Sedimentary Characteristics of Shallowwater Deltas in Chang8 Subsection of Yanchang Formation, Jiyuan area

Key words: 
• cell parameters

Abstract: Jiyuan area is in the middle of the western Ordos Basin and its sedimentary facies is controlled by provenance from northeast, northwest and south directions in Late Triassic epoch. Detailed studies have been made on the rock types, sedimentary structures, and direction of provenance, paleontology and sedimentary facies characteristics of Chang8 subsection of Yanchang Formation of Upper Triassic of Jiyuan area by an integrated analysis of cores, thin sections, well log and other data. During the deposition of the Chang8 subsection of Yanchang Formation, the topography was gentle and stream actions were weak and the depositional system of shallowwater delta facies was developed. The sedimentation and sandbodies distribution were controlled by the raise and fall of the lake level and the direction of provenance obviously which were significant for hydrocarbon accumulation. It can be summarized as follows: ①The shoreline moved to provenance directions quickly and stripshaped or birdfootshaped subaqueous distributary channel sandbodies developed in the periods of high water levels; ②The delta plain developed and the front sandbodies came from different provenance converged in the center of the study area in the periods of low water levels; ③The sandbodies deposited in the periods of high water levels were favorable to develop lithologic reservoirs than those deposited in low water levels because the former distributed as sheetshaped which formed lithologic barriers.