Accumulation Characteristics Of Biodegradation Gas From Heavy Oil In Sanhecun Subsag Of Zhanhua Depression

Biodegradation gas has the advantage of shallow burial depth and easy to obtain,which has bright exploration prospects.Recent years large amounts of heavy oil reservoirs and the associated biodegradation gas reservoirs have been discovered in Zhanhua Depression,but it’s uncommon sense that considering them as a whole and studying their accumulati on characteristics.Based on the characteristics and formation mechanism of the heavy oil and biodegradation gas of Sanhecun Subsag in Zhanhua Depression,this thesis focuses on the heavy oil-gas migration system,summarizes the accumulation characteristics of hydrocarbon and establishes a shallow gas accumulation model,which effectively provides a theoretical guidance for the exploration practice.The research of geological characteristics indicates that the oil in Shahejie Formation is primary heavy oil derived from argillaceous source rocks in Es₄^s of Bonan Subsag while the oil in Neogene is biodegraded heavy oil derived from source rocks in Es₃,and the main causes of oil densification are biodegradation,oxidation and deasphalt process of oil during immigration.The shallow gas has a mixing characteristic of transformed thermogenic gas and secondary biogenic gas.Geochemical analysis and dissection of representative reservoirs indicate that heavy oil reservoirs biodegradation gas reservoirs have close relations both in formation mechanism and distribution.The research of migration system indicates that the characters of fractures in plane and in profile are various and fractures activity matched the accumulation periods well.Sanhecun deposited braided—meandering river sedimentary system in Neogene which features for fine sandstones and silt sandstones reservoirs.The provenance is from Chenjiazhuang Uplift in the southeast and the sand body spreads in NW direction.The sand body connectivity of Ngx strata is better than Ngs in both plane and profile,but the sand-bearing percent is lower than other districts in Jiyang Basin,which leads to the formation of structural-lithologic reservoirs.The main types of Neogene unconformity are parallel,overlap and folded fault unconformity in Sanhecun Subsag,and the main structural patterns of unconformity are sand-mud-sand,silt-mud-silt and silt-mud.It’s easy to form double pathways for hydrocarbons on and below the unconformity.The research of hydrocarbon accumulation regulations indicates that the hydrocarbon sources are sufficient,the reservoir-caprock assemblage is excellent and the types of traps are multiple in Sanhecun Subsag,which leads to great exploration potentials.The main controlling factors of gas accumulation includes:firstly,shallow gas migrated along fractures and slight structural ridges;secondly,the distribution of gas reservoirs are controlled by fractures;thirdly,flood plain sediments are good caprocks for shallow gas.Heavy oil-shallow gas accumulation model in Neogene has typical features for low sand-bearing percent and meshwork-carpet accumulation,which means that in the first hydrocarbons reached the Neogene skeletal sand bodies and unconformities through vertical migration along master fault Gunan Fault Belt,then migrated in lateral towards Chenjiazhuang Uplift.During migration the crude oil was thickening constantly and generated biodegradation gas,while the gas migrated and accumulated in shallow layers along secondary faults.