Hydrocarbon Potential and Mechanism of Organic Matter Enrichment of Oligocene Oil Shales in Bahcecik-Izmit Area (NW Turkiye)

Abstract

Oligocene oil shales in the Bahcecik-Izmit area (NW Turkiye) are one of the alternative unconventional resources. Oil shales are rarely exposed to the high temperatures necessary for converting kerogen to oil. However, they can yield substantial quantities of combustible hydrocarbons through unconventional techniques. This study examines the determination of hydrocarbon potential and the investigation of various geochemical processes that affect the factors controlling source rock deposition of Bahcecik-Izmit oil shales. Rock-eval pyrolysis analysis was applied to a total of 13 outcrop shale samples. The total organic carbon (TOC) content of studied shales ranges from 2.62 to 6.64 wt., with an average of 3.67 % wt. Kerogen type is mainly Type I (the hydrogen index (HI); 706-984 mgHC/gTOC). The Tmax values varying from 434 to 444oC, parameters of S2/S3 (with an average of 28.2), Production index (PI; with an average of 0.02), and related diagrams indicate that studied shales are in the thermally immature-early mature stage. Based on the S2 (with an average of 29.4 (mg HC/g TOC), genetic potential (GP; with an average of 29.9 mg HC/g TOC), and related diagrams Bahcecik-İzmit oil shales have very good-excellent hydrocarbon potential. In addition, studied shales were deposited in a lacustrine environment with a humid-warm paleoclimatic condition in oxic-suboxic environments based on several redox indicators such as V systematics (V/Sc, V/Ni, V/(V+Ni)), Ni/Co, Th/U, U/Th, and Mo/Mn ratios. The major controlling factor for organic matter accumulation can be O2-deficient conditions in the depositional environment. While a low sedimentation rate (based on the w(La)N/w(Yb)N and ΣREE values) during deposition leads to the dilution of organic matter, the paleoproductivity (based on productivity index of Ba/Al) and detrital material input (based on markers of detrital fluxes Ti, Si, Zr, Th) may have played a role in organic matter enrichment. The clay minerals may not influence organic matter preservation (based on the lack of correlation between clay-forming elements and TOC). In the end, a depositional model was established.   Keywords: Oil shales, hydrocarbon potential, organic matter enrichment, depositional model, Bahcecik-Izmit (NW Turkiye)