Accumulation Period and Dynamic Accumulation Process Analysis of Zhenwu Oilfield in Gaoyou Sag

Key words: 
• accumulation process /
• accumulation period /
• Zhenwu oilfield /
• Gaoyou sag /
• Subei basin

Abstract: Zhenwu oilfield, located at the midsouthern part of Gaoyou Sag of Subei basin, is the largest oil field that has been discovered in Subei basin, which has the characteristics of developed long, large oilbearing area and high enrichingoil degree. The oilfield is located at the downthrow block of Zhen 2 fault in the southern part of the deep sag, and it is to hydrocarbon generating sags.The oil mainly distributes in three layers including the first member of the Sanduo Formation, the first & second member of the Dainan Formation. Some petroleum also distributed in the first member of the Funing Formation. Reservoir rocks are mostly fine sandstone and anisometric sandstone. Combining with various geological features, the authors analyse accumulation period and the dynamic accumulation process of Zhenwu oil field by ways of structural evolution, crude oil features and fluid inclusion. The structural evolution history shows that fault activity is strong from Dainan Formation deposition to early Yancheng Deposition. Earth crust uplifted and formation eroded resulted in serious destruction of oil reservoir at the late of Sanduo Formation, then fault continued activity and petroleum migration. The sag continued to accept deposition and fault activity stops at the late of Yancheng Formation deposition, and the petroleum migration rate slowed down. It is favorable for petroleum reservation when the formation thickness was big at the late deposition. 
From the source rock evolution history , there are two times of petroleum generation during which the source rock of the second member of the Funing Formation enter the sag is at a great quantity of petroleum generation at the late and last stage of Sanduo Formation when the source rock of the fourth member of the Funing Formation had not reached oil threshold and only generated a little petroleum, so it was the oil mainly generated by the source rock of the second member of the Funing Formation. But most of the reservoirs were damaged by the influence of the regional tectonic activities at the late of Sanduo period. When the Yancheng Formation was depositing, the source rock of second & fourth member of the Funing Formation again reached maturation which result in secondary hydrocarbon generation.
Meantime, the characteristics of fluid inclusions in Zhenwu oil field show that it mainly developed one phase of fluid inclusion which organic inclusion and its associated brine inclusion mostly develop at the quartz secondary outgrowth cementation and there is only one peak temperature area. Associated with burial history of individual well, the authors find that the time of petroleum migration and accumulation was the time of the second member of Sanduo Formation deposition which had a certain difference with accumulation time inverted by structural evolution and source rock evolution. At last, with the analysis of petroleum dynamic accumulation process by ways of accumulation periods, the authors reach conclusion that there were two accumulation periods in Zhenwu oilfield, firstly, the second member of the Funing Formation generated, migrated and accumulated before the late of Sanduo Formation, but the next structural uplift at the late of Sanduo Formation damaged the early oil reservoirs. At the EarlyMiddle of Yancheng Formation, it mainly is the time of the fourth member of the Funing Formation reburied and secondary oil generation, which is the essential period of petroleum migration and accumulation. From the the late of Yancheng Formation on, it is the essential period of petroleum preservation.