| Guided by theories of unconventional oil and gas reservoir geology and fuzzy mathematics,this paper intensively studied the petrological characteristics,microscopic reservoir space,reservoir properties,densification and diagenesis,and the coupling of compaction and compaction in the tight reservoirs of the Yanchang Formation in the southwestern Ordos Basin,basing on a large number of core observations,thin section identification,scanning electron microscopy,energy spectrum analysis,field emission scanning electron microscopy,and general mercury intrusion and constant rate mercury injection experiments.On this basis,this dissertation deeply discussed the controlling factors of tight reservoirs and conduct fuzzy comprehensive evaluation of the tight reservoirs in this area.We had achieved the following results and understanding:The sedimentary facies of the tight reservoirs of the Yanchang Formation in Ordos Basin were dominated by the delta front subfacies and semi-deep lake-deep lacustrine facies.The petrological characteristics were characterized by fine grain size,moderate size classification,poor roundness,and high heterogeneity.The prominent characteristics of dense mica reservoirs in this area were high levels and outstanding illite cementation content.Compact reservoirs had small pores,fine throat,and poor physical properties.The porosity distribution ranged from 8%to 11%,the average porosity was 8.22%,the permeability was between 0.01 and 1×10-3?m2,and the average permeability was 0.27×10-3?m2.Porosity and permeability of tight oil reservoir had a relatively strong correlation.However,in the process of burial and diagenesis of tight oil reservoirs,the relationship between permeability and porosity was not constant.The key difference between the physical properties of tight oil reservoirs and conventional reservoirs lied in the fact that under similar porosity,the permeability of tight oil reservoirs was much lower than that of conventional reservoirs.This was due to the high content of illite in the tight oil reservoirs of the area.Illite was bridging in the pores and channels and caused them severely blocked,which affect the seepage ability of the reservoir.The permeability of the reservoir was greatly reduced during the period of large formation of illite,and this period was the key to the formation of tight reservoirs.Therefore,the study of permeability change and porosity of tight oil reservoirs were equally important.There was a significant positive correlation between the physical properties of dense reservoirs and the grain size.The larger the grain size,the better the physical properties.The porosity and permeability of the northern and western parts of the study area were generally high.This was due to the fact that the northern and western parts of the plane were dominated by the delta front,and the grain size of the sediments was relatively coarse,while the southern part was dominated by the lakes.The difference in physical properties between the dry layer and the oil layer and the oil and water layer was not obvious.The factors determining the formation of tight reservoirs as the oil layer,the dry layer or the oil and water layer were mainly the characteristics of the microscopic reservoir space and the pore structure of the rock.It was also related to the distance from the source rock and the dredging of the fracture in the reservoir.In tight reservoirs,intergranular dissolved pores,intragranular dissolved pores,and austenite intercrystalline micropores were common,among which intergranular micropores were dominated by intergranular microlites of illite and kaolinite.Although intercrystalline pores could be used as an effective hydrocarbon storage space,their connectivity was poor.Although the dissolved pores in the tight reservoir were relatively common,due to the compactness of the reservoir and poor seepage,the scope of the dissolution was not large,and the connectivity of the dissolved pores was poor.The pore size of tight oil reservoirs was mostly in the micron?>1?m?and nanoscale?<1?m?,and the pore size was significantly smaller than that of conventional reservoirs.The overall characteristics of the pore-throat configuration of tight reservoirs were as follows:the average throat of tight oil reservoirs was 0.08?m,good separation,and good pore-throat connectivity.The positive correlations between the maximum connected throat adius,the median radius,the maximum mercury saturation and the particle diameter were obvious,and the peak throat of fine sandstone was significantly larger than silty fine sandstone.Using correlation analysis,factor analysis and cluster analysis,eight parameters including porosity,permeability,discharge pressure,and maximum throat radius were selected as indexes for evaluation of pore structure,and the pore structure of tight reservoirs was divided into Class I,II,III,IV.The four pore structures exhibited different characteristics from the characteristics of porosity,permeability,expulsion pressure,median radius,and capillary pressure curve.The pore structure of tight oil in the Yanchang Formation of Ordos Basin was dominated by Type I and Type II,followed by Type III and Type IV.In the original sedimentary conditions with fine grain size and high content of hetero-bases,the densifying diagenesis in the study area were dominated by compaction and cementation.Compaction performance was particularly evident.Tight oil sandstone was rich in granularity,heterogeneity,and mica content,and was strongly compacted.The phenomenon of compaction under the microscope