Abstract:

Oil reservoirs in the Chang-8¹ member of the Yanchang Formation of the Longdong area in Erdos Basin are typical low-permeability reservoirs.An observation of their diagenesis and bitumen occurrence and an analysis of the paleo-fluid indicate a multistage diagenesiswith at least three times ofoil charge in the reservoirs of the member.The previous study results were employed in the analysis of the reservoir features.Based on an analysis of dynam ic conditions，a reliable oil accumulation pattern is proposed.During the late Jurassic when the reservoirs still had favorable porosity and permeability，hydrocarbons generated by the quality source rocks of the Chang-7 ³ member entered the carrier system of the Chang-8 ³ member.The hydrocarbonsm igrated laterally due to buoyancy and accumulated for the first time.Later，with the tilting of the basin，the accumulated oil and gas escaped.At the same time，the continuous compaction and cementation lowered the permeability of the reservoirs.Since the early Cretaceous，the hydrocarbons mainly m igrate and accumulate laterally along the residual pathways that are oil-wet due to long-term contactwith oil.

Key words: diagenetic process, carrier system, reservoiring pattern, low-permeability reservoir, Xifeng oilfield, Longdong area, Ordos Basin