| The Ordos Basin is an important petroliferous basin and an oil and gas producing base in China, which has a huge amount of oil and gas resources. But more than90%of the oil and gas resources found there so far accumulate in low permeable reservoirs mainly characterized as low permeability and low production in development phases. With the further development of exportation and development, Changqing oilfield has paid more attention to the reservoirs with the permeability of about0.5×10-3μm2for the future work, with the rising of the percentage and amount of reserves in low or ultra-low permeability reservoirs. So it has been a great issue and challenge to be solved urgently, that how to efficiently evaluate and develop the low and ultra-low permeability oil-gas reservoirs to improve the producing degree and use value of the reserves in low permeability reservoirs.Reservoir evaluation is a core part during the oil-gas field development. The previous research in region of Longdong in the southwest ordos basin only used static data, and focused on single factor analysis of reservoir characteristics, such as Chang8oil-bearing formation’s petrological and mineralogical characteristics, microscopic heterogeneity, diagenesis and its effect on low permeability. While the low permeability reservoir of Chang8oil-bearing formation occurs in complex geological conditions, the heterogeneities of which are very strong vertically and horizontally, the corresponding work need to be refined and deepened, comprehensive research through multidisciplinary.Taking the region of Longdong as example, this thesis is aimed at the analysis of low permeability reservoir of Chang8oil-bearing formation, including geological background for its forming, conditions for its deposition and diagenesis, and its macro-and microeconomic characteristics, screens effective evaluation indicators, and evaluates the low permeability reservoir comprehensively. Based on the extensive investigation of the research status and latest evolution of low permeability reservoirs in the world, this thesis amply gathers the data in the research area, including geology, drilling, testing and analytical data, production dynamic data, and so on. Directed with the theories of sequence stratigraphy, petroleum geology, sedimentology, reservoir geology, oil and gas field development geology and geological statistics, this thesis comprehensively researches the sandstone reservoir characteristics of Chang8oil-bearing formation through multidisciplinary, such as sedimentary facies, diagenetic facies, macro and micro heterogeneity, optimizes the classification evaluation parameters for the low permeability reservoir, determine the classification standard, and evaluate the vertical and horizontal distributing feature and the contribution to oil-gas reserves of the low permeability reservoir with different levels.1. Stratigraphic classification and correlationGuided by high-resolution sequence stratigraphy, applying the method of correlation using high frequency cycle, according to stratigraphic contact relations, depositional sequence or cycle, lithological association, and so on, determine the marker bed, and chose standard wells to build comprehensive columnar sections to divide Chang8oil-bearing formation, on the basis of which the single layers in skeleton well profiles are correlated in all region. Chang8oil-bearing formation is divided into Chang81and Chang82sand group, the former is subdivided into Chang811,Chang812and Chang813from top to bottom, and the later is subdivided into Chang821, Chang822from top to bottom.2. Sedimentary facies analysisOn the basis of core observation and microsection appraisal, combing the logging data, the sedimentary facies of Chang8oil-bearing formation is analyzed in key wells. Together with the electrofacies and sand content analysis, with the method of highlighting underwater distributary channel, determine the Section and plane distribution characteristics of sedimentary microfacies in each layer.The delta front subfacies and prodelta subfacies of braided river delta is developed in Chang8oil-bearing formation, the former is mainly underwater distributary channel microfacies and underwater distributary interchannel microfacies with secondary estuary dam microfacies, and the later is divided into prodelta microfacies.The types of sand bodies in delta front are mainly cutting river, stack river, isolated river dam, river, river dam and isolated dam, among which the cutting river is best sand body for permeability and accumulation with thick and widely distributed sand body.Underwater distributary channel is the main microfacies to accumulate the oil, in which in each layer the average ratio of the total thickness of oil layers to the total thickness of oil layers in researched area is0.889.3. Diagenetic facies analysisBased on core observation, thin section identification and mercury injection analysis and so on, the author analyzes lithologic characteristics, pore types and pore throat combination of Chang8oil-bearing formation’s reservoirs, determines the types and phases of diagenesis and the evolution history of porosity, groups the types of diagenesis, analyzes the distribution characteristics in single well, vertically and horizontally, and evaluates the diagenetic facies in different layers and regions.The lithology of Chang8reservoir is mainly fine-medium grained, very fine-fine grained feldspar lithic sandstone, mainly composed of diagenetic minerals with low compositional and textural maturity. The content of diagenetic mineral is18.5%, and the matrix only1.6%. The compositions of diagenetic mineral are mainly authigenic chlorite and calcite with the content of5.7%and5.1%respectively. The visible pores in sandstone are mainly intergranular pores with secondary feldspar dissolved pores, and micropores are mainly distributed in matrix and cement. The content of intergranular pores is1.6%, which mainly distribute in sandstones composed of chlorite cementation with thick single layer. The pore throat combination in sandstone is mainly mesoporous-micro throat type. The visible pores are commonly tetrahedron and polyhedron surrounded by cement, and the throats are flake-bending sheet interspace among grains (open width μm<1). The micropores are pores and bundle shape throats.The sandstone has undergone many kinds of diagenesis dominated by compaction pressure solution and cementation, and is in the substage A2of stage A of mesogenetic stage now. The order of different kinds of diagenesis is roughly compaction