Evaluation of Late Paleozoic Over-High Maturity Source Rocks in the Erlian Basin and Its Adjacent Areas

Abstract

The evaluation of Paleozoic source rocks during oil and gas exploration is important yet difficult. This study qualitatively and quantitatively evaluated the potential of Late Paleozoic source rocks through organic geochemical analyses of 18 outcrop samples from the Linxi, Zhesi and Benbatu formations in the Erlian Basin and its adjacent areas. The results revealed that Late Paleozoic source rocks are currently in a stage of over-high maturity. The three source rock intervals recorded low organic carbon contents (TOC) and low hydrocarbon generation potential. The abundance of organic matter (OM) was corrected to account for the effects of weathering and to calculate the original carbon content. The original hydrocarbon generation potential was determined using a chemical kinetics method. The average original TOC contents (TOC₀) of the Linxi, Zhesi and Benbatu formations were 0.89, 0.28 and 0.17%, respectively, with average original hydrocarbon generation potentials (S₂₀) of 2.171, 0.939 and 0.409 mg/g, respectively. The original abundance of OM met the standard of an effective source rock, and the mudstone of the Linxi Formation reached the standard of an effective source rock. The results of saturated hydrocarbon gas chromatography (GC) analysis using the Late Paleozoic source rocks in the Erlian Basin and its adjacent areas revealed obvious “bimodal” characteristics, and the main carbon peaks were distributed near C_17–19 and C_23–25. These results suggested that the sources of OM were mainly low-level aquatic organisms, which are able to generate oil and gas. The obvious differences between the residual and original hydrocarbon generation potentials indicated that the Late Paleozoic source rocks generated and discharged hydrocarbons. Notably, the original hydrocarbon generation potential of the Linxi Formation mudstone is high.