The paper summarized the hydrocarbon accumulation pattern in the formation based on analysis of logging and core data

relationship among lithology

physical property

electrical property and oil-bearing situation

as well as formation test and production test results from the Chang-9 reservoir in the Yanchang Formation of the Hongjingzi block in Jiyuan oilfield

Ordos Basin.It is suggested that the oil and gas generated in source rocks of the Chang-7 layer were mainly driven by the excess flow pressure inside and migrated to the Chang-9 reservoirs through pores and fractures.However

the hydrocarbon expulsion in Chang-7 was not strong enough to fully fill up the Chang-9 reservoirs.In addition the bottom water widely developed in the Chang-9 reservoirs impeded the further downward migration of hydrocarbons.The top barriers inside the Chang-9 provided the sealing conditions necessary for hydrocarbon accumulation in the reservoirs

resulting in the enrichment of some hydrocarbons in interlayers inside the Chang-9.Hydrocarbons might first accumulate in top layers of the Chang-9

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where the physical properties were more favorable due to the dual control of lithology and physical properties.The relationship between structure features and formation testing as well as production testing results demonstrates that the structural relief has very little effect on oil and gas accumulations in the Chang-9 reservoirs.