales is rather high.

The average hydrogen index (HI) values of the the Beypazari, Seyitomer, Himmetoglu, Hatildag, Golpazari and Bahcecik oil shale samples are 822, 648, 723, 720, 781 and 812 mgHC/gTOC, respectively. Such high HI values show that (Type I kerogen) oil shales have a potential to generate oil [21, 39]. Furthermore, S2/S3 rate is found to be rather high for all the areas, and this rate higher than 5 also indicates that these oil shales can generate oil [21].

Therefore, the formation of crude oil was not possible due to immature nature of the oil shales, although hydrocarbon potential of the oil shales of all the areas is high.

Conclusions

The average TOC values of the Beypazari, Seyitomer, Himmetoglu, Hatildag, Golpazari and Bahcecik oil shales are determined to be 8.91; 9.17; 15.81; 4.75; 5.17 and 7.15%, respectively. These TOC values show that organic matter content of the oil shales is very high.

HI-OI, HI-[T.sub.max] and [S.sub.2]-TOC kerogen type classification diagrams indicate that the Beypazari, Himmetoglu, Golpazari and Bahcecik oil shales contain Type I kerogen; the Seyitomer and Hatildag oil shales contain also Type I kerogen dominantly and Type II kerogen in small quantities. These kerogen types indicate the dominance of algal (lacustrine) organic matter. Gas chromatograms of the Himmetoglu, Hatildag, Golpazari and Bahcecik oil shales all show bimodal distributions. The dominance of low carbon-number n-alkanes indicates that these oil shales consist mainly of algal and less of terrestrial organic matter. The oleanane index for the Hatildag, Golpazari and Bahcecik oil shales also indicates that there is very little input of terrestrial organic matter.

The PY values calculated for the Beypazari, Seyitomer, Himmetoglu, Hatildag, Golpazari and Bahcecik oil shales are 78.99; 50.21; 124.07; 37.49; 43.98 and 59.54 mgHC/g rock. Basing on these values, it is concluded that the oil shales are the source rocks with high hydrocarbon potential. High HI values and high [S.sub.2]/[S.sub.3] ratios of oil shale samples from all fields show that oil shales have a potential to generate oil.

The [T.sub.max] values of pyrolysis of the Beypazari, Seyitomer, Himmetoglu, Hatildag, Golpazari and Bahcecik oil shales are 429, 423, 432, 436, 431 and 442[degrees]C, respectively. According to these values, all the oil shales are immature. In addition to gas chromatograms, m/z 217 and m/z 191 mass chromatograms also indicate that these oil shales are immature. Therefore, their alteration to form crude oil was not possible due to immature nature, although the oil shales of all the areas are characterized by high hydrocarbon potential.

The Pr/Ph ratios of the Beypazari, Seyitomer, Himmetoglu, Hatildag, Golpazari and Bahcecik oil shales show that the Beypazari, Seyitomer and Hatildag oil shales have been deposited in an anoxic and the Himmetoglu, Golpazari and Bahcecik oil shales in a suboxic environment.

Gammacerane was recorded in higher abundances for the Himmetoglu, Hatildag, Golpazari and Seyitomer oil shales and in medium abundances for the Beypazari and Bahcecik shales. According to the gammacerane presence and other parameters, all the oil shales may be considered to have been deposited in a saline environment.

The [C.sub.25]/[C.sub.26] tricyclic terpane ratio lower than 1.0 calculated for the Hatildag oil shales and the existence of [C.sub.26] tricyclic terpane without recording [C.sub.25] tricyclic terpane for the other areas indicate that all the oil shales are the products of a non-marine depositional environment. Basing on the data about lithology and fossils of the oil shales in the former studies performed in these areas, it can be said that they have deposited in a lacustrine environment. The biomarker and the other data obtained in this study also indicate a lacustrine depositional environment.

Basing on the whole geochemical data which is mentioned above in detail, the oil shales studied from Northwest Anatolia, were deposited in a similar depositional environment and conditions.

