Geochemical characterization of source rocks and oils from the eastern part of the Precaspian and Pre-Uralian Basins (Kazakhstan): Palaeoenvironmental and palaeothermal interpretation

The generative potential, depositional environment and thermal maturity of organic-rich rocks from the eastern part of Precaspian and Pre-Uralian Basins (Kazakhstan) were determined by Rock–Eval and molecular analyses. Source rocks of the Precaspian and Pre-Uralian Basins were deposited in a marine environment. Late Carboniferous deposits from the slope located west of the Janajol Platform and the Pre-Uralian Basin contain Type II algal kerogen in limestones. Early and Mid-Viséan and Early Permian turbidites contain Type III kerogen composed of terrestrial plants transported and deposited by turbidity currents, or a mixture of Types II and III kerogens. Source rocks from the Carboniferous and most of the Early Permian were deposited in a reducing environment, favouring organic matter preservation; however, the continental influence increases towards the eastern areas in the Pre-Uralian Basin and the Suture zone. The onset of the oil window occurs at a depth of 3200–4000 m. In Early Permian rocks, thermal maturity increases from the west in the Precaspian Basin to the east in the Pre-Uralian Basin and the Suture zone under the Urals Nappe. The Carboniferous rocks in the Precaspian Basin are of lower thermal maturity than the Early Permian rocks of the Pre-Uralian Basin. A multivariate statistical principal components analysis (PCA) using source dependent biomarker parameters suggests that Viséan oil correlates with Viséan source rocks from the west, the Late Carboniferous oil from the Janajol Platform correlates with Late Carboniferous source rocks from the Pre-Uralian Basin, and the Early Permian oil in the Precaspian is probably a mixture of oil from Early Permian and deeper Early and Late Carboniferous source rocks. Oil from a suprasaliferous reservoir (Late Permian) is close in composition to the Early Permian oil. Models of palaeotemperatures and genesis of hydrocarbons in two sections are consistent with the migration of oil and gas from greater depth and from the suture zone under the thrust of Urals to the west.