Study On Reservoir Characteristics And Physical Property Controlling Factors Of Paleogene In Eastern Mahai Area

The eastern Mahai Area is located in the eastern part of Mabei Uplift in the northern margin of Qaidam Basin.Paleogene reservoirs develop a variety of sedimentary facies,and the sedimentary heterogeneity is strong,resulting in large differences in physical properties of different layers and regions of the reservoirs.However,the current researches on reservoir characteristics and physical property controlling factors in the study area are not deep enough,which restricts the deployment of future exploration work.In order to solve these problems,based on core,seismic,logging and experimental test data,the researches have been conducted on reservoir sedimentary characteristics,rock and mineral characteristics,diagenetic characteristics,physical properties and controlling factors,and prediction of favorable reservoir areas.The main achievements and understandings are as follows :(1)The sedimentary facies types and characteristics of Paleogene reservoirs are determined,and the spatial distribution characteristics of sedimentary facies in each layer are clarified.Alluvial fan deposits are mainly developed in the Lulehe Formation of the eastern Mahai Area,and fan margin subfacies are developed in the southern margin and the westsouthwest margin of the study area.Most of them are channel filling sedimentary microfacies,and the edge is channel runoff sedimentary microfacies.The distribution of sheet flow deposits in the study area is small.The lower part of Xiaganchaigou Formation mainly develops braided river-meandering river deposits.The braided river channel deposits are mainly distributed in the middle of the study area,and the river channel changes obviously.The heart beach deposits are developed at the intersection of the river channels,and the flood plain deposits are developed in the other areas.By the upper period of Xiaganchaigou Formation,meandering river channel deposits developed in the NE-SW direction in the middle,and the rest areas were flood plain deposits.(2)The rock and mineral characteristics and diagenetic characteristics of Paleogene reservoirs are expounded.The lithology of Paleogene reservoirs is mainly argillaceous siltstone and fine sandstone,followed by siltstone and fine conglomerate.Pore cementation is the main type,there is base cementation.The structural maturity is poor,and the debris composition shows that the component maturity increases first and then decreases gradually from bottom to top.Diagenesis types mainly include compaction,cementation,dissolution and metasomatism.The diagenetic stage of Paleogene reservoirs is early diagenetic stage B-middle diagenetic stage A,mainly distributed in middle diagenetic stage A.The diagenetic evolution sequence of the Lulehe Formation in eastern Mahai Area is mechanical compaction-early calcite cementationfeldspar dissolution / kaolinite precipitation-authigenic quartz-early hydrocarbon fillingfeldspar dissolution / late hydrocarbon filling-late calcite cementation.The diagenetic evolution sequence of the lower part of Xiaganchaigou Formation reservoirs is mechanical compactionearly calcite cementation-early hydrocarbon filling / feldspar dissolution / kaolinite precipitation-authigenic quartz / late hydrocarbon filling-late calcite cementation.(3)The pore structure and physical properties of Paleogene reservoirs are clarified.The reservoirs space of Paleogene reservoirs in eastern Mahai Area is mainly fractures,residual intergranular pores and secondary dissolved pores.The fractures in the Lulehe Formation are developed,and the reservoirs space is mainly fractures,residual intergranular pores and secondary dissolved pores.The fractures in the lower part of Xiaganchaigou Formation are not developed,and the reservoirs space is mainly residual intergranular pores and secondary dissolved pores.The pore throat structure is poor,the throat is thin,and the pore throat connectivity is not good.The reservoirs have the characteristics of low porosity and low permeability.By calculating the average reservoirs physical properties of the Lulehe Formation and the lower part of Xiaganchaigou Formation in each well,it is found that the reservoirs physical properties of the Lulehe Formation are better than those of the lower part of Xiaganchaigou Formation.(4)The controlling factors of physical property of Paleogene reservoirs are clarified.Sedimentary micro-relative reservoir property has obvious control effect on macroscopic.Compaction is the most important factor of porosity loss of Paleogene reservoirs in the study area,cementation damage to reservoirs is weak,dissolution is weak.The main factors affecting physical properties are sedimentary factors,compaction and cementation.(5)The favorable block distribution of Paleogene reservoirs is pointed out.The reservoirs in the Lulehe Formation and the lower part of Xiaganchaigou Formation are divided into three types,among which type Ⅰ reservoirs is the most favorable reservoir.The type Ⅰ reservoir of the Lulehe Formation is distributed in the middle and north east of the study area,and the distribution range is small.The type Ⅱ reservoir is distributed in most areas of the study area.The type Ⅲ reservoir is distributed in the edge of the study area.The type Ⅱ favorable reservoir in the lower part of Xiaganchaigou Formation is distributed in the middle and west of the study area.The type Ⅱ favorable reservoir is distributed in a small range.The type Ⅲ reservoir is distributed in a banded distribution,and is distributed in most areas of the study area.