Acknowledgements

This work was supported by the Research Fund of Karadeniz Technical University (KTU) and AAPG Foundation. Analyses were carried out by TPAO (Turkish Petroleum Corp.) Organic Geochemistry Labs. Some samples were obtained from MTA (General Directorate Mineral Research and Exploration). In addition, this study was supported by Senior Geologist H. Iztan (TPAO), Dr. N. Tuysuz (KTU) and Lec. A. Abduloglu (KTU). We thank all these people and organizations.

Presented by M. V. Kok Received June 20, 2007

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R. KARA GULBAY *, S. KORKMAZ

Department of Geological Engineering Karadeniz Technical University 61080, Trabzon, Turkey

* Corresponding author: e-mail kara@ktu.edu.tr
Table 1. Peak definitions of steranes of m/z 217 mass chromatograms

Peak No    Compound

      1    [C.sub.27] 13[beta](H),17[alpha](H)-Diasterane (20S)

      2    [C.sub.27] 13[beta](H),17[alpha](H)-Diasterane (20R)

      3    [C.sub.27] 13[alpha](H),17[beta] (H)-Diasterane (20S)

      4    [C.sub.27] 13[alpha](H),17[beta] (H)-Diasterane (20R)

      5    [C.sub.28] 13[beta](H),17[alpha](H)-Diasterane (20S)

      6    [C.sub.28] 13[beta](H),17[alpha](H)-Diasterane (20R)

      7    [C.sub.28] 13[alpha](H),17[beta] (H)-Diasterane (20S)

      8    [C.sub.27] 5[alpha](H),14[alpha](H),17[alpha](H)-
           Sterane (20S)+
           [C.sub.28] 13[alpha](H),17[beta] (H)-Diasterane (20S)

      9    [C.sub.27] 5[alpha](H),14 [beta](H),17[beta](H)-
           Sterane (20R)+
           [C.sub.29] 13[beta](H),17[alpha](H)-Diasterane (20S)

     10    [C.sub.27] 5[alpha](H),14[beta](H),17[beta](H)- Sterane
           (20S)+
           [C.sub.28] 13[alpha](H),17[beta](H)-Diasterane (20R)

     11    [C.sub.27] 5[alpha](H),14[alpha](H),17[alpha](H)-
           Sterane (20R)

     12    [C.sub.29] 13[beta](H),17[alpha](H)-Diasterane (20R)

     13    [C.sub.29] 13[alpha](H),17[beta](H)-Diasterane (20S)

     14    [C.sub.28] 5[alpha](H),14[alpha](H),17[alpha](H)-
           Sterane (20S)

     15    [C.sub.28] 5[alpha](H),14[beta](H),17[beta](H)-Sterane
           (20R)+ [C.sub.29]

     16    [C.sub.28] 5[alpha](H),14[beta](H),17[beta](H)-
           Sterane (20S)

     17    [C.sub.28] 5[alpha](H),14[alpha](H),17[alpha](H)-
           Sterane (20R)

     18    [C.sub.29] 5[alpha](H),14[alpha](H),17[alpha](H)-
           Sterane (20S)

     19    [C.sub.29] 5[alpha](H),14 [beta](H),17[beta](H)-
           Sterane (20R)

     20    [C.sub.29] 5[alpha](H),14 [beta](H),17[beta](H)-
           Sterane (20S)

     21    [C.sub.29] 5[alpha](H),14[alpha](H),17[alpha](H)-
           Sterane (20R)

     22    [C.sub.30] 5[alpha](H),14[alpha](H),17[alpha](H)-
           Sterane (20S)

Table 2. Peak definitions of tritepanes of m/z 191 mass chromatograms

Peak No.

       1    [C.sub.19] Tricyclic Terpane

       2    [C.sub.20] Tricyclic Terpane

       3    [C.sub.21] Tricyclic Terpane

       4    [C.sub.22] Tricyclic Terpane

       5    [C.sub.23] Tricyclic Terpane
            (18,19-Bisnor-13[beta](H),14[alpha](H) Cheilanthaneane)

