Main Controlling Factors Of Deep Natural Gas Accumulation In The Dehui Depression Of The Songliao Basin

According to the previous studies and comprehensive theories of sedimentology,organic geochemisty and geophysics,the indepensible forming conditions of deep natural gas including source rocks,reservoirs,cap rocks,pathway system and accumulation period characteristics were systematically analyzed in this thesis.Through the analysis on the typical deep natural gas reservoirs,the main controlling factors of deep natural gas accumulation are ascertained and the favorable areas of deep natural gas exploration are predicted.The results show that the deep source rocks of the Dehui Depression are characterized by high organic matter abundance.The types of organic matter are mainly Type II₁-III.The organic matters are at mature to highly mature stages.The source rocks of Huoshiling formation are the best in quality,followed by the Shahezi formation,and Yingcheng formation is relatively poor.The rock types of deep reservoirs are dominated by glutenite and andesite,which are characterized by both low composition maturity and textural maturity.The porosity ranges from 5%to 10%and the permeability is low.The reservoir beds are tight with strong heterogeneity.The deep reservoir is interpreted as tight reservoir as a result of the intense compaction and cementation,which caused the porosity and permeability decreasing,while the dissolution effect improved the physical property to some degree.The deep mudstone cap rocks have the characteristics of favorable sedimentary environment,large accumulation thickness,large single layer thickness and wide distribution area.The deep formation pressure is much higher than the normal formation pressure,and pressure coefficient of local area is as high as 1.5.It can be used as good local cap rocks of overpressure sealing.The deep sand bodies have the characteristics of wide distribution area,large single layer thickness,good connectivity with the direction of material source,and poor connectivity crossing source direction.The transport coefficient of the sand bodies in the Huoshiling formation is the largest and the Yingcheng formation is the smallest.And the average value of the transport coefficient of the sand bodies is greater than 400,which can serve as a good channel for oil and gas migration.The unconformity surfaces at the top of Yingcheng formation and the top of Shahezi formation are the most extensive regional unconformity surface in the Dehui Depression,which is closely related to oil and gas accumulation.The"three layer structure"of the unconformity has the characteristics of larger thickness near the source area and smaller thickness away from the source area.The transport efficiency of the unconformity at the top of Yingcheng formation is higher than that at the top of Shahezi formation.The distribution of faults in the Dehui Depression is characterized by zonal and cluster distribution.The faults act as obvious dominant migration pathway,which are very favorable for oil and gas accumulation,and is the key factor of deep natural gas accumulation.The main accumulation periods of deep natural gas are the sedimentary periods of Quantou formation and Qingshankou formation to Nenjiang formation.Widely distributing source rocks,high quality reservoirs with large area distribution,good cap rocks and complex pathway system are the main controlling factors of deep natural gas accumulation in the Dehui Depression.Based on the research results of accumulation conditions,the analyses on the typical deep natural gas reservoirs and the main controlling factors of deep natural gas accumulation were completed,and 4favorable zones in the Dehui Depression were predicted and 9 favorable areas can be further divided finally.