Geochemical Characteristics and Oil–Source Correlation Analysis: A Case Study of Jurassic to Paleogene Source Rocks and Paleogene Oils in the Tajik Basin

The Tajik Basin is rich in oil and gas resources, but no analysis of the geochemical characteristics of source rocks and oil has been conducted since the Soviet era, leaving its hydrocarbon generation potential unclear. Through field outcrop sampling and organic geochemical analyses, the geochemical characteristics of the Middle–Lower Jurassic, Cretaceous, and Paleogene source rocks and Paleogene oils were systematically evaluated, and oil–source correlation was conducted using cluster analysis. This study clarified the hydrocarbon generation potential of source rocks in different stratigraphic units and identified differences in the properties of northern and southern oilfields. Results show the significant vertical differentiation in hydrocarbon potential, with the sub-salt Lower–Middle Jurassic as the primary gas-prone source rock. Organic matter type dictates “oil generation in supra-salt, gas generation in sub-salt” pattern. Analysis of biomarkers and carbon isotopes effectively distinguishes the features of different source rocks and oils. The organic matter in the supra-salt Cretaceous and Paleogene source rocks contains a relatively high contribution from aquatic organisms, whereas the sub-salt Middle–Lower Jurassic source rocks are dominated by terrigenous higher plant input from fluvial–deltaic and swamp facies. The oils form southern oilfields exhibit the characteristics of high-sulfur heavy oils, while the oils form northern oilfields are low-sulfur medium-gravity oils. The supra-salt Paleogene oils show marine source characteristics, distinctly different from the Middle–Lower Jurassic coal-bearing source rocks. This indicates that there is no clear evidence for a significant contribution from the Jurassic coal-bearing source rocks to the Paleogene oils. Cluster analysis and biomarker-related parameters indicate that the closest geochemical affinity was observed with Paleogene oils in the northern oilfield and the analyzed Cretaceous-Paleogene source rocks.