       6    [C.sub.24] Tricyclic Terpane

       7    [C.sub.25] (22S+22R) Tricyclic Terpane

       8    Tetracyclic Hopane (Secohopane)

       9    [C.sub.26] 22(S) Tricyclic Terpane

      10    [C.sub.26] 22(R) Tricyclic Terpane

      11    [C.sub.28] Tricyclic Terpane

      12    [C.sub.29] Tricyclic Terpane

      13    [C.sub.27] 18[alpha](H)-22,29,30-Trisnorneohopane (Ts)

      14    [C.sub.27] 17[alpha](H)-22,29,30-Trisnorhopane (Tm)

      16    [C.sub.30] Tricyclic Terpane

      18    [C.sub.29] 17[alpha](H),21[beta](H)-30-Norhopane

      19    [C.sub.29] Ts (18[alpha](H)-30-Norneohopane

      21    [C.sub.29] 17[beta](H),21[alpha](H)-30-Normoretane

      22    Oleanane

      23    [C.sub.30] 17[alpha](H),21[beta](H)-Hopane

      24    [C.sub.30] 17[beta](H),21[alpha](H)-Moretane

      25    [C.sub.31] 17[alpha](H),21[beta](H)-30-Homohopane (22S)

      26    [C.sub.31] 17[alpha](H),21[beta](H)-30-Homohopane (22R)

      27    Gammacerane

      28    Homomoretane

      29    [C.sub.32] 17[alpha](H),21[beta](H)-30,
            31-Bishomohopane (22S)

      30    [C.sub.32] 17[alpha](H),21[beta](H)-30,
            31-Bishomohopane (22R)

      31    [C.sub.33] 17[alpha](H),21[beta](H)-30,31,
            32-Trishomohopane (22S)

      32    [C.sub.33] 17[alpha](H),21[beta](H)-30,31,
            32-Trishomohopane (22R)

      33    [C.sub.34] 17[alpha](H),21[beta](H)-30,31,32,
            33-Tetrakishomohopane (22S)

      34    [C.sub.34] 17[alpha](H),21[beta](H)-30,31,32,
            33-Tetrakishomohopane (22R)

      35    [C.sub.35] 17[alpha](H),21[beta](H)-30,31,32,33,
            34-Pentakishomohopane (22S)

      36    [C.sub.35] 17[alpha](H),21[beta](H)-30,31,32,33,
            34-Pentakishomohopane (22R)

Table 3. Results of Rock-Eval/TOC analyses

Sample     Depth            TOC               [S.sub.1]
No         (m)              (%)               (mgHC/g rock)

BB-1 Well (Beypazari-Ankara)
ABB-1      50.35            1.85              2.16
ABB-4      85.60           18.01              7.54
ABB-5      95.50            6.47              7.70
ABB-6      99.65            8.95              5.61
ABB-8     108.20            5.98              3.33
ABB-10    113.90            7.92              6.04

1037 Well (Beypazari-Ankara)
BP-2       94.30           12.58              6.36
BP-9      165.30            9.52              3.61
Average                     8.91              5.29

BS-5 Well (Seyitomer-Kutahya)
KS-4       46.00            3.09              1.57
KS-5       52.80            6.46              3.34
KS-7       61.50            2.22              1.16
KS-8       62.50           24.92             14.28
Average                     9.17              5.09

88/3 Well (Himmetoglu-Bola)
BH-2       15.70            3.02              1.43
BH-5       64.50            5.85              1.89
BH-7       81.40           11.64              2.60
BH-8       86.50           35.58             14.30
BH-10      93.55           10.61              3.32
BH-12     100.30           28.14             15.73
Average                    15.81              6.55

G-2 Well (Hauldag-Bola)
BHD-1       58.00           2.04              0.94
BHD-4      121.50           2.53              0.23
BHD-5      166.65           8.26              1.06
BHD-7      191.60           6.48              2.27
BHD-9      207.00           3.84              0.46
BHD-10     221.00           5.36              2.80
Average                     4.75              1.29

Golpazari (Bilecik) measured stratigraphical section
BG-8       44.70            3.21              0.63
BG-7       35.60            2.54              0.77
BG-6       31.25            2.06              0.53
BG-4       20.00           10.16              3.56
BG-2       10.15           10.34              3.25
BG-1        4.55            2.71              1.15
Average                     5.17              1.65

