晚二叠世盐亭—蓬溪拉张槽东段特征及其对四川盆地中部长兴组油气成藏的控制作用

Characteristics of the east section of the Late Permian Yanting-Pengxi intracratonic sag and its controlling effects on the hydrocarbon accumulation of the Changxing Formation in the middle Sichuan Basin

长期以来,四川盆地中部上二叠统长兴组油气勘探尽管已取得发现,但并未形成大规模产能。现今对川中地区长兴组礁滩体分布特征、晚二叠世"盐亭—蓬溪"拉张槽东段边界特征以及长兴组成藏耦合关系并不清晰。结合最新地震资料,精细刻画出盐亭—蓬溪拉张槽东段最新边界,总结分析四川盆地中部长兴组礁滩体分布规律与油气地质特征。研究表明:①盐亭—蓬溪拉张槽东段南西侧以发育条带状生屑礁滩复合体为主要特征,定名为磨溪生屑滩高带。北东侧以发育透镜状生物礁为主要特征,定名为广安生物礁高带。②盐亭—蓬溪拉张槽对川中地区长兴组厚度及礁滩体分布具有控制作用。拉张槽边界两侧长兴组厚度具有明显差异,礁滩体主要发育在拉张槽两侧长兴组中上部地层中。③盐亭—蓬溪拉张槽对川中地区上二叠统—下三叠统烃源岩分布具有控制作用,拉张槽内部长兴组厚度减薄,而飞仙关组一段厚度明显增大,具备较优质烃源岩发育条件。④四川盆地中部是以下寒武统为主的多源供烃模式,具有垂侧向复合输导系统,长兴组为次生古油藏—原生气藏/次生气藏模式。综上认为,长兴组油气地质特征相较下古生界更为复杂,四川盆地中部长兴组古油藏规模有限,气源不足是现今未大规模成藏的主要原因。烃源岩/优势气源区—输导系统—优质礁滩体储层耦合是寻找长兴组油气勘探"甜点"区的关键。

For a long time,although the Upper Permian was discovered in the Changxing Formation,no large-scale production capacity has been formed.The reef-shoal distribution characteristic of the Changxing Formation,the border of the Late Permian Yanting-Pengxi intracratonic sag and the hydrocarbon accumulation relationship are still not clear.With the latest seismic data,this paper describes the latest boundary of the eastern section of the Yanting-Pengxi intracratonic sag in detail,and summarizes the distribution of reef-shoal body and the geological characteristic of hydrocarbon for the Changxing Formation in the central Sichuan Basin.The results show that:(1)The southwestern side of the Yanting-Pengxi intracratonic sag is characterized by the development of the strip-like complex reef-shoal,which is named as the High Belt of the Moxi Shoal Platform Margin.The northeast side mainly developed the tundra-like reef as the main feature,it is named as the High Belt of the Guang’an Reef Platform Margin.(2)The Yanting-Pengxi sag has a controlling effect on the thickness of the Changxing Formation and the distribution of reef-shoal in the central Sichuan Basin.There are significant thickness differences of the Changxing Formation on the both sides of the intracratonic sag boundary.The reef-shoal are mainly developed in the middle and upper strata of the Changxing Formation.(3)The Yanting-Pengxi Intracratonic sag has a controlling effect on the source rock distribution of the Upper Permianto the Triassic in the central Sichuan Basin.The thickness of the Changxing Formation in the intracratonic sag reduced,while the thickness of the Feixianguan Formation obviously increased,which have the development conditions of high quality source rocks.(4)With multisource hydrocarbon supply model of the Lower Cambrian black shale leading and the vertical-lateral seal bypassing system,the Changxing Formation in the Sichuan Basin is secondary paleo-oil reservoir-original gas reservoir/secondary gas reservoir models.In summary,the geological characteristics of the Changxing Formation in the central Sichuan Basin is more complex than those in the Lower Paleozoic.The paleo-oil reservoirs of the Changxing Formation are currently not a large scale,so that the lack of gas source is the main reason for the lack of large-scale reservoir accumulation.The coupling relationship of good source rocks/gas source areas-seal bypassing system-reef-shoal reservoirs is the key point for'dessert'zone exploration of the Changxing Formation.