Mingxiao Sun

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.

Oil–source relationships in the Tarim Basin remain controversial because Paleozoic oils may record contributions from multiple marine source rock systems. We compiled 207 source rock trace element, 184 source rock rare earth element (REE), 44 crude oil trace element, and 33 crude oil REE records to evaluate elemental proxies in an overmature marine carbonate petroleum system. Lower Cambrian source rocks generally record more reducing conditions than the Saergan Formation, although both groups are compositionally variable. PERMANOVA shows significant separation in V/Ni–Ni/Co and Ni/Mo–Mo/Co spaces (R² = 0.4675 and 0.6381; p < 0.001), but PERMDISP indicates unequal dispersion only for V/Ni–Ni/Co. Thus, Ni/Mo–Mo/Co provides the more robust source rock separation. Some Ordovician reservoir oils are compositionally compatible with the Lower Cambrian field, whereas others occupy intermediate or outlying positions. However, whole-rock and ashed oil analyses represent different elemental pools, precluding unique genetic assignment. REE parameters have limited discriminatory capacity because source rock fields overlap and oil data are incomplete and strongly study dependent. V–Ni–Mo–Co relationships should therefore be used as complementary evidence together with biomarkers, stable isotopes, and regional petroleum geology.

Marine source rocks in the Tarim Basin are generally highly mature to overmature, and conventional organic geochemical parameters may not fully reflect their original organic matter accumulation under such conditions. In this study, a comprehensive dataset including major elements, trace elements, and rare earth elements was used to systematically evaluate paleoproductivity variations, spatial distribution patterns, and geochemical controls of the Lower Cambrian (Є1) source rocks in the Tarim Basin. The results show that paleoproductivity in the Lower Cambrian source rocks exhibits clear stratigraphic and regional differentiation. Vertically, the lower intervals are characterized by relatively higher paleoproductivity, whereas the overlying intervals show a gradual decrease in productivity-related signals. Spatially, paleoproductivity in northwestern Tarim was generally higher and more variable than that in eastern Tarim, reflecting differences in depositional environments and material supply among different parts of the basin. These variations were jointly controlled by basin paleogeography, redox conditions, hydrothermal input, and terrigenous supply. Further analysis indicates that inorganic elements can provide complementary information on nutrient supply, export productivity, and organic matter preservation. Among these proxies, P and Ba and their excess indicators record enhanced nutrient availability and increased organic matter export, respectively; however, their variations may be decoupled from TOC because they are strongly influenced by redox conditions and post-depositional processes. In contrast, Ni, Cu, and Zn show relatively weaker enrichment but may better reflect the preserved organic matter component under reducing conditions. This study emphasizes that the application of inorganic geochemical proxies to highly mature marine source rocks requires integrated consideration of lithological heterogeneity, redox conditions, hydrothermal influence, terrigenous input, and diagenetic modification. This work represents a quantitative, data-based review and statistical reanalysis of published geochemical datasets.