鄂尔多斯盆地陕北地区上三叠统延长组不同级次层序界面的识别

Identification of sequence boundaries of different orders in the Upper Triassic Yanchang Formation in northern Shaanxi,Ordos basin

以高分辨率层序地层学理论为指导,通过对鄂尔多斯盆地陕北地区上三叠统延长组典型露头剖面及岩心观察、描述的基础上,结合大量测井曲线,详细地分析了延长组不同级次层序界面的识别标志。结果表明:鄂尔多斯盆地陕北地区上三叠统延长组由1个超长期旋回,4个长期旋回,9个中期旋回及若干个短期旋回组成。由于印支运动的影响,在研究区形成了两个区域性的构造运动升降面(SLSB2和SLSB1),该界面为超长期旋回的顶、底界面。长期旋回层序为一套区域性湖进-湖退沉积序列,界面是低角度的侵蚀不整合面和与其相应的整合面。研究区3个长期旋回层序界面自下而上依次为:区域性基准面抬升所形成的水进界面,相当于"李家畔页岩";区域性的泥岩、凝灰岩标志层,相当于长6油层组底部的k2标志层;区域性的相转换界面,相当于长4+5和长3的分界面。中期旋回和短期旋回层序界面主要为不同级次的湖泛面、冲刷侵蚀面、岩石类型或相组合的转换面、岩石相内部的层理变化界面、砂泥岩厚度旋回性变化界面等。在界面识别的基础上,对研究区沉积旋回及界面的空间配置进行了分析,认为不同级次界面的识别是高精度层序地层研究的核心,对于油田勘探和开发具有至关重要的作用。

The indications for identification of sequence boundaries of different order are analyzed in detail according to the theory of high-resolution sequence stratigraphy and on the basis of the observations and descriptions of the main field outcrops and cores of the Upper Triassic Yanchang Formation in northern Shaanxi, Ordos basin, combined with abundant well logging curves. The results indicate that there are one super long-duration depositional cycle, four long-duration depositional cycles, nine medium-duration cycles and some short-duration cycles in the Yanchang Formation. Affected by the Indosinian movement, two regional uplift and subsidence surfaces （SLSB2 and SLSB1） of tectonic movement formed in the study area and the two sequence boundaries are the top and bottom of a super long-duration cycle. The long-duration cyclic sequence is a regional lake transgressive-regressive depositional sequence and its boundary is a low-angle erosion unconformity and its corresponding conformity. There are three long-duration cyclic sequence boundaries in the study area; they are in ascending order： （1） the transgressive boundary formed by regional base-level elevation, which is approximately equivalent to the ＂Lijiapan shale＂; （2） the regional marker bed of mudstone and tuff, which is approximately equivalent to marker bed k2 located at the bottom of oil layer set Chang 6; and （3） the regional facies transition boundary, which is approximately equivalent to the boundary between layer sets Chang 4＋5 and Chang 3. The medium-and short-duration cyclic sequence boundaries are lake flooding surfaces of different orders, erosion surfaces, transition surfaces of rock types or facies groupings, surfaces of bedding change within rock facies and boundaries of cyclic change in thickness of sandstone and mudstone. On the basis of identifying boundaries, the depositional cycles and spatial distribution of boundaries in the study area are analyzed. The identification of boundaries of different orders is the key to the study of high-resolution sequence stratigraphy and plays an important role in petroleum exploration and exploitation.