Two typical accumulation mechanismsof marine natural gas reservoirs in China: Case comparison between Tarim Basin and Sichuan Basin

Marine basin contains abundant natural gas resources in China, especiallythe Palaeozoic is becoming a hotspot of natural gas exploration, wherea series of large gas fields have been constantly discovered. It isbroad consensusthathigh-post mature source rocks (SRs)is gas-prone and thermal sulphatereduction (TSR) could accelerate the hydrocarbon cracking, which would be the dominant origin of gas enriched in the Palaeozoic of petroliferous Basin in China. In addition, these cracking gas is rich in H₂S and CO₂, due to the marine source rocksare rich inargillaceous sedimentary and sulfates. Nevertheless, the accumulations of gas reservoirs were obviously different between two giant marine craton in China. Sichuan basin is abundance of marine cracking gas, which was mainly generated around 257-205Ma. But it is rarely found marine oil reservoirs in Sichuan basin. In contrast, abundance of marine condensates are rich in Tarim Basin. Gas washing and migration fractionation effect would be the major formation mechanism of secondary condensates, which coexist with dry gas and normal oil reservoirs. These various reservoirs orderly distribute around source rocks in the Manjia'er sag of Tarim Basin. The origin of marine gases was mainly mixed from the hydrocarbon cracking derived via TSR and the different stages of thermal evolution of marine SRs, as well as portion of humic gas. Therefore, the carbon isotopic composition of diverse gas reservoirs is quite complex and the phenomenon of isotopic reversal δ¹³C₁>δ¹³C₂is pervasive. Eventually, two typical accumulation mechanismsof marine natural gas reservoirshave been established. It could be inferred the exploration of cracking gas is given priority in SichuanBasin and marine secondary condensatesshould be paid more attention during the petroleum exploration in Tarim Basin. (paper)