四川盆地西部中二叠统栖霞组天然气成藏特征及主控因素

Characteristics and main controlling factors of gas accumulation of the Middle Permian Qixia Formation in western Sichuan Basin

近年来,四川盆地西部(简称川西地区)中二叠统栖霞组油气勘探取得重大突破,展现出四川盆地栖霞组巨大的勘探潜力。为理清川西地区栖霞组天然气成藏期次、成藏演化、成藏主控因素和成藏特征等关键地质问题,为栖霞组下一步的勘探提供支撑,通过分析研究大量钻井、测井及实验分析测试等资料认为:(1)川西地区中二叠统栖霞组天然气甲烷含量一般大于96%,天然气干燥系数大于0.99,呈高演化的特征,是典型干气,主要为原油二次裂解气。(2)栖霞组天然气δ^(13)C_(1)介于-31.33‰~-28.31‰,δ^(13)C_(2)介于-28.50‰~-25.20‰。气气对比认为川西地区栖霞组天然气不存在志留系龙马溪组烃源岩的贡献,是以寒武系筇竹寺组烃源输入为主,中二叠统烃源输入为辅的混合天然气。(3)通过对所识别出流体包裹体的均一温度分析,识别出4期油气成藏事件。第Ⅰ期和第Ⅱ期分别为中—晚三叠世和晚三叠世的古油藏成藏时期;第Ⅲ期为早—中侏罗世古油气藏成藏时期;第Ⅳ期为晚侏罗世—早白垩世古气藏成藏时期。(4)成藏主控因素为优越的烃源岩条件、优质的储层发育及可供油气持续聚集成藏的构造—岩性圈闭。(5)川西北双鱼石—中坝地区推覆体下盘隐伏前缘带及川西南平落坝—大兴场地区具备形成大气区的优越成藏条件组合,是下一步川西地区栖霞组勘探的最有利区带。

In recent years,great breakthroughs have been made of the Middle Permian Qixia Formation in western Sichuan Basin,showing great exploration potential.A large number of well drilling,logging,and lab test data are studied to determine the key geological difficulties,such as the stages,evolution,main controlling factors and characteristics of gas accumulation of Qixia Formation in western Sichuan Basin,so as to provide technical support for the exploration in the near future.The study results show that:(1)The methane content and the gas drying coefficient of natural gas of Qixia Formation are more than 96%and 0.99 respectively,showing characteristics of high maturity,which is a typical dry gas and pyrolysis gas generated by the secondary cracking of crude oil;(2)The gas isotope ofδ^(13)C_(1)ranges from-31.33‰to-28.31‰,while that ofδ^(13)C_(2)from-28.50‰to-25.20‰.The gas-source correlation indicates that the natural gas of Qixia Formation is mainly a mixed type gas reservoir generated by source rock of the Cambrian Qiongzhusi Formation and supplemented by the Middle Permian,but no contribution from source rock of the Silurian Longmaxi Formation;(3)The identified fluid inclusions recorded four stages of hydrocarbon accumulation events.The first and second stages were paleo oil reservoir accumulation in the Middle-Late Triassic and the Late Triassic respectively;The third stage was paleo oil reservoir accumulation in the Early-Middle Jurassic;The fourth stage was paleo gas reservoir accumulation from the Late Jurassic to the Early Cretaceous;(4)The main controlling factors of hydrocarbon accumulation include the large-scale high-quality source rocks,high-quality reservoirs and structural-lithologic traps that supply accommodation for continuous gas accumulation;(5)The concealed front zone of the footwall of the nappe in Shuangyushi-Zhongba area in northwestern Sichuan Basin and Pingluoba-Daxingchang area in southwestern Sichuan Basin have superior conditions for gas accumulation,which are the most favorable areas for further exploration of Qixia Formation in western Sichuan Basin.