Occurrence Mechanism And Movable Resource Potential Of Alkaline Lacustrine Shale Oil In The Fengcheng Formation,Mahu Sag

The Lower Permian Fengcheng shale in the Mahu Sag of the Junggar Basin has favorable oil-bearing property and great resource potential.However,this set of shales was formed in an alkaline lacustrine environment with complex lithologic and lithofacies association.The sweet spots are spread,making it challenging to assess advantageous lithofacies reservoir.At present,the movable resource potential of shale oil in the Fengcheng Formation is not clear.Basing on investigating the characteristics of alkali-lacustrine shale of the Fengcheng Formation and identifying lithofacies in the study area,this thesis carried out research on shale geochemistry,reservoir properties,shale oil occurrence mechanism,and movable resource potential.The main achievements were as follows.The lithofacies,reservoir space and evolution model of the Fengcheng shale were determined by SEM,gas adsorption and high-pressure mercury injection experiments.The effective reservoir spaces in the shale include intergranular pores,intragranular pores,and fracture pores,while the intraparticle pores and organic pores are mostly ineffective reservoir spaces.According to mineral composition,the Fengcheng shale can be divided into siliceous shale,calcareous shale,and mixed shale.Additionally,the reservoir characteristics of different lithofacies shales were discussed by qualitative and quantitative methods.Among them,meso-pores and macropores are well developed in siliceous shales,and pore-throat distribution is uniform with good connectivity.The macropore development and pore connectivity of calcareous shales and mixed shales are relatively poor,and the pore size is small.The models of pore evolution of shale with various lithofacies are created combined with the diagenetic evolution.Siliceous shale experienced weak compaction in the diagenetic process and was characterized by relatively large residual intergranular pores and intragranular dissolution pores.Porosity decline in calcareous shale is mainly caused by strong cementation,while that in mixed shale is dominated by compaction.This type of lithofacies has many intergranular pores of clay mineral and the worst reservoir physical properties.An improved pyrolysis method was used to correct the amount of retained oil in different lithofacies,and the impacts of organic and inorganic components on the occurrence state of retained oil were analyzed.Quantitative analysis reveals that the ratio of retained oil and free oil in siliceous shale is the highest,while that in calcareous shale and mixed shale is relatively low.The pore size analysis before and after extraction with organic solvent indicates that micro-nano inorganic pores developed in shale are effective storage space for hydrocarbons,with pore size larger than 5nm being the predominant occurrence space.The distribution space of retained oil varies with different lithofacies.Larger pore scale in siliceous shale can create greater room for the occurrence of abundant retained oil.The retained oil is mainly concentrated in macropores and mesopores.To the contrary,calcareous shale and mixed shale are relatively poor pore network,and liquid hydrocarbons is mainly enriched in the mesoporous and microporous.The lower limit of pore throat radius and the amount of the mobile oil with different lithofacies was analyzed by using NMR experiment and geochemical index.The lower pore size of mobile oil in siliceous shale,calcareous shale,and mixed shale are 8 nm,13nm,and 28 nm,respectively.Additionally,the limit of shale oil mobility in the Fengcheng shale was defined as the threshold of free oil content reaching at least 2.0 mg/g.The mobile oil index(MOI)is proposed,and it is pointed out that MOI required by different shale lithofacies to reach the mobility of retained oil is different.It should be greater than0.5 for siliceous shale,greater than 0.6 and 0.66 for calcareous shale and mixed shale,respectively.Through the closed system thermal simulation experiment,the activation energy of liquid hydrocarbon generated from kerogen pyrolysis by alkaline minerals is reduced,and liquid hydrocarbon with high content of saturated hydrocarbon can be produced at relatively low maturity.The peak of hydrocarbon production is delayed,but the effect on hydrocarbon yield is minimal.The oil production of the Fengcheng shale is 45.00×10~8t by hydrocarbon generation thermal simulation method.Considering the occurrence state and movable characteristics of retained oil within shale,the volume method and Monte Carlo simulation were used to determine the quantity of resources.Under the 50%probability,total retained oil,free oil,and mobile oil resources of the Fengcheng shale in the northern area of the Mahu Sag were calculated to be 13.18×10~8t,6.12×10~8t,and2.03×10~8t,respectively.