Maturity of organic matter
In the present study, using pyrolysis Tmax (Peters et al., 2005; Peters and Moldowan, 1993) and Tmax-based vitrinite reflectance values (Ro) (Jarvie et al. (2001) were evaluated the thermal maturity of organic matter. According to Tmax values the limestones samples located in Bağlarbaşı, Gülaçar, Ünlüpınar, Mescitli, and Yuvacık areas are thermally immature – mature (Table 1). Considering PI values (0.08 to 0.5, Jarvie and Tobey, 1999), all the samples in the Bağlarbaşı (except B16), Mescitli (except M10), and Yuvacık (except Y7) areas are oil production window, while the samples in the Gülaçar and Ünlüpınar areas are immature - oil production window (<0.08 to 0.5).
In addition, HI-Tmax diagram ( Lafargue et al., 1998; Qiao et al., 2021) was used to define the maturity of organic matter and all samples were settled in immature – mature areas (Fig. 3). According to Tmax-based vitrinite reflectance values, the thermal maturity of all the samples ranges from immature – oil window level (Jarvie et al., 2004). Although thermal maturity values range from immature to mature, most of the samples are mature and oil window level. Therefore, burial history within study areas was stable, affecting the bottom and top of the sequence level the same. In addition, high thermal maturity in the many samples may have induced the decrease of the TOC ingredients of the limestones and the extraction of generated hydrocarbons (Adegoke et al., 2015).
Organic matter source and source rock characteristics
Rock-Eval analysis results have been used to define organic matter that was conserved in the studied limestones. The limestone samples are characterized by low to moderate HI and moderate to high oxygen index (OI) values. According to HI and OI values, and the HI-Tmax diagram (Fig. 3), the organic matter is interpreted to contain chiefly Type III kerogen (gas prone) and Type II kerogen (oil – gas prone) for Bağlarbaşı and Gülaçar, Type III kerogen (gas prone) and Type II kerogen (oil – gas prone and oil-prone) for Ünlüpınar, and Type II kerogen (oil – gas prone and oil-prone) for Mescitli and Yuvacık areas. This means that the organic matter derived from mixed marine – terrigenous Type II/III kerogen is able of generating oil and gas.
Because of low and fair TOC values (<0.2 % is low for limestones, 0.2 to 0.5 % is fair for limestones, Beaumont and Foster, 1999), the limestones in the Bağlarbaşı, Gülaçar, Ünlüpınar, and Mescitli areas are characterized as poor to fair source rock potential, while limestones in the Yuvacık area are featured as poor to good source rock potential (Table 1) and in the TOC vs. S1 + S2 diagram (Fig. 4), all the samples (except Y13) were settled poor source rock area. However, the fact that S1/TOC ratios are greater than 0.1 in most of the samples in all areas (except Gülaçar area) indicates that the limestones threw out hydrocarbons. On the other hand, according to TOC vs S1 diagram (Fig. 5), the limestone samples produced by hydrocarbons are autochthonous, and there is no organic contamination (Wehner, 1989). In addition, the values of TOC were generally positively correlated with concentrations of Ni and V (elemental of productivity), and not correlated with that of Sr and Ba (detrital input proxies) (Adeboye et al., 2022) (Fig. 6) supports the approach that the organic matter in the studied samples is autochthonous.
The Tmax vs. PI (Fig. 7) indicates that most of the Mescitli, Gülaçar, and Yuvacık area limestone samples can be thought of as well-drained rocks that were extracted and originated hydrocarbon efficiently from the limestone macro and microfractures (Figs. 8a and b).
Paleo-depositional environment
paleo-redox condition
The total sulfur ingredient associated with organic matter in sediments is an imagined level of marine impact. Huang and Pearson (1999) suggested that high organic sulfur ingredient is deduced to be taken from sources deposited in hypersaline and reducing marine situations, and Berner and Raiswell (1983) reported that low sulfur concentrations are characteristic of a freshwater environment (Adegoke et al., 2015). Sulfur value is less than 0.5% exhibiting non-marine sediments, while between 0.5% and 1.5%, and high 1.5% indicating slightly and strongly seawater affected, respectively (Sykes, 2001; 2004). The distribution of the organic sulfur in the limestone samples ranges from 0.21 – 0.55 % in the Ünlüpınar, 0.13 – 0.62 % in the Mescitli, and 0.04 – 0.63 % in the Yuvacık areas, while the organic sulfur values are less than 0.5 % in the Bağlarbaşı and Gülaçar areas (Table 2). Organic sulfur values contained in the limestone indicate both a non-marine environment and a marine environment lightly affected by seawater.
In addition, some trace element data such as Ni and V are used to assess paleo-redox conditions during sedimentation of the depositional environment. It is considered that Ni and V are important indicators for the redox conditions throughout sedimentation because Ni is generally enriched in comparison with V in the oxic environment (Galarraga et al., 2008; Bechtel et al., 2001; Peters and Moldowan, 1993; Barwise, 1990; Saydam Eker, 2021). Galarraga et al. (2008) suggested that V/Ni ratios changing from 1.9 to 3 exhibit deposition under suboxic conditions, while V/Ni ratios >3 exhibit deposition under anoxic conditions. The V/Ni ratios range from 4.29 to 13.3 in the Bağlarbaşı, 5.79 to 11.3 in the Gülaçar, 1.55 to 3.84 in the Ünlüpınar, 1.27 to 53.3 in the Mescitli, 1.47 to 3.62 in the Yuvacık (Table 2). According to these ratios, limestones in the Bağlarbaşı and Gülaçar deposited under anoxic conditions, in the Mescitli, Ünlüpınar, and Yuvacık deposited under oxic to anoxic conditions. In addition, as seen in Figure 9, the concentrations of Ni and V of all limestone samples indicate terrestrial, marine-terrestrial, and marine anoxic shale-carbonate environment.
Paleo-water salinity
Barium and Sr are known as the empirical significant determiner of paleo-water salinity (Adegoke et al., 2015; Wang, 1996; Deng and Qian, 1993; Lui et al., 1984; Liu, 1980). Strontium and Ba concentrations are lower in freshwater than in seawater (Saydam Eker et al., 2016; Li et al., 2016; Reimann and Cartitat, 1998). Therefore, A high Sr/Ba ratio indicates a high salinity environment, thus the Sr/Ba ratio (>1) reflects the relationship of a marine effect on sediment (Shao et al., 1998). The Sr/Ba ratios of the limestone samples change between 2.4 and 37.8 in the Bağlarbaşı, 56.3 and 278 in the Gülaçar, 18.8 and 35.8 in the Ünlüpınar, 7.1 and 109.1 in the Mescitli, and 0.55 and 6.7 in the Yuvacık areas (Table 2). These data reflect high saline water during a deposition for Bağlarbaşı, Gülaçar, Mescitli, Ünlüpınar areas and low to high saline water for the Yuvacık area, indicating the depositional environment of terrestrial and marine conditions is influential. The plot of Sr/Ba vs. V/Ni ratio (Fig. 10) was used to indicate salinity stratification during the deposition of limestone, indicating that the samples are under the influence of salinity and reducing conditions.