Organic geochemical properties of jura-triassic aging rocks in the south part of Caspian basin (Kazakhstan)

Primorsk-Emba region is one of the main oil and gas regions of the Caspian Basin. The oil and gas resources of the Primorsk-Emba region range from 5 to 12 billion tons of oil and 2 to 6 trillion m³ of natural gas. This study primarily concentrates on investigating the organic geochemistry and petroleum geology characteristics of sedimentary units that generated oil in the central Primorsk-Emba province. The study area is discussed in two sections as the East and West sections. 20 rock samples taken from the Jurassic units in the western part of the study area are characterized by Rock-Eval pyrolysis analysis. Using TOC, S1, S2, S3, Tmax, HI, OI, PI, PY and S2 / S3 values organic matter amount, hydrocarbon production potential, type of organic matter, the maturity of organic matter were determined. According to the Rock-Eval and TOC results Jurassic aged rocks generally have a petroleum potential ranging from weak to excellent, the organic matter is between Type II (oil-prone), Type II-III (gas-oil) and Type III (gas-prone), and the degree of maturation is an immature-mature stage. Biomarker analyzes were also performed on oil samples. 9 oil samples were collected from West, 42 oil samples were collected from East part were characterized by geochemical methods including gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS). N-alkanes and isoprenoids were evaluated by high-resolution gas chromatography (GC-HR), terpanes (hopanes), steranes / diasteranes and aromatic hydrocarbons were evaluated by gas chromatography-mass spectrometry (GC-MS). The GC and GC-MS data obtained, it has been determined whether the paleoenvironment characteristics of the study area, hydrocarbon potential, organic matter type, maturity level of organic matter and whether it is affected by biodegradation. Similarities between the East and West sections are revealed. Distribution of n-alkanes in the Gas Chromatogram showed that no biodegradation was observed in oil samples, source rock deposited in a marine environment under reducing conditions and an organic matter that occurred were generated by marine carbonates. According to Pr / Ph, Pr / nC17, Ph / nC18, C26 / C25 TT, C31 R / C30 H and low homohopan index ratios, the deposition environment is a marine / reducing and C29 / C30, DBT / P ratios the oil is derived from carbonated source rock. Gamaseran index showed sea water with normal salinity during the accumulation of oil. Based on maturity parameters such as Ts / Tm, Ts / (Тm + Ts), C29 20S / (20S + 20R) and C29 ββ / (αα + ββ) steran ratios, MPI-1 and 4-MDBT / 1-MDBT values subtracted from GC-MS measurement based on aromatic hydrocarbons, studied oils are mature and located on the oil generation window. According to the vitrinite reflection (Ro) analysis, it was determined that rock samples were between thermally immature-mature for hydrocarbon production and oil samples were advanced mature. According to C28 / C29, norcholestane (NCR)/nordiacholestane (NDR) biomarker age parameters for the studied oils and carbon isotope (δ13C), oil samples are generally of Mesozoic (Triassic-Jurassic) origin, but there are also levels corresponding to the Paleozoic (Permian) late stages. The starplots of oil samples showed similar source origin and oils from reservoirs are genetically related.