四川盆地下寒武统筇竹寺组烃源岩有机质差异富集机制与天然气勘探领域

Differential mechanisms of organic matter accumulation of source rocks in the Lower Cambrian Qiongzhusi Formation and implications for gas exploration fields in Sichuan Basin

四川盆地以下寒武统筇竹寺组为烃源岩的海相高演化天然气系统是全球这一领域的一个典范,为进一步深化地质理论认识,拓展勘探领域,针对当前川中、川东北2个热点勘探区块,通过地球化学方法,从古生产力、氧化还原条件、陆源输入三方面入手,对不同区域该套烃源岩质量及其有机质富集机制展开了差异对比研究,探讨了天然气勘探领域。结果表明:川中GS17井、ZJ2井所在的裂陷槽内深水陆棚相烃源岩有机碳含量整体高于川东北小洋坝浅水陆棚相烃源岩,两者有机质富集均主要受控于氧化还原环境;川中烃源岩形成于缺氧环境,相较于川东北烃源岩所形成的贫氧环境更有利于有机质的保存;川东北烃源岩形成过程中陆源输入对有机质的稀释作用尤为显著,烃源岩质量不如川中烃源岩。筇竹寺组天然气系统的勘探仍需围绕裂陷槽及周缘展开,同时盆地周缘的深水陆棚地区也值得加以重视,且常规与非常规天然气资源并重。

The marine facies highly mature natural gas system with the Lower Cambrian Qiongzhusi Formation as the source rock in the Sichuan Basin is a global model in this field.To deepen the geological understanding of high quality source rocks and the natural gas exploration areas in the Sichuan Basin,taking two hot exploration blocks in the central and northeastern parts of Sichuan Basin as examples,the differential mechanisms of organic matter accumulation in source rocks of the Qiongzhusi Formation were studied,and some gas exploration areas were identified by the means of geochemical methods from three perspectives,including paleoproductivity,redox conditions and input of terristral organic matters.The TOC of source rocks within deep-water shelf in the intracratonic rift sampled from the Gaoshi 17 and Zhongjiang 2 wells in the central Sichuan were higher than that of source rocks within the shallow-water shelf sampled from the Xiaoyangba outcrop,northeastern Sichuan.The organic matter accumulations of both sample sets were mainly controlled by redox condition,but in comparison,the source rocks in the central Sichuan Basin were formed in relative anoxic environment,which was more conducive for the preservation of organic matter than the suboxic environment in northeastern Sichuan Basin.In addition,the dilution effection by terrestrial organic matter input was particularly significant during the formation of source rocks in the northeastern Sichuan Basin,leading to poorer source rock quality.The exploration of natural gas system in the Qiongzhusi Formation still needs to be carried out closely around the intracratonic rift and its surrounding margin in the future,and the deep-water shelf area around the basin also deserves attention.Both conventional and unconventional natural gas resources should be considered.