showed that the debris particles had a directional arrangement,and the particle contact was mainly line contact.Rigid particles could be broken,plastic debris,mica and intergranular miscellaneous bases were significantly compacted.Throughout the burial process,the compaction effect had greatly reduced the pore permeability of the reservoir.Cementation was also an important reason for the poor physical properties of dense reservoirs in this area.The degree of siliceous cementation in cementation was not high,carbonate cementation was generally developed,and the type of cementation was dominated by late iron carbonate cementation.It was worth noting that the clay minerals in the tight reservoirs of this area were very common,and the impact on the permeability was particularly prominent.It could make the effective large pores and throats into small pores and inefficient throats with poor seepage ability.This was an important reason for the poor seepage capacity of tight reservoirs in this area.Based on the discussion and analysis of diagenesis and the formation order of authigenic minerals,combined with the environment of mineral formation and fluid properties of reservoirs,the characteristics of each diagenesis were integrated to determine the diagenetic evolution sequence of tight reservoirs in the study area.According to the sequence of cementation-dissolution in the tight reservoir,the diagenetic process of the tight reservoir was divided into four stages:early compaction stage;alkaline cementation stage;acidic dissolution stage and iron calcite and iron dolomite cementation stage.For these four stages,the physical properties of tight reservoirs were simulated.Because compaction was the most important cause of porosity reduction in tight reservoirs,it was necessary to allocate the amount of compaction first.Based on the calculation of the total amount of compaction,according to the changes in the depth of burial corresponding to each stage,the amount of compaction was distributed proportionally to the three stages of early compaction stage,alkaline cementation stage and acidic dissolution stage.According to calculations,the compaction loss porosity in the above three stages was 18.39%,4.29%,and 6.74%,respectively.Through the statistics of the content of cement,the porosity of pores and the ratio of microporosity,the quantitative analysis of porosity evolution was performed,and the porosity of the reservoir at the end of each stage was estimated to be 19.01%,16.88%,13.42%,and10.1%,respectively.Because the relationship between permeability and porosity was not a single function in the process of burial of tight reservoirs,a simulation model of pore-infiltration correlation was established at different periods,and the permeability was estimated by the porosity at the end of each stage.The calculated results were1.33×10-3?m2,0.43×10-3?m2,0.27×10-3?m2,0.152×10-3?m2,respectively.The measured air permeability of the tight reservoir sample in this area was 0.18×10-3?m2,and the relative error of the fitting permeability was 16%.The simulation results showed that the permeability of the reservoir decreased rapidly during the alkaline cementation stage and reached the limit of the tight reservoir at the end of the acidic dissolution stage.Through the dynamic recovery of the history of tight reservoirs and burial history and the history of hydrocarbon generation,it was presumed that a large-scale ripening oil filling had begun before the reservoir reached permeability threshold.After the formation of dense reservoirs,oil and gas filling did not stop immediately.The time relationship between densification and reservoir-forming should be almost at the same time,oil accumulation continued after densification.Based on the analysis of the control factors of tight reservoirs,twelve evaluation indicators related to the four parameters of petrological characteristics???,pore characteristics???,reservoir properties???,and pore structure???were selected to comprehensively evaluate the reservoirs.The results of calculation by stratigraphic analysis indicated that the primary controlling factors affecting the evaluation of tight reservoirs were:reservoir properties>pore structure>pore characteristics>petrological characteristics?0.4233>0.2705>0.1608>0.1453?.According to the comprehensive weight,the influence of each secondary index on the total target level could also be judged.Among them,porosity,permeability,and face ratio had the greatest impact on the total target layer.By using the analytic hierarchy process and fuzzy mathematics methods,a fuzzy AHP comprehensive evaluation model for tight reservoirs was established.The membership function calculations were performed for each of the five blocks in the study area,and the evaluation grade of each block was calculated according to the principle of maximum membership degree.The Liwan-Huanxian area was preferred as a favorable area for the comprehensive evaluation of tight reservoirs,and was the preferred area for further exploration and development of tight oils.The lithology of the Liwan-Huanxian area was dominated by feldspathic litho fine sandstone,with good sorting,moderate levels of miscellaneous and mica,and relatively high face ratios,with an average of 2.57%.The physical properties of the core showed good porosity and permeability,the average porosity was11.42%,and the average permeability is 0.35×10-3?m2.The reservoir space was mainly composed of intergranular pores and dissolved pores,containing a small amount of micropores,with thick pores and throats and good connