The Sedimentary Phase Characteristics and Evolution Rule of Chang 9 in Huanxian-Zhengning Area

The paper chooses Chang 9 of triassic Yanchang Formation as the study object, which is in Huanxian-Zhengning area of the Ordos Basin. Based on field outcrops, core observations and log data analysis, the sedimentary phase characteristics and evolution rule of Chang-9₁ and Chang-9₂ were analyzed from the aspects of point, line and surface.The study shows that the sedimentary phase type in Huanxian-Zhengning area is braided river delta phase and lacustrine phase.The braided river delta plain subphase is the most typical in the outcrop area, while the delta front subphase is the dominant phase zone of the study area.Chang-9₂ sand body is more developed than Chang-9₁ according to the longitudinal evolution;sand body in the west is more developed than that in the southwest and south. During the deposition period of Chang-9₂, the braided river delta, further divided into four smaller deltas, is roughly stretching in the east-west and the northeast-southwest direction. The braided river delta sedimentary phase in Chang-9₁ sedimentary period has much inheritance than that of in Chang-9₂.