and pressolution-early calcite cementation-chlorite cementation-siliceous cementation-kaolinite cementation-middle calcite cementation-feldspar increase-illite cementing-corrosion-authigenic quartz filling.The initial porosity of sandstone is30～37%, but8.3～17.2%in place now. During the diagenetic process, the reduced porosity due to compaction and pressolution is1.3～20.4%, the reduced porosity due to cementation is3.3～19.5%, and the increased porosity due to dissolution is0.2～2.5%.Seven types of diagenetic facies have been grouped in the sandstone, including early cementation, compaction and pressolution-micropores, compaction and cementation-micropores, chlorite cementation-intergranular micropores, corrosion-intergranular micropores, chlorite cementation-intergranular pores, and corrosion-dissolution and intergranular pores. Chlorite cementation-intergranular micropores and chlorite cementation-intergranular pores are favorable diagenetic facies due to good reservior properties and wide distribution (distribution frequency>10%), corrosion-dissolution and intergranular pores and corrosion-intergranular micropores are relatively favorable diagenetic facies with distribution frequency4%, and the type of compaction and cementation-micropores is adverse diagenetic facies with bad reservoir properties and wide distribution (distribution frequency64.5%).4. Analysis of reservoir heterogeneityBased on the characteristics of pressure mercury curve, the Chang8reservoir can be divided into3types and6sub-types. The main feature of type1reservoir is low threshold pressure and relatively high permeability, with the porosity above10%and permeability above lmd. According to the distribution feature, type1can be divided into poorly sorted and well sorted sub-types. The main feature of type2reservoir is medium threshold pressure and permeability, with the porosity8%-10%and the permeability0.3-1mD. According to threshold pressure and distribution feature, type2can be divided into lower threshold pressure, well sorted and higher threshold pressure, well sorted sub-types. The main feature of type3is high threshold pressure and low permeability, with the porosity below10%and permeability below0.3mD. According to threshold pressure, permeability and sorting type, type3can be divided into high threshold pressure, low peameability, well sorted and very high threshold pressure, very low permeability, poorly sorted sub-types.The result of sensitivity report revealed that the Chang8reservoir are weak velocity sensitivity, weak-no water sensitivity, weak-no salinity sensitivity, strong-very strong soil acid sensitivity and medium-weak hydrochloric acid sensitivity.From the studies of the variation of sandstone grain size, grain sequence and physical property in single sand layer, the studies of interlayer distribution feature and the studies of the vertical variation of permeability, it was indicated that lithology has certain control function to physical property, but it is not the only control factor.The statistic anylysis of interlayer varation and heterogeneity coefficient of permeability from28wells indicate that the varation coefficient has the feature of double peaks, within which weak heterogeneity and strong heterogeneity make a high percentage, and the heterogeneity coefficient has the feature of single peak, within which weak heterogeneity makes a high percentage. Generally speaking, the interlayer heterogeneity of Chang8reservoir is not obvious.Using the constructed flow unit discrimination function, the Chang8reservoir can be divided into A, B, C and D4types, in which type A is confined to certain area and isolately distributed, type B and C are mainly distributed along with river channel, type D are mainly distributed at the edge of river channel.5. Comprehensive Reservoir EvaluationAn evaluation system and evaluation process of0.3mD-low-permeability reservoir is set up, suitable for research area and even the whole Ordos Basin. Physical parameter is the key parameter, lithologic parameter and oiliness are important parameters, and pore structure is a reference data. Hereby the C-8sandstone reservoir of the research area is divided into5kinds:Class Ⅰ reservoir (super-low permeability reservoir) is the best reservoir, Class Ⅱ reservoir (Class Ⅰ ultralow permeability reservoir) is favorable reservoir, Class Ⅲ reservoir (Class Ⅱ ultralow permeability reservoir) is comparatively favorable reservoir, Class Ⅳ reservoir (Class Ⅲ ultralow permeability reservoir) is unfavorable reservoir and Class Ⅴ reservoir is useless reservoir.The planar distribution of Reservoir of the research area is controlled by sedimentary facies zone. The horizons of ultralow permeability reservoirs grow fixedly, and the heterogeneity is strong, thus different kinds of sandstone formations distribute and superpose. In the longitudinal direction large-thickness subagueous distributary channel sandstone have stronger inheritance in small layers. Along the flow direction, the extended distance of sand bodies is relatively long, constituting the main reservoirs. Along the vertical flow direction, the sandbody present lenticular and the extended distance of sand bodies is relatively near. Different kinds of reservoirs distribute in ring zones on the plane. Class Ⅰ reservoir is located in the kern of the host sand body. Secondary reservoirs surround Class Ⅰ reservoir successively. The good reservoirs of the east Gansu Province is concentrated in Chang811and Chang822generally and Chang811is better than Chang822. Secondly, Chang812is better than Chang821and Chang812is better than Chang823.After clarifying the planar distribution and the vertical distribution of different kinds of reservoirs, the reserve evaluation of different kinds of reservoirs is analyzed. The reserve of different small layers and different kinds of reservoirs is calculated. In the east Gansu Province the total oil and gas reserves of Chang8are44963.58×104t. The oil and gas reserves of Class Ⅰ reservoir is996.88×104t, account for2.22%of the total reserve. The oil and gas reserves of Class Ⅱ reservoir is4440.61×104t, account for9.88%of the total reserve. The oil and gas reserves of Class Ⅲreservoir is21022.49×104t, account for46.75%of the total reserve. The oil and gas reserves of Class Ⅳ reservoir is18533.6×104t, account for41.22%of the total reserve. The oil and gas reserves of Class Ⅲ reservoir and Class Ⅳreservoir is account for87.97%of the total reserves, which is the main oil and gas development object. |
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