“塔里木盆地油气资源”攻关项目检查评估会在涿州召开

Lateral Accretion Cross-bedding in the Ahe Formation of Lower Jurassic at Kuche Depression,Tarim Basin

塔里木盆地库车拗陷下侏罗统阿合组主要由砂岩和含砾砂岩组成,夹少量粉砂岩及泥岩。属辫状三角洲水下分流河道沉积。其中普遍发育一种大型交错层,其层系厚一般为0.5～2m,常见相邻层系呈反向倾斜,在纵向上倾向反复交替变换,对7条剖面详细观察表明,这两组倾斜方向均与区域古流向近于垂直。经研究,这种交错层是水道侧向迁移而引起的砂体侧向加积所形成的。其特点是:①层系的整体形态呈板状,而细层呈“S”形弯曲,向上、向下明显收敛:②侧积交错层的某些细层内部可见次一级交错层或交错纹理以及滑塌构造:③层系厚度与沉积物粒度无关:④同一层系内各细层的粒度和成分有较大差异。侧积交错层出现的沉积相包括曲流河、辫状河、三角洲,辫状三角洲及扇三角洲,但发育程度各有不同,以辫状三角洲水下分流河道最为发育。

Mainly consisting of sandstones and pebbled sandstones with minor siltstone and mudstone partings, the lower Lower Jurassic Abe Formation in Kuche depression is subaqueous distributary channel deposit of braided delta. In Ahe Formation there developed a kind of large scale cross-bedding, which is characterised by an average set thick of 0.5 to 2 m, frequently occuring reverse dip bedding of adjacent sets, repeatedly and alternately changing of dip directions vertically Through detained observation of 7 sections, it is clear that the two dip directions are not in line with that of the regional paleoflow, but approximately perpendicular to it This kind of cross-bedding results from lateral accretion of sandbodies induced by the lateral migration of channels and its characteristics are: ①the overall shape of its sets is tabular, but its laminae are sigmoid, clearly converging both upward and downward. ②there exists secondary cross-bedding or cross laminations as well as slump structures in the inner part of some 'laminae'. ③its set thick has nothing to do with the grain size of the sediments and represents the depth of the lateral migrating channel. ④the grain size and composition of each lamina within the same set are different each other greatly. Lateral accretion cross-bedding also exists in Triassic System and other strata of Jurassic System. Lateral accretion cross-bedding are common in subaqueous distributary channel of braided delta, sometimes are found in meandering river, braided river, braided delta and fan delta in the different extent.