Geochemical Characteristics and Hydrocarbon Generation Potential of Paleozoic Petroleum Source Rocks Under the Influence of Orogeny: A Comparative Study of the Eastern Caspian Depression and the Northern Margin of the Tarim Basin

The eastern Pre-Caspian Depression and northern margin of the Tarim Basin are two Paleozoic craton-based basins that were impacted by different late Paleozoic collisional systems—the Uralides and the South Tian Shan. A comparative analysis was performed on their Paleozoic petroleum source rocks as part of this research project. This comparison utilized an identical methodological process across both basins. Data on total organic carbon content (TOC), Rock-Eval pyrolysis and vitrinite reflectance (Ro) were obtained from 15 wells (eight in the eastern Pre-Caspian Depression and seven in the Tarim Basin). In addition to these new datasets, published regional databases and sediment accumulation rates calculated at 40 locations were used. The Pre-Caspian source rocks contained TOC values ranging from 1.9 wt% to 6.0 wt% (average = 3.5 wt%) and had hydrogen indices (HI) ranging from 279 to 500 mg hydrocarbons per gram TOC; they were characterized as oil-prone Type II kerogens and were mature to late-mature (Ro = 0.85–1.40%). The Tarim source rocks contained less TOC (range = 1.2–4.8 wt%; average = 2.6 wt%) and had HI values that were generally lower and highly variable (range = 80–350 mg hydrocarbons per gram TOC); they were composed of Type I–II to III kerogens and were significantly more mature than those found in the Pre-Caspian Basin (range = Ro = 0.80–2.00%), with some areas reaching the dry-gas window. Sediment Accumulation Rates (SARs) were approximately equal in magnitude (maximum SARs ranged from 60 to 64 m per million years) but differed in terms of spatial distribution. While SARs were primarily focused along the southern margin of the Tian Shan mountain range in the Tarim Basin, SARs were relatively evenly distributed throughout the intracratonic eastern Pre-Caspian Depression. Therefore, it can be concluded that the type of orogenic system is responsible for determining when source rocks enter the oil and gas windows, providing a transferable methodology for distinguishing oil- from gas-prone fairways within orogen-flanked cratonic basins.