Investigation of the organic matter properties and hydrocarbon potential of the Sivas Basin, Central Eastern Anatolia, Turkey, using Rock-Eval data and organic petrography

Abstract

Abstract The main goal of this study is to assess the organic geochemical properties and hydrocarbon generation potential of the Tertiary units in the Sivas Basin which is one of the largest sedimentary basins in the Central Anatolia, Turkey. In this investigation, 200 representative samples were collected from the Tertiary sedimentary rock outcrops. In addition, 15 scaled columnar sections were measured and 187 samples were also collected from these sections. Hydrocarbon generating potential and the maturity levels of the Tertiary units along with the geological history in this basin were determined by means of the total organic carbon (TOC) analysis, Rock-Eval pyrolysis, vitrinite reflectance (Ro) and the organic petrographical observations. Kerogen slides of 96 samples containing high TOCs were prepared to determine the organic matter types. Information about maturity degrees were also obtained using the spore fluorescence colours and the vitrinite reflection (Ro). According to the results of analyses, TOC values of the Hafik (0.05–0.17%), Kalekoy (0.01–0.12%), Yagmurluseki (0.01–0.54%) and Selimiye (0.01–017%) Formations are very low and other geochemical parameters are not encouraging to perform further investigations for these formations. Because of these reasons, they may not be accepted as source rocks. However, the Bozbel (0–1.43%) and Karayun (0.07–7.5%) Formations display high TOC values indicating poor to moderate source rock characteristics with S1+S2 values up to 1.03 mg HC/g rock for the Bozbel and up to 23.45 mg HC/g rock for the Karayun Formation. Organic matter types of the Bozbel and Karayun Formations are mainly Type-III and Type-II/III in the HI–OI and HI–Tmax diagrams, respectively. Samples of the other formations include only Type-III kerogen, in general. The maturation degree of the Bozbel Formation varies in a broad range, from immature to mature levels in the majority of samples. The Karayun Formation is generally in the early mature interval. Consequently, both formations have a limited gas generation potential. However, the Sivas Basin is still worth to explore for hydrocarbons, because all samples were collected from outcrops and type sections close to the basin margins, not from boreholes or from the deeper parts of the basin. Thickness of the sedimentary units is found as 8–10 km in average in the previous studies. On the other hand, the deepest parts of the basin containing such a thick sedimentary deposits have not been drilled, yet. It is possible to find out more encouraging results in case of drilling deep exploration wells.