GEOGRAPHY

Revealing that methane emissions were an important factor in the late Permian mass extinction


The team of Wang Yunpeng, a researcher at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, collaborated with Zhou Zheng’s team, a researcher at Lancaster University in the United Kingdom, Dr. Qin Shengfei from the PetroChina Institute of Exploration and Development, and Dr. Greg Holland from the University of Manchester, and found that high-temperature methane emissions from large igneous provinces that triggered the cracking of ancient oil reservoirs may be an important factor in the mass extinction of the Late Permian. The study was published online Nov. 12 in Nature Communications. Chen Chengsheng, a postdoctoral fellow at Guangzhou Institute of Geochemistry, is the first author of the paper, and Wang Yunpeng and Zhou Zheng are the corresponding authors.

The Late Permian Mass Extinctions (LPME) are considered the worst biosphere crisis in geological history. Two separate extinction events, the Guadalupian-Lopingian Extinction (GLE) and the Permian-Triassic Extinction (PTE), caused the extinction of more than 90% of species on Earth. These two extinction events are currently thought to be closely related to intrusions, eruptions and large-scale greenhouse gas emissions in the Emeishan and Siberian igneous provinces.

Although the above views have been widely accepted, the sources and emission mechanisms of greenhouse gases related to them are not fully understood, which is one of the current international research hotspots. Solid bitumen, as the final product of cracking and methane formation in ancient oil reservoirs, is widely distributed in the upper Yangtze region (such as the Sichuan Basin) and the Siberian region (such as the Tunguska Basin) affected by the large igneous province. Therefore, the high-temperature methane emissions from the cracking of ancient oil reservoirs in large igneous provinces may be an important cause of global climate change and mass extinction induced in geological history. However, there is still insufficient evidence to support this view, and its universality has been questioned.

In response to this problem, the researchers used methane cluster isotope (monodeuterium) and noble gas isotope methods to systematically study the Dengying Formation-Longwangmiao Formation (Z2dn-1l) Natural gas genesis, and the impact of high-temperature methane emissions on the late Permian mass extinction caused by the cracking of ancient oil reservoirs triggered by large igneous provinces was discussed.

The researchers found that the isotope temperature of natural gas methane clusters in Anyue gas field (average 249-17/+19~256-20/+22°C, the highest recorded temperature can reach 269-20/+22°C) is significantly higher than the current reservoir temperature (140~165°C), the peak temperature of crude oil cracking (160~180°C) and the reservoir simulation temperature (200~220°C) at the maximum burial depth in the Late Cretaceous. Chemical kinetic calculations based on thermal simulation experiments show that the temperature conditions required to form extremely dry natural gas (C1/ΣC2-5= 583~3019) from Anyue gas field by crude oil cracking exceed 250°C, which is also consistent with the isotope temperature of methane clusters. The above results show that the cracking of ancient reservoirs and the formation of high-temperature methane are affected by abnormal thermal events in addition to normal burial.

Noble gas isotopic characteristics indicate that the Dragon King Temple Formation (1l) Almost all of the helium (He) in natural gas comes from the contribution of the earth’s crust (>99.7%), while there is a significant contribution of mantle source He (4.80~38.54%) in the natural gas of the lamp shadow group (Z2dn), indicating that the light shadow group is obviously directly affected by the hydrothermal fluid of the mantle source. At the same time, the discrete distribution characteristics of radiogenic argon (Ar*) in the natural gas of the Dengying Formation also reflected the mixing of rare gases from shell mantle sources in different proportions in the gas reservoir, while Ar* and He in the natural gas of the Longwangmiao Formation both showed a single crustal source. This result indicates that the difference in hydrothermal activity intensity and formation configuration caused the difference of noble gas isotopes in the two sets of reservoirs, that is, the Lamp Shadow Formation was closer to the substrate as a lower formation and the Wuzhusi Formation (1q) Thick layers of mud shale barrier, so may have experienced more direct and intense hydrothermal influences. The above results show that the Sichuan Basin has suffered directly from the “hot spot” event, namely the mantle pillar-large igneous province of the Emei Mountains.

According to the yield relationship between crude oil cracker methane and solid bitumen (1.09 L/g) and the average abundance of solid bitumen in the reservoir (about 1.0 wt%), the research team further estimated that the emissions of high-temperature cracking methane induced by Emeishan igneous rock province can reach 1.44 trillion tons, and the greenhouse effect is equivalent to 40.41 trillion tons of CO2 emissions, which is more than twice the CO2 release from Emeishan igneous rock province, which is enough to trigger drastic global climate change and biological extinction. Finally, the team notes that similar methane emissions could also occur in areas affected by the Great Igneous Province of Siberia (STLIP) (estimated to have methane emissions of more than 10 trillion tons), which in turn played an important role in the end-Permian mass extinction.

This study gives key evidence of large-scale cracking of ancient reservoirs in central Sichuan in Emeishan Igneous Province (ELIP), estimates methane emissions from ancient reservoir cracking in Emeishan and Siberian Great Igneous Provinces, and points out that high-temperature methane emissions from large igneous provinces that trigger ancient reservoir cracking may be an important factor in the late Permian mass extinction. In addition, the research results have important theoretical significance for re-understanding the influence of Emeishan Igneous Province on the oil and gas bearing system in the Sichuan Basin, including the special thermal system, oil and gas genesis, oil and gas type, oil and gas phase and resource distribution of the basin.

The above research work was jointly funded by the Strategic Leading Science and Technology Project (Category A) of the Chinese Academy of Sciences, the National Natural Environment Research Council of the United Kingdom, the National Natural Science Foundation of China and the Guangdong Provincial Foundation. In addition, Dr. Chen Chengsheng was funded by the China Scholarship Council (CSC) during the joint training period. (Source: China Science News, Zhu Hanbin, Deng Turian)

Related paper information:https://doi.org/10.1038/s41467-022-34645-3



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