139 / 2025-04-06 17:10:47

Relationship between methane prokaryotes and sedimentary organic matter formation in black shales

Late Paleozoic,High-quality hydrocarbon source rocks,Lucaogou Formation,Junggar Basin,Methane prokaryotes

Theme 4: Microbial and Biogeochemical Evolution through Time > Session 4a: Control of Microbial and Biogeochemical Cycling on Hydrocarbon Formation and Critical Metal Mineralization

The Junggar Basin, a large superimposed petroliferous basin in northwestern China, hosts high-quality hydrocarbon source rocks within the Late Paleozoic Lucaogou Formation (~290 Ma). These saline lacustrine deposits exhibit exceptionally high total organic carbon (TOC) contents (up to ~40%) and provide compelling evidence of intense microbial methane cycling, including putative methanogenic archaea fossils, carbonates with δ¹³C values exceeding +5‰, and regular hopanes with δ¹³C as low as −63‰ (Sun et al., 2022). These features make the Lucaogou Formation an ideal case study for investigating prokaryotic influences on SOM formation. Our investigation shows that methane prokaryotes co-evolved with environmental and biological variations. Mudstone intervals with the strongest methane microbial activities (3β-MeHI = 10.2±1.2 %) correspond to a stratified water column, relative to the relatively tight intervals (3β-MeHI = 6.0±0.8 %). These strata also exhibit Tasmanite bloom and contain abundant carbonaceous microfossils resembling methanogenic archaea within fossilized Tasmanite cysts. Notably, microbial methane activities showed no direct and remarkable correlation with bulk TOC abundance but was associated with enhanced hydrocarbon potential (S₁ + S₂ = 47.4 ± 37.6 mg HC/g rock), oil-prone kerogen (HI = 644 ± 191 mg HC/g TOC; Type I–II), and higher hydrocarbon-generating capacity (PG/TOC = 724 ± 226 mg HC/g TOC), indicating a significant contribution to the oil-generating organic matter. The mechanism is proposed that methanogens improve the kerogen type of the hydrocarbon source rock by modifying the degradation-resistant organic matter and forming less branched high-abundance aliphatic hydrocarbons, which are more oil-prone. We conclude that prokaryotic enhancement of SOM formation likely depends on depositional conditions; stratified water columns—with oxic surface layers and anoxic bottom waters—appear particularly favorable for this process and mechanism.