Study On The Petrologic Characteristics And Evaluation Of The Pyroclastic Rock Reservoir

Tamtsag basin is located in the eastern Mongolia, which is mesozoic continentalextensional faulted basin. Tamtsag basin of Mongolia and Hailar Basin of Chinabelong to the same basin: Hailar-Tamtsag basin. Tanan Depression is one of the mostimportant second tectonic units with NE orientation. Besides, Tanan Depression is ahalf graben-like depression with steep slope in the east and gentle slope in the west.Pyroclastic rock is found in Tongbomiao Formation and Nantun Formation. In recentyears, oil and gas resources have been found in pyroclastic rock, so pyroclastic rock isconsidered as one of new exploration targets. Pyroclastic rock is a kind of transitionalrock between normal volcanic rock and normal sedimentary rock, with a poorreservoir property, high content of clay minerals,content of the strong reservoirheterogeneity.It is obvious that this type of reservoir is difficult for exploration.Thus,it is necessary to research the reservoir characteristics and its influence factors,evaluate the reservoir quality in order to provide theoretical basis for reservoirexploration and development.Taking the pyroclastic rock of Tongbomiao Formation and Nantun Formation fromTanan Depression in Tamtsag Basin for example, the lithology, diagenesis, reservoirspaces, physical properties, pore structure, reservoir sensitivity characteristics,controlling factors and reservoir assessment were researched by observation on rocks,casting thin sections and scanning electron microscope, based on the theory ofsedimentology and reservoir geology. The achievement and main understanding are as follows：The pyroclastic rock is mainly composed of welded pyroclastic rocks，ordinarypyroclastic rocks，sed pyroclastic rocks and pyroclastic sedimentary rocks. Thepyroclastic rock types of Tongbomiao Formation mainly includes welded tuff, weldedbreccias,tuff, volcanic breccias,sedimentary tuff and tuffaceous sandstone(conglomerates).Among these, tuffaceous sandstone (conglomerates) is the mostabundant rock, the second most abundant types are tuff and welded tuff. Thepyroclastic rock types Nantun Formation1stsection mainly includes welded tuff, tuff,sedimentary tuff and tuffaceous sandstone (conglomerates), tuffaceous sandstone(conglomerates) is the most abundant rock. The pyroclastic rock types of NantunFormation2ndsection only includes tuff and tuffaceous sandstone (conglomerates),tuffaceous sandstone (conglomerates) has absolute superiorityDiagenesis includes compaction, cementation, dissolution, dissolution,metasomatism, sinter, devitrification and recrystallization. The diagenetic processesare as follows: Welding and corrosion,clay coating, devitrification andmicrocrystalline quartz, mechanical compaction,smectite,illite and chlorite,dissolution, kaolinite and albite, calcite, ferrocalcite, dawsonite, ankerite. Thediagenetic facies are including: tufaceous cementaion，corrosion，authigenic minealcoating, pore filling, carbonate cementaion. The diagenetic stage is in middlediagenetic A.Pyroclastic reservoir spaces can be divided into four types: primary pores, primarycracks, secondary pores, secondary cracks, including10sub types, dominated bysecondary dissolved pore. Meanwhile, the pyroclastic rock belongs to low-extra lowporosity, extra low permeability reservoir. The porosity and permeability have a goodcorrelation.From the best to the worst, the order of reservoir physical property of TongbomiaoFormation are as followed: west faulted buried-hill tectonic zone,middle faultedburied-hill tectonic zone and west sub-depression,middle sub-depression,eastfaulted-nose tectonic zone,east sub-depression.That of Nantun Formation1st are asfollowed: east faulted-nose tectonic zone, middle sub-depression, west sub-depression,east sub-depression.Two abnormal high porosity belts are found at thelocations of1900m-2100m and2600m-2900m,one abnormal high permeability isfound at the location of1900m-2100m.The pore structure Heterogeneity of Tongbomiao Formation and Nantun Formationis strong.The pore structure can be divided into four class:class Ⅰ, class Ⅱ, class Ⅲ,class Ⅳ.The pore structure of Nantun Formation2nd section is better than that ofTongbomiao Formation and Nantun Formation1st section.The water sensitivity differs when rock types changes; salinity sensitivity ismoderate–strong, critical salinity is5126mg/L;velocity sensitivity is moderate-weakto weak; alkaline sensitivity is moderate-weak to moderate-strong, critical alkalinity is7.43；acid sensitivity changes widely.Reservoir qualities are affected by lithology and lithofacies, tectonic movement,diagenesis, etc. And the dissolution contributes a lot to the qualities of reservoir.Porosity，permeability，granular，carbonate content,clay minerals comtent, radius ofpore throat, expulsion pressure and SHgmax are chosen;and Grey CorrelationAnalysis Method is used to evaluate the reservoir,the results of evaluation are dividedinto four levels：Ⅰ，Ⅱ， Ⅲ，Ⅳ；with a content of34.27%25.87%，25.87，13.99%respectively.The north part of reservoir quality in middle sub-depression and middlefaulted buried-hill tectonic zone is better than that of south part;on the contrary, westfaulted buried-hill tectonic zone and east sub-depression are opposite.The bestreservoir quality locates in the south part of west faulted buried-hill tectonic zone.