Organic Geochemistry of Source Rock and Crude Oil in the Paleozoic and Mesozoic Petroleum Systems in SE Anatolia (Türkiye): Oil–Source Rock Relationships With Biomarker Approaches

ABSTRACT This study investigates the source rock and organic geochemical properties of Paleozoic and Mesozoic petroleum systems in southeastern (SE) Anatolia. It focuses on the Bedinan and Dadaş (Paleozoic) and the Derdere, Karababa, Karaboğaz, and Sayındere (Mesozoic) formations. Total organic carbon (TOC)/Rock Eval, gas chromatography (GC), and gas chromatography‐mass spectrometry (GC‐MS) analyses were performed on crude oil and source rock samples, and the oil–source rock and oil–oil relationships were determined using the n ‐alkane, isoprenoid, and biomarker data obtained from these analyses. Measured TOC values for the Bedinan and Dadaş formations are 0.59–2.5 wt.% and 1.47–3.64 wt.%, respectively. Mesozoic formations showed TOC ranges of 0.76–1.48 wt.% (Derdere), 0.42–0.82 (Karababa), 0.90–3.36 wt.% (Karaboğaz), and 1.06–1.74 wt.% (Sayındere). The Paleozoic Bedinan and Dadaş formations constitute active source rocks exhibiting variable quality, ranging from poor to good. These units predominantly generate natural gas, with subordinate oil generation. Their organic matter (OM) is characterized by Type II, II/III, and III kerogen, and their thermal maturity corresponds to conditions within the oil window. The Mesozoic Derdere Formation is fair, while the Karababa, Karaboğaz, and Sayındere formations are good source rocks. Source rocks from the Mesozoic era have the capacity to generate oil due to their Type II kerogen content, and these formations are active source rocks with maturity levels within the oil formation window. The source rocks of the Paleozoic petroleum system (Bedinan and Dadaş formations) differ significantly from the source rocks of the Mesozoic petroleum system (Derdere, Karababa, Karaboğaz, and Sayındere formations) and crude oil samples, exhibiting higher higher pristane (Pr)/phytane (Ph), Ts/(Ts+Tm), diasterane/sterane, C 24 /C 23 tricyclic terpane (TT), C 29 norhopane (NH)/C 30 hopane (H), secohopane (SH)/C 30 hopane (H), C 26 /C 25 TT ratios, C 29 sterane (S), C 29 monoaromatic steroid (MAS), C 30 diahopane (C 30 *) contents, and lower C 22 /C 21 TT, C 35 /C 34 homohopane (HH), and dibenzothiophene/phenanthrene (DBT/P) ratios. Crude oil samples are characterized by biomarker parameters such as low Pr/Ph, absence of C 30* , high C 28 /C 29 sterane ratios and C 35 /C 34 HH, and others, which align closely with Mesozoic formations, especially Karababa, Karaboğaz, and Sayındere. Biomarker distribution suggests that the crude oils originated from a Cretaceous carbonate source rock deposited in an anoxic marine environment with low clay content. Cluster analysis of biomarker data confirms a strong correlation between the Mesozoic formations and the crude oils, particularly with Karababa, Karaboğaz, and Sayındere. Paleozoic formations show low correlation and distinct organic geochemical properties, indicating they are not the source of the analyzed crude oils.