下扬子逆冲推覆构造后缘凹陷油气富集关键要素及有利勘探方向

Key factors and favorable exploration directions for oil and gas enrichment in back margin sag of thrust nappe in Lower Yangtze

通过开展系统的构造特征、沉积环境、油气富集规律分析,在钻探新发现的新资料基础上,对下扬子北缘逆冲推覆构造后缘凹陷油气富集规律和有利勘探方向进行了研究。研究结果表明:(1)下扬子北缘逆冲推覆构造后缘凹陷中油气显示表现为“三气两油”特征,即二叠系大隆组和孤峰组页岩气、龙潭组煤层气和致密砂岩气、大隆组页岩油和三叠系殷坑组灰岩裂隙油。(2)以宣城凹陷为代表的下扬子北缘逆冲推覆构造后缘凹陷带油气富集受3个关键要素控制,即后缘凹陷块体稳定,已形成的油气体系具有较好的保存环境;该地区二叠系大隆组(P3d)和孤峰组(P2g)沉积环境属深水陆棚环境,是有利的沉积相带,发育2套富有机质的烃源岩层系;顶、底板有效封存条件均较好,烃源岩上部有三叠系泥灰岩地层覆盖,下部有栖霞组生物碎屑灰岩为垫层,顶、底地层控制了纵向油气的散逸。(3)研究区下三叠统—上二叠统自上而下依次富集泥灰岩裂隙油、页岩油、页岩气、煤层气、致密砂岩气,整体呈“上油下气”多种非常规油气同生共存的特征,形成了特殊的油气富集模式。(4)宣城凹陷与南陵凹陷、句容凹陷和常州凹陷具有相似的地质条件,是下一步页岩油气勘探的首选有利区。

Through systematic analysis of structural characteristics,sedimentary environment and hydrocarbon enrichment law,on the basis of newly discovered drilling data,the oil and gas enrichment law and favorable exploration direction of the back margin sag of thrust nappe in northern margin of Lower Yangtze were studied.The result shows that:(1)The oil and gas in the sag is characterized by“three gas and two oil”,namely,shale gas of Permian Dalong Formation and Gufeng Formation,coalbed methane and tight sandstone gas of Longtan Formation,shale oil of Dalong Formation and limestone fracture oil of Triassic Yinkeng Formation.(2)The oil and gas enrichment in the sag,represented by Xuancheng Sag,is controlled by three key factors:The sag is a stable block and the formed oil and gas system has a good preservation environment;The sedimentary environment of Dalong Formation(P3d)and Gufeng Formation(P2g)in this area is a deep-water shelf environment,which is a favorable sedimentary facies zone,with two sets of organic matter rich source rock series developed;The effective sealing conditions of the top and bottom are good,with the upper part of the source rocks covered byTriassic marl formation,and the lower part being of bioclastic limestone of Qixia Formation,controlling the longitudinal oil and gas escape.(3)The Lower Triassic to Upper Permian strata in the study area are enriched in marl fracture oil,shale oil,shale gas,coalbed methane and tight sandstone gas from top to bottom,showing the characteristics of“upper oil and lower gas”multiple unconventional oil and gas coexisting together,forming a special oil and gas enrichment pattern.(4)Xuancheng sag has similar geological conditions to Nanling sag,Jurong sag and Changzhou sag,making it the preferred favorable area for next shale oil and gas exploration.