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

Abstract：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.