Bahgecik (Izmit) measured stratigraphical section
IB-12      33.50            6.26              1.73
IB-8       21.70            7.24              2.49
IB-7       19.10            9.19              2.37
IB-5        9.90            9.20              2.51
IB-2        4.60            6.27              1.50
IB-1        3.40            4.72              0.71
Average                     7.15              1.89

Sample     [S.sub.2]        [S.sub.3]         Kerogen
No         (mgHC/g rock)    (mgC[O.sub.2]/    Type
                            g rock)           [S.sub.2]/
                                              [S.sub.3]
BB-1 Well (Beypazari-Ankara)
ABB-1       14.37           1.13              12.72
ABB-4      145.84           6.41              22.75
ABB-5       54.02           1.69              31.96
ABB-6       76.11           1.58              48.17
ABB-8       53.54           1.40              38.24
ABB-10      62.43           1.88              33.21

1037 Well (Beypazari-Ankara)
BP-2       115.80           3.88              29.85
BP-9        67.49           2.21              30.54
Average     73.70           2.52              30.93

BS-5 Well (Seyitomer-Kutahya)
KS-4        21.84           2.03              10.76
KS-5        42.46           3.14              13.52
KS-7        15.70           1.30              12.08
KS-8       130.49          12.07              10.81
Average     52.62           4.64              11.79

88/3 Well (Himmetoglu-Bola)
BH-2        19.51           1.96               9.95
BH-5        45.54           2.18              20.89
BH-7        87.37           4.37              19.99
BH-8       287.21           9.33              30.78
BH-10       70.57           4.55              15.51
BH-12      194.93           8.15              23.92
Average    117.52           5.09              20.17

G-2 Well (Hauldag-Bola)
BHD-1      12.54            0.93              13.48
BHD-4      18.66            0.87              21.45
BHD-5      62.96            1.49              42.26
BHD-7      58.88            1.12              52.57
BHD-9      13.64            1.34              10.18
BHD-10     50.52            1.81              27.91
Average    36.20            1.26              27.98

Golpazari (Bilecik) measured stratigraphical section
BG-8       23.46            1.60              14.66
BG-7       20.36            0.77              26.44
BG-6       14.00            1.38              10.14
BG-4       92.04            2.82              32.64
BG-2       83.33            3.13              26.62
BG-1       20.79            1.44              14.44
Average    42.33            1.86              20.82

Bahgecik (Izmit) measured stratigraphical section
IB-12      46.82            1.20              39.02
IB-8       58.50            0.85              68.82
IB-7       66.94            1.75              38.25
IB-5       76.35            1.13              67.57
IB-2       57.31            0.78              73.47
IB-1       40.00            0.86              46.51
Average    57.65            1.10              55.61

Sample     (PY)             (PI)              [T.sub.max]
No         [S.sub.1] +      [S.sub.1]/        ([degrees])
           [S.sub.2]        ([S.sub.1]+
           (mgHC/g rock)    [S.sub.2])

BB-1 Well (Beypazari-Ankara)
ABB-1      16.53            0.13              428
ABB-4     153.38            0.05              438
ABB-5      61.72            0.12              433
ABB-6      81.72            0.07              426
ABB-8      56.87            0.05              437
ABB-10     68.47            0.09              411

1037 Well (Beypazari-Ankara)
BP-2       122.16           0.05              437
BP-9       71.10            0.05              423
Average    78.99            0.08              429

BS-5 Well (Seyitomer-Kutahya)
KS-4       23.41            0.07              421
KS-5       45.80            0.07              425
KS-7       16.86            0.07              424
KS-8      114.77            0.10              421
Average    50.21            0.08              423

88/3 Well (Himmetoglu-Bola)
BH-2       20.94            0.07              430
BH-5       47.43            0.04              436
BH-7       89.97            0.03              433
BH-8      301.51            0.05              433
BH-10      73.89            0.04              433
BH-12     210.66            0.07              424
Average   124.07            0.05              432

G-2 Well (Hauldag-Bola)
BHD-1      13.48            0.07              427
BHD-4      18.89            0.01              442
BHD-5      64.02            0.02              441
BHD-7      61.15            0.04              440
BHD-9      14.10            0.03              427
BHD-10     53.32            0.05              437
Average    37.49            0.04              436

Golpazari (Bilecik) measured stratigraphical section
BG-8       24.09            0.03              432
BG-7       21.13            0.04              428
BG-6       14.53            0.04              426
BG-4       95.60            0.04              442
BG-2       86.58            0.04              435
BG-1       21.94            0.05              420
Average    43.98            0.04              431

Bahgecik (Izmit) measured stratigraphical section
IB-12      48.55            0.04              440
IB-8       60.99            0.04              443
IB-7       69.31            0.03              436
IB-5       78.86            0.03              442
IB-2       58.81            0.03              448
IB-1       40.71            0.02              444
Average    59.54            0.03              442

Sample     (HI)             (OI)              Residual
No         [S.sub.2]/TOC    [S.sub.3]/TOC     Carbon
           (mgHC/g TOC)     (mgC[O.sub.2]/
                            gMC)

BB-1 Well (Beypazari-Ankara)
ABB-1      776              61                0.48
ABB-4      809              35                5.23
ABB-5      834              26                1.33
ABB-6      850              17                2.14
ABB-8      895              23                1.25
ABB-10     788              23                2.22

1037 Well (Beypazari-Ankara)
BP-2       913              30                2.50
BP-9       708              23                3.60
Average    822              30                2.34

BS-5 Well (Seyitomer-Kutahya)
KS-4       706              65                1.14
KS-5       657              48                2.65
KS-7       707              58                0.82
KS-8       523              48               12.86
Average    648              55                4.37

88/3 Well (Himmetoglu-Bola)
BH-2       646              64                1.28
BH-5       778              37                1.90
BH-7       750              37                4.15
BH-8       807              26               10.46
BH-10      665              42                4.46
BH-12      692              28               10.59
Average    723              39                5.47

G-2 Well (Hauldag-Bola)
BHD-1      614              45                0.92
BHD-4      737              34                0.96
BHD-5      762              18                2.93
BHD-7      908              17                1.39
BHD-9      355              34                2.67
BHD-10     942              33                0.92
Average    720              30                1.63

Golpazari (Bilecik) measured stratigraphical section
BG-8       730              49                1.21
BG-7       801              30                0.78
BG-6       679              66                0.85
BG-4       905              27                2.20
BG-2       805              30                3.13
BG-1       767              53                0.89
Average    781              43                1.51

Bahgecik (Izmit) measured stratigraphical section
IB-12      747              19                2.22
IB-8       808              11                2.16
IB-7       728              19                3.42
IB-5       829              12                2.63
IB-2       914              12                1.37
IB-1       847              18                1.33
Average    812              15                2.19

Table 4. Biomarker parameters calculated from m/z 217 and m/z 191
mass chromatograms

Parameters                                   Field name
                                      Beypazari        Seyitomer

[C.sub.35]/([C.sub.31]-[C.sub.35])
Homohopane Index                      nd               9%
[C.sub.35]/[C.sub.34] Homo-           nd               0.83
hopane Ratio
[C.sub.25]/[C.sub.26]
Tricyclic Terpane                     nd               nd
Ratio
Gammaceran Index
[Gammacerane/[C.sub.30]               0.13             0.54
Hopane]
Oleanane Index
[(Oleanane/[C.sub.30]                 nd               nd
Hopane)x100]
[C.sub.29]/[C.sub.30] Hopane Ratio    0.55             1.58
[C.sub.27],[C.sub.28],[C.sub.29]      17, 24,          39, 20,
Sterane Percent                       59%              41%
Normal-sterane, Iso-
sterane,                              37, 54,          32, 38,
Rearranged-sterane                    9%               30%
Diasterane/Sterane                    61               127
Ratio
Sterane/Hopane Ratio                  1.10             3.80
20S(20S+20R)
Sterane Ratio ([C.sub.29])            nd               0.45
[beta][beta]/([beta][beta]+
[alpha][alpha])
Sterane Ratio ([C.sub.29])            nd               0.51
22S/(22S+22R)
Homohopane Ratio                      0.55             0.56
([C.sub.31])
Ts/(Ts+Tm)                            0.5              0.32
Moretane/Hopane                       0.21             0.64
Ratio

Parameters                                Field name
                                      Himmetoglu       Hatildag

[C.sub.35]/([C.sub.31]-[C.sub.35])
Homohopane Index                      12%              15%
[C.sub.35]/[C.sub.34] Homo-           nd               0.91
hopane Ratio
[C.sub.25]/[C.sub.26]
Tricyclic Terpane                     nd               0.82
Ratio
Gammaceran Index
[Gammacerane/[C.sub.30]               0.32             0.48
Hopane]
Oleanane Index
[(Oleanane/[C.sub.30]                 nd               14%
Hopane)x100]
[C.sub.29]/[C.sub.30] Hopane Ratio    0.61             0.34
[C.sub.27],[C.sub.28],[C.sub.29]      49, 11, 40%      37, 28,
Sterane Percent                                        35%
Normal-sterane, Iso-
sterane,                              68, 21, 11%      44, 43,
Rearranged-sterane                                     13%
Diasterane/Sterane                    22               8.5
Ratio
Sterane/Hopane Ratio                  2.02             2.49
20S(20S+20R)
Sterane Ratio ([C.sub.29])            0.11             0.19
[beta][beta]/([beta][beta]+
[alpha][alpha])
Sterane Ratio ([C.sub.29])            0.22             0.19
22S/(22S+22R)
Homohopane Ratio                      0.44             0.36
([C.sub.31])
Ts/(Ts+Tm)                            0.25             0.21
Moretane/Hopane                       0.48             0.22
Ratio

Parameters                                   Field name
                                      Golpazari        Bahcecik

[C.sub.35]/([C.sub.31]-[C.sub.35])
Homohopane Index                      15%              2%
[C.sub.35]/[C.sub.34] Homo-           nd               0.45
hopane Ratio
[C.sub.25]/[C.sub.26]
Tricyclic Terpane                     nd               nd
Ratio
Gammaceran Index
[Gammacerane/[C.sub.30]               0.31             0.08
Hopane]
Oleanane Index
[(Oleanane/[C.sub.30]                 7%               8%
Hopane)x100]
[C.sub.29]/[C.sub.30] Hopane Ratio    0.24             0.98
[C.sub.27],[C.sub.28],[C.sub.29]      37, 25,          20, 22,
Sterane Percent                       38%              58%
Normal-sterane, Iso-
sterane,                              66, 15,          70, 19,
Rearranged-sterane                    19%              11%
Diasterane/Sterane                    4.6              24
Ratio
Sterane/Hopane Ratio                  2.93             0.90
20S(20S+20R)
Sterane Ratio ([C.sub.29])            0.04             0.12
[beta][beta]/([beta][beta]+
[alpha][alpha])
Sterane Ratio ([C.sub.29])            0.16             0.08
22S/(22S+22R)
Homohopane Ratio                      0.49             0.36
([C.sub.31])
Ts/(Ts+Tm)                            0.17             0.19
Moretane/Hopane                       0.32             0.14
Ratio

[C.sub.35]/(C31-[C.sub.35]) Homohopane Index: (35+36) /
(25+26+29+30+31+32+33+34+35+36)
(m/z 191)

[C.sub.35]/[C.sub.34] Homohopane Ratio: (35+36)/(33+34) (m/z 191)

Diasterane/Sterane Ratio: [(1+2)/(8+11)]x100 (m/z 217)

Sterane/Hopane Ratio: (8+9+10+11+14+15+16+17+18+19+20+21)(m/z217) /
(18+23+25+26+29+30+31+32)(m/z191)

20S(20S+20R) Sterane Ratio: 18/(18+21) (m/z 217)

[beta][beta]/([beta][beta]+[alpha][alpha]) Sterane Ratio: (19+20)/
(19+20+18+21) (m/z 217)

22S/(22S+22R) Homohopane Index (C32): 29/(29+30) (m/z 191)

Moretane/Hopane Ratioi: 24/23 (m/z 191)

nd: not determined
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Author:Gulbay, R. Kara; Korkmaz, S.
Publication:Oil Shale
Date:Dec 1, 2008
Words:8586
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