四川盆地开江-梁平海槽周缘晚二叠世长兴期台缘礁滩沉积结构及其差异性

Sedimentary architectures of reef-shoal facies at the platform margin during Changxing times of the Late Permian around Kaijiang-Liangping trough in the Sichuan Basin and their differences

四川盆地开江-梁平海槽周缘长兴组生物礁滩型气藏是川东地区天然气勘探开发的主要对象,目前对海槽周缘台缘生物礁滩沉积结构及其差异性研究较少。综合岩心、薄片、测井和地震等资料,在海槽周缘地震地层格架对比基础上,分析海槽周缘台地前缘坡度角变化、生物礁滩岩性组成、成礁旋回、外部轮廓、垂向叠加样式、横向迁移、空间展布特征及其差异性。结果表明:①开江-梁平海槽形成于晚二叠世断陷期,受近北西走向的基底正断层活动控制,在海槽周缘发育近U形展布的台缘带,台缘古构造格局具有“西北陡峭水深、东南宽缓水浅”的特征。②海槽周缘台缘沉积结构包含4种类型,分别为断控陡坡-加积镶边结构、沉积陡坡-加积镶边结构、缓坡坡折-退积条带状结构以及极缓坡-迁移离散状结构。不同类型台缘结构间互式分布在海槽周缘,西侧和北侧台缘沉积结构以断控或沉积陡坡-垂向叠置型为主,东侧和南侧变为缓坡-退积迁移型。③成因机制上,构造古地貌和海平面变化联合控制了研究区的台缘沉积结构及其差异性。构造古地貌控制台缘带的构成样式及生物礁的发育空间,而海平面的旋回变化控制台缘生物礁滩沉积结构和迁移叠置。

The reef-shoal gas reservoirs of Changxing Formation around the Kaijiang-Liangping trough in the Sichuan Basin are the major targets of natural gas exploration and development in eastern Sichuan Basin.However,few studies have been seen on the sedimentary architectures of reef-shoal around platform margin and their differences as of now.An integrated analysis of cores,thin sections,well logs,and seismic data,is applied to disclose the changes of slope angle at the front,lithologic composition of organic reef-shaol facies,reef cycle,external contour,vertical superimposition pattern,lateral migration,spatial distribution characteristics and differences around the trough,coupled with the seismic stratigraphic framework around the trough.The results are as follows.First,the Kaijiang-Liangping trough formed during the Late Permian rifting,when the NW-trending tensional basement-rooted faults were active,has nearly U-shaped platform marginal zones developed around.The palaeotectonic pattern of platform margin is characterized by“deep water in the steep northwest and shallow water in the wide and gentle southeast”.Second,the sedimentary architectures of platform margins around can be classified into four types,that is,the fault-controlled steep slope type with accretion rim,sedimentary steep slope type with accretion rim,gentle slope break type with retrograding strip,and extremely gentle slope type with discrete migration.These architectures are in alternative distribution around the trough,the western and northern platform margin dominated by fault-controlled or vertical superimposed steep slope type and the eastern and southern by extremely gentle slope type with discrete migration.Third,tectonic palaeogeomorphology and sea level eustacy jointly control the formation of sedimentary architectures of the platform margin in the study area and their differences.Among others,the former controls the pattern of the platform margin and accommodation of organic reef,and the latter controls the sedimentary architecture,migration and superposition of reef-shoal.