Sequence stratigraphy and sedimentary filling model of Chang-8 oil layer of Yanchang Formation in southwestern Ordos Basin

Comprehensively based on the analysis of outcrop, core, logging, seismic and test data of Chang-8 oil layer of Yanchang Formation in southwestern Ordos Basin, the sequence stratigraphic framework, sedimentary characteristics, spatial distribution of sedimentary filling and the sedimentary evolution model controlled by the sequence of Chang-8 oil layer are investigated. The results show that in Chang-8 oil layer, the lowstand system tract sediments of the Chang-7-Chang-8 sequence is mainly developed, which can be further divided into 6 parasequence sets and 15 parasequences. Sequence stratigraphic correlation shows that the spatial distribution of the stratum is stable, no obvious thickness change and less topographic relief. A sequence structure pattern of gentle slope is thus developed on the margin of lacustrine basin. The sedimentary sand body is characterized by low composition maturity and structure maturity, two segments of grain structure, well-developed sedimentary structures formed by rapid deposition and wave transformations, and density flow deposition with traction flow feature. In addition, it can be visibly seen that shallow water wormholes and plant roots are developed, indicating a shallow water delta environment. Sediments have multi-phase flood density flow sedimentation characteristics controlled by seasonal climate change. The vertical, sectional and planar distribution characteristics of sedimentary fillings reveal that the channels of delta front diverge frequently in the front zone, thus forming the networking distribution pattern under the background of gentle slope, shallow water and rapid deposition. Progradation of delta mainly occurred during the early period of Chang-8 oil layer deposition. The supply of provenance was reducing, so the construction ability of delta was weakened under the background of lake level rising. The channels were highly restrictive, thus the extensive vertical superposition was widely developed at sand body bar. During the late period of Chang-8 oil layer deposition, the subaqueous distributary channels and mouth bars were subject to relatively strong reforming. The grain size of the sediments became finer and the lateral distribution of sand bodies was strengthened. The lateral sheet-spreading distribution of sand bodies is enhanced, dominated by aggradation and progradation. On the whole, the Chang-8 oil layer recorded a complete cycle of lowstand filling sequence of continental lacustrine basin from regression to transgression. © 2019, Editorial Office of ACTA PETROLEI SINICA. All right reserved.