再谈四川盆地上三叠统层序地层划分方案

Re-discussion on the sequence division schemes of Upper Triassic formations in the Sichuan Basin

川西坳陷上三叠统须家河组在与邻区香溪群的对比中,目前流行4种地层划分方案,一直以来,对这几种划分方案都存在着激烈的争论。为此,应用层序地层学观点,利用中石油与中石化两大集团的勘探成果,建立层序地层序列,利用野外露头、古生物、年代地层等资料,结合钻、测井层序地层分析与井震标定,提出须家河组与香溪群对比的最新方案,即:①川西—川中须五段对比香四、香五、香六段,须四段对比香三段,须三段对比香二段;②须二段、小塘子组与马鞍塘组向东逐层上超至无沉积,在坳陷西坡的北、中段,须六段遭剥蚀,至南段须三段侧变为香二段;③须二段、小塘子组与马鞍塘组侧变为海相香一段。分析认为,底界面T52是解决安县运动界面与层序地层方案分歧的关键。在安县运动底界面T52之上地震波由NE往SW前积,之下由SW往NE前积,坳陷中心在彭州鸭子河一带。由西往东须家河组与香溪群为T4与T6两条地震波组控制的楔形体,其内以转换面T52为界分别发育各向东、西变薄的楔形体。最后,论述了湖盆东进与南迁过程中沉积中心迁移、物源水系变迁,以及相带展布与勘探领域转换的证据。

There are 4 popular schemes for stratigraphic division in the sequence-stratigraphic correlation between the Upper Triassic Xujiahe Formation and the adjacent Xiangxi Group in the western Sichuan Depression.However,a strong controversy exists all the time over these schemes.In view of this,a sequence division is established with the aid of the exploration achievements by both Sinopec and PetroChina.Various data such as outcrop,palaeontoloy and chronostratigraphy are used,in combination with drilling data analysis,logging sequence stratiraphic analysis and well-seismic calibration,to propose a new scheme for the sequence-stratigraphic correlation between the Xujiahe Formation and the Xiangxi Group.(1) The 5th member of the Xujiahe Formation in the western-central Sichuan Basin correlates with the 4th,5th and 6th members of the Xiangxi Group,the 4th member of the Xujiahe Formation correlates with the 3rd member of the Xiangxi Group,and the 3rd member of the Xujiahe Formation correlates with the 2nd member of the Xiangxi Group.(2) The 2nd member of the Xujiahe Formation,the Xiaotangzi and Ma'antang formations onlap layer by layer toward east and finally pinchout;The 6th member of the Xujiahe Formation is eroded in the north and central parts of the west slope zone in the depression and the 3rd member of the Xujiahe Formation changes laterally into the 2nd member of the Xiangxi Group in the southern part of the west slope zone in the depression.(3) The 2nd member of the Xujiahe Formation,the Xiaotangzi and Ma'antang formations change laterally into the marine 1st member of the Xiangxi Group.It is believed that the lower boundary T52 is the key to eliminate the discrepancy between the Anxian Movement interface and stratigraphic sequence division scheme.Seismic waves above the lower boundary T52 of the Anxian Movement show progradational reflection configuration in the NE-SW direction,while that below show progradational reflection configuration in the SW-NE direction.The depocenter of the depression occurs in the Yazihe area of Pengzhou.From west to east,the Xujiahe Formation and the Xiangxi Group occur as a wedge controlled by T4 and T6 interfaces,within which a eastward thinning wedge and a westward thinning wedge occur with the conversion surface T52 as the boundary.Finally,this paper discusses the migration of depocenter and provenance during the prograding of the lake basin,the distribution of facies zone,and the change of exploration direction.