Reservoir Description And Evaluation Of Chang-8Oil-bearing Layer In Qingyang Area,Southwest Ordos Basin

The sandstone of Chang-8oil-bearing in Qingyang area is typical low permeability reservoir which is the research focus today. It’s important to expolre the area effectively that finding the favorable exploration block basing on reservoir description and evaluation.In this paper, exhaustive description and evaluation is carried out on Chang-8reservoir in Qingyang area by a systematic study on sedimentary facies, characteristics of petrology, physical property, pore texture and diagenesis of reservoir,basing on core observation,thin section analysis, SEM, grade image analyse of section,pore image analyse of section, high-pressure mercury penetration,physical property testing,inclusion temperature testing， analysis of X-ray diffraction and so on methods. The next favorable exploration area also pointed out.The following conclusions are obtained:Characteristics of sedimentary facies:At Chang-8phase, the sedimentary facies of Qingyang area is typical delta front subfacies,and can individe into subsea distributary channel and interdistributory cove microfacies. The sedimentary facies and sand body extend along the SE-NW and S-N direction.The size of channel and coverage of thin sand body at Chang-81phase is larger than that at Chang-81phase.Characteristics of petrology of reservoir:The rock type in research area is main lithic arkose and feldspathic lithic sandstone, with fine and medium-fine grain.The grain of Chang-81reservoir is coarser than that of Chang-82. The average volume of the fillings is12.62%. And the fillings are mainly made up of carbonate mineral and chlorite.Characteristics of pore texture of reservoir:The types of reservoir pore are mainly intergranular pore and feldspar dissolved pore. The average pore radius is60.8μm and average throat radius is0.34μm, which are very small. The average throat radius of Chang-82reservoir (0.18μm) is far less than that of Chang-81reservoir (0.40μm).Characteristics of physical property of reservoir:The resulut of core physical property testing indicates that the average porosity and the average permeability is8.63%and0.70×10-3μm2so it is typical low and extra-low porosity, extra-low and ultra-low permeability sandstone reservoir. By means of experience in statistics and mercury injection test, the minimum physical property value of effective reservoir is ensured porosity6%and permeability0.10×10-3μm2. The plan of available porosity and permeability indicates that the coverage and continuity of higher value of Chang-81is lager than that of Chang-82.Characteristics of diagnesis:The reservoir undergoes intensly compaction, cementation, metasomatism and denudation.The stage is generally up to the stage A of the middle diagenetic.The result of research on diagenetic porosity evolution indicates that the average natural porosity is38.03%. After compaction, the average porosity is21.58%. Compaction makes the natural porosity decrease43.15%. After cementation, the average porosity is6.89%. cementation makes the natural porosity decrease39%. Compaction and cementation are the immediate cause making the porosity decrease. And denudation produces4.23%secondary pore, which makes the porosity increase10.23%. The denudation is the main factor that makes the physical property better.The affecting factor of physical property of reservoir:Sedimentary microfacies are the dominating element. The feature of fine grain and much ductility component in reservoir is the main foundation of tight sandstone reservoir.The intense compaction and cementation is the main cause making the reservoir tight.The denudation produces secondary pore, making the physical property better.A number of nanometer thoat is the main microscopic cause making the reservoir tight.Exhaustive evaluation of reservoir:The reservoir was divided into four types. Type Ⅰ, Ⅱ,Ⅲ is the favourable, less favourable and poorer effective reservoir which could provide industrial oil-production in the current technical conditions. Type IV is the non-effectivet reservoir. The coverage and continuity of type Ⅰ,Ⅱ,Ⅲ of Chang-81is far better than that of Chang-82.Basing on comprehensive evaluation of reservoir and well testing, two best favorable blocks, whose coverage is morn than45km2,and four better favorable blocks,whose coverage is between15and45km2,and eight favorable blocks,whose coverage is less than15km2,are confirmed.