Constraining multi-stage and protracted oil generation of alkaline lacustrine source rocks by heteroatomic compounds

Abstract

It is unclear whether source rocks deposited in ancient alkaline lakes undergo multiple stages of hydrocarbon generation, particularly of oils. In this study, the lower Permian Fengcheng Formation in the Junggar Basin, China, was investigated using the geochemistry of heteroatomic species, with a focus on N-containing compounds, by negative-ion electrospray high-resolution Fourier transform ion cyclotron resonance mass spectrometry. The maturity parameters based on N-containing compounds (e.g., C_0–5/C_15–35-DBE 12 N₁ and C_0–5/C_15–35-DBE 15 N₁) and aromatic hydrocarbons (TMNr) reveal progressive generation of mature to highly mature crude oils. The polymerization index P1 ([DBE 18 + DBE 15]/DBE 12_N₁), alkylation index R1 (RC₆₋₃₅/RC₀₋₅), and paleo-salinity index (β carotane/nC_max) of the Fengcheng Formation source rocks suggest that the high salinity inhibited molecular polymerization of organic matter and prolonged the oil generation peak, which produced highly mature crude oils in the late stages of hydrocarbon generation. Therefore, alkaline lacustrine source rocks undergo multiple stages of protracted oil generation, which results in a longer oil window and higher threshold for gas generation than in the case of the classic Tissot model. The results highlight the oil generation potential of deep basins containing alkaline lacustrine source rocks and highly mature organic matter, which also have a lower gas-generation potential than previously thought.