Abstract: This paper analyzes the geologic conditions of hydrocarbon accumulation in the Chang-9 reservoir of Ordos Basin.The oil in the Chang-9 reservoir is mixed oil sourced from both the overlying Chang-7 and Chang-9 itself.However,oil in the Chang-9 reservoir in Zhidan area is only sourced from Chang-9 source rocks,without any contribution from the Chang-7 source rocks.The oil mainly occurs in the first segment of Chang-9 oil reservoir.Reservoirs are mainly of distributary channel sandstone of braided river delta plain facies and underwater distributary channel sand of delta front facies,with some turbidites of semi-deep lake facies.Lithology of Chang-9 reservoir is dominated by lithic arkose with well-developed primary inter-granular pores,low porosity and permeability.The physical properties of the Chang-9 oil reservoirs show great lateral variation.In Jiyuan area,the sandstone feature in coarse grain,well-developed inter-granular pores and good poroperm characteristics.In contrast,in Luochuan area,the carbonate cemented sandstone has poor poroperm characteristics due to poor development of pores.According to the relationship between source rocks and reservoirs,two source rock-reservoir combination patterns were identified in Chang-9 oil reservoirs,i.e."upper sourcing" pattern and "indigenously sourcing" pattern.Jiyuan area is the most favorable for further exploration based on comprehensive analysis.

Key words: accumulation condition, Chang-9 oil reservoir, Yanchang Formation, Ordos Basin