Study On The Hydrocarbon Accumulation Characteristics And Dominant Controlling Factors In Eastern Mahai Area

The Eastern Mahai Area is located in the northern margin of the Qaidam Basin and the eastern wing of the northern Mabei Area.It is adjacent to the Gaxi-Yuka hydrocarbongenerating sag with good oil source conditions.In recent years,oil and gas has been found,but the exploration effect is not significant.The understanding of its hydrocarbon accumulation law and main controling factors is limited,which restricts the further exploration.Therefore,it is of great significance to analyze the factors affecting oil and gas accumulation and distribution in the region,which is conducive to accelerating the exploration process in Eastern Mahai Area.In view of this problem,core observation,logging data analysis,seismic data analysis,oil source correlation experiment,fluid inclusion fluorescence observation and temperature measurement experiment were used to study the reservoir forming characteristics and main control factors in Eastern Mahai Area.(1)The development characteristics of source rocks around the Eastern Mahai Area and the development potential of source rocks are clarified.The source rocks in the Gaxi-Yuka sag are abundant,and there are three types of source rocks : mudstone,carbonaceous mudstone and coal.The mudstone has high organic matter abundance,good type and high maturity,reaching the oil generation window with high oil generation potential.It is a good source rock,while the carbonaceous mudstone and coal are subject to the organic matter abundance and type which are medium source rocks.(2)The development characteristics of reservoir and caprock in Eastern Mahai Area are expounded.Metamorphic rock reservoirs are developed in Proterozoic,which are low porosity and low permeability reservoirs.The main storage space is structural fractures generated by tectonic activities.The Lulehe Formation and the Lower Xiaganchaigou Formation are clastic reservoirs,and the physical properties of the Lulehe Formation are better than those of the Lower Xiaganchaigou Formation.The mudstones with wide area and large thickness are developed at the top and upper sections of the Lower Xiaganchaigou Formation,which are high-quality regional caprocks.The porosity and permeability of the Proterozoic bedrock itself are poor,which can also effectively prevent oil and gas diffusion.(3)The characteristics of hydrocarbon accumulation in Eastern Mahai Area are expounded.Structural reservoirs and structural-stratigraphic reservoirs are developed in the Eastern Mahai Area.Oil and gas come from the northern Gaxi-Yuka sag.The Oil and gas generation time is33-25 Ma and 19-8Ma,located in N + Q period.Fault is the key for oil and gas to enter the Eastern Mahai Area from the hydrocarbon generation area.The plane distribution results of maturity parameters and methylphenanthrene parameters of biomarkers show that oil and gas migration has two directions : one is directly from the northern Gaxi-Yuka depression,and the other is the redistribution of oil and gas in Mabei area,which migrates from Mabei to Mahaidong area.(4)The main controlling factors of hydrocarbon accumulation in Eastern Mahai Area are revealed.The well-developed source rocks provide the basis for oil and gas generation;Structural activity controls the effective entry and improvement of oil and gas reservoirs by generating faults and corresponding structural fractures.Good reservoir conditions provide effective space for hydrocarbon accumulation.(5)The development range of favorable blocks in Eastern Mahai Area was determined.The favorable blocks in Eastern Mahai Area are divided into three categories,among them I and II are important favorable blocks.The favorable areas of Proterozoic II in Eastern Mahai Area are SG1 well and SG101 well area.The favorable areas of Lulehe Formation I are S3 C and S3 well area.The favorable areas of Proterozoic II are middle SG1 well area and southeast fault nose structure.The favorable areas of Xiaganchaigou Formation I are northeast S3 C well area.The favorable areas of Proterozoic II are middle structural height and southeast fault nose.The comprehensive evaluation results show that the next favorable exploration direction is fault nose structure located in the southeast.