川东北地区二叠系—三叠系多期台缘演化规律及勘探方向

Evolution and exploration direction of Permian-Triassic multiphase platform margin in northeast Sichuan Basin

基于等时层序格架,综合钻井和地球物理资料,对四川盆地开江—梁平海槽二叠系—三叠系多期台缘的演化规律进行了研究,探讨了不同时期的台缘类型、礁滩发育模式及展布特征。研究结果表明:(1)基于龙潭组和吴家坪组同时异相的沉积特征,以最大海泛面和地层旋回一致性为依据,对川中地区和开江—梁平地区的地层界线进行调整,可建立区域等时地层格架。(2)基于对龙岗地区茅口组—长兴组台缘带的重新刻画,证实开江—梁平海槽在茅口组二段沉积期出现台缘雏形,在吴家坪组沉积期持续发育,长兴组沉积期礁体规模最大,最后在飞仙关组沉积期逐渐消亡;台缘带的演化经历了茅口组沉积期的单斜缓坡模式、吴家坪组沉积期的远端变陡缓坡模式、长兴组沉积期的镶边碳酸盐台地模式。(3)开江—梁平海槽东、西两侧均有多期台缘叠置的发育特征,海槽西侧发育茅口组、吴家坪组、长兴组3期台缘,海槽东侧主要发育吴家坪组、长兴组2期台缘;根据生物礁的地震反射特征和不同时期生物礁的垂向叠置模式,识别出加积型生物礁和前积型生物礁,并预测了不同类型生物礁的分布规律。(4)四川盆地在二叠纪—三叠纪形成的多个NW向裂陷均具备形成多期台缘的构造背景,多期台缘叠加的生物礁滩储层是今后四川盆地的一个重要勘探领域。

Based on the isochronous sequence framework,drilling and geophysical data,this paper investigates the evolution of the Permian-Triassic multiphase platform margin in Kaijiang-Liangping trough of Sichuan Basin,and also explores the platform margin types,reef-shoal development patterns and distribution characteristics in different periods.The results show as follows.(1)Based on the sedimentary characteristics of contemporaneous heterotopic facies between Longtan Formation and Wujiaping Formation and the consistency of the maximum flooding surface and stratigraphic cycle,the regional isochronous stratigraphic framework can be established by adjusting the stratigraphic boundaries in central Sichuan Basin and Kaijiang-Liangping area.(2)Based on re-describing the platform margin belt of Maokou Formation and Changxing Formation in Longgang area,it was confirmed that the platform margin prototype appeared at the Kaijiang-Liangping trough during the sedimentary period of Member 2 of Maokou Formation,continued to develop during the sedimentary period of Wujiaping Formation,reached the maximum reef size during the sedimentary period of Changxing Formation,and finally gradually died out during the sedimentary period of Feixianguan Formation.The evolution of the platform margin belt has experienced three development patterns,i.e.,the gentle-dipping monoclinal slope in the sedimentary period of Maokou Formation,the gentle slope with steepening distal end in the sedimentary period of Wujiaping Formation,and the rimmed carbonate platform in the sedimentary period of Changxing Formation.(3)The east and west sides of Kaijiang-Liangping trough have the development characteristics of multiphase platform margin superposition.Three stages of platform margins of Maokou Formation,Wujiaping Formation and Changxing Formation were developed on the west side of the trough,and two stage of platform margins of Wujiaping Formation and Changxing Formation were mainly developed on the east side of the trough.According to the seismic reflection characteristics of reefs and the vertical superposition patterns of reefs in different periods,aggradational and progradational reefs were identified,and the distribution of different reef types were predicted.(4)The NW-trending rifts formed in the Permian-Triassic in Sichuan Basin have the tectonic setting of forming multiphase platform margins.The reef-shoal reservoirs superposed on multiphase platform margins will be an important exploration field in Sichuan Basin in the future.