四川盆地中泥盆统和中二叠统天然气地球化学特征及成因

中泥盆统和中二叠统是近年四川盆地天然气勘探获得重大突破的领域,但其天然气来源尚不明确,影响下一步勘探部署决策。开展了中泥盆统和中二叠统天然气地球化学、储层沥青及烃源岩生物标志物等特征的综合研究。结果表明:①中泥盆统和中二叠统天然气均属于二次裂解的干气,甲烷含量>86%,含少量乙烷、丙烷等烃类气体及少量的氮气、二氧化碳、硫化氢等非烃气体,不同区域的天然气成熟度存在差别;②天然气的δ13C1值为-35.7‰^-27.3‰、δ13C2值为-38.7‰^-26.6‰、δ13C3值为-37‰^-26.5‰、δ2HCH4值为-141‰^-125‰、δ2HC2H6值为-164‰^-112‰,为腐泥型气和以腐泥型为主的混合型气;③不同区域天然气δ13C1值、δ13C2值和δ2HCH4值的差异,与其来源于不同时代烃源岩贡献比例大小有关。川西北地区双鱼石构造中泥盆统和中二叠统、川西南部地区中二叠统、川中古隆起中二叠统天然气主要源于寒武系筇竹寺组和中下二叠统烃源岩,川西南部中上二叠统火山岩天然气主要源于筇竹寺组烃源岩,川西北地区河湾场构造、川东地区和川南地区中二叠统天然气主要源于志留系龙马溪组烃源岩。研究成果对四川盆地中泥盆统和中二叠统下步天然气勘探部署决策具有重要的指导意义。

Geochemical characteristics and genesis of Middle Devonian and Middle Permian natural gas in Sichuan Basin,China

Major breakthroughs have been made for natural gas exploration of Middle Devonian and Middle Permian formations in Sichuan Basin in recent years,but its natural gas source is still not clear,affecting the next exploration deployment decision. A comprehensive study is conducted on the geochemical characteristics of Middle Devonian and Middle Permian natural gas,the biomarkers of reservoir bitumen and source rocks. The results show that these natural gases are secondary cracking dry gas,including more than 86% of methane content,a small amount of ethane,propane and non-hydrocarbon gas such as nitrogen,carbon dioxide and hydrogen sulfide. The natural gas maturity varies slightly in different regions. The values of natural gas δ13C1=-35.7‰ to-27.3‰,δ13C2=-38.7‰ to-26.6‰,δ13C3=-37‰ to-26.5‰,and δ2HCH4=-141‰ to-138‰,δ2HC2H6=-164‰ to-112‰,indicate the genetic type of natural gas is the sapropelic type and the mixture type dominated by sapropelic type. The difference of natural gas values of δ13C1,δ13C2 and δ2HCH4 in different regions is related to the contribution ratio of source rocks from different ages. The gases of Middle Devonian and Middle Permian of Shuangyushi Structure in Northwest Sichuan,Middle Permian of Southwest Sichuan and Middle Permian of Paleo-uplift in Central Sichuan are mainly derived from Cambrian Qiongzhusi Formation and Middle-Lower Permian source rocks. Middle-Upper Permian natural gases of volcanic reservoir in Southwest Sichuan is mainly from Qiongzhusi source rock. Middle Permian natural gases of Hewanchang structure in Northwest Sichuan,Eastern and Southern Sichuan are mainly derived from Silurian Longmaxi source rock. The research results have important guiding significance for developing exploration and deployment solutions for natural gas of Middle Devonian and Middle Permian formation in Sichuan Basin.