The Mechanism For Sedimentation And Diagenesis Of Chang2 Subformation Of Upper Triassic, JiYuan District Of Ordos Basin

The Yan-Chang formation of Upper Triassic in Ordos basin is mostly detritus sediments derived from rivers and lakes sedimentary environment on setting of inland basin. It is divided into 10 subformations containing oil and gas by its lithologic, electric, and paleontological characters.We have obtained abundance of information about its sedimentary structure and lithologic characters by observing bore cores of Chang2 subformation, and combining with the information from well-log curves, and a great deal of thin section observation, heavy mineral analysis, porosity and permeability data, stratum thickness, et al. In the end, we get following achievements:The sedimentary facies of Chang2 subformation in JiYuan district are reticulation fluvial. During the time from Chang2~3- Chang2~1, the ratio for thickness of sand bodies to thickness of stratum decreased, with fine granularity, gentle grade of riverbed, and shrinking sandbodies. The environment of JiYuan district had evolutated into marsh in the terminal of Chang2 subformation. At the most time of Chang2 subformation, the material province of JiYuan district is mainly located in northeast to some degree.The carbonate cementation is mostly negative to development of reservoir, however, kaolinite, chlorite and silica cementation are favored to preservation of reservoir space from many aspects. The mechanism for conservation and evolvement of pore of Chang2 from diagenesis are as following: a) early quartz cementation during diagenesis; b) lining of chlorite in pores in early period; c) dissolution of feldspar by freshwater during the Indosinian period, which is the most important diagenetic mechanism in Chang2 subformation in our target area. Moreover, the secondary pores increased violently near surface of unconformity.The intergranular pores are very low, whereas the secondary pores are very high in JiYuan district compared with the entire basin. During the whole diagenetic stage, the role of the secondary pores are very important for the improvement of reservoir quality.The intergranular relatively pores prevailed in the lower part of Chang2 subformation than the upper part, on the contrary, the upper part contained more sencondary pores than the lower part of Chang2 subformation. the reason leading to...