多级坡折带对颗粒滩储层的控制及有利勘探区带——以四川盆地乐山—龙女寺古隆起南斜坡寒武系洗象池组为例

Control of hierarchic slope-break zone on grain shoal reservoirs and favorable exploration zones:A case study of Cambrian Xixiangchi Formation in the south slope of Leshan-Longnüsi paleouplift,Sichuan Basin,SW China

基于二维、三维地震和测录井资料,开展四川盆地乐山—龙女寺古隆起南斜坡中上寒武统洗象池组井-震地层充填及滩控储层分布规律研究,以揭示地层充填、古地貌与规模颗粒滩的成因联系。结果表明:①研究区洗象池组渐次向乐山—龙女寺古隆起超覆充填,仅在古隆起剥蚀尖灭线附近,地层因削顶而急剧减薄,因此识别出2个超覆坡折带和1个削蚀坡折带,并将区内洗象池组由下至上划分为4段;②底超顶削的地层充填模式表明,洗象池组厚度可反映其沉积前古地貌,揭示了研究区具有向东南倾伏的、多级坡折控制的单斜式地貌特征;③规模颗粒滩及滩控储层纵向上主要发育于洗三段和洗四段中,横向上主要发育于多级超覆坡折带附近;④超覆坡折带与北西倾向的逆断层密切相关,中晚寒武世华南板块西缘与羌塘—喜马拉雅地块聚合,导致扬子板块西北缘快速抬升,并向东南扩展,形成逐渐倾伏的多级坡折地貌,进而形成多条北东向展布的规模颗粒滩储层发育带。结合油气测试结果,预测滩控储层有利勘探区带面积可达3340 km^(2)。

Based on 2D and 3D seismic data and well logging data,this paper studies the distribution of well-seismic stratigraphic filling and shoal controlled reservoirs of Upper Cambrian Xixiangchi Formation in the south slope of Leshan-Longnüsi paleouplift in the Sichuan Basin,to reveal the genetic relationship between stratigraphic filling,paleogeomorphology and large-scale grain shoal.(1)The Xixiangchi Formation in the study area is overlapped and filled gradually to the Leshan-Longnüsi paleouplift,but gets thin sharply due to truncation only near the denudation pinch-out line of the paleouplift.Therefore,two overlap slope break belts and one erosion slope break belt are identified,and the Xixiangchi Formation is divided into 4 members from bottom to top.(2)The filling pattern of the overlapping at the base and erosion at the top indicates that the thickness of Xixiangchi Formation can reflect the pre-depositional paleogeomorphology,and reveals that the studied area has a monoclinal geomorphic feature of plunging to southeast and being controlled by multistage slope break belts.(3)The large-scale grain shoals and shoal controlled reservoirs are developed longitudinally in the third and fourth members of the Xixiangchi Formation,and laterally in the vicinity of the multistage overlap slope break belts.(4)Overlap slope break belts are closely related to northwest trending reverse faults.The convergence of the western margin of the South China Plate with the Qiangtang-Himalaya massif in the middle–late Cambrian led to the rapid uplift of the northwestern margin of the Yangtze Plate and the expansion to the southeast,forming a gradually plunging multistage slope break paleogeomorphology and subsequently several northeast-trending belts of large-scale grain shoal reservoirs.Combined with oil and gas test results,it is predicted that the favorable exploration zone of the grain shoal controlled reservoirs covers an area of 3340 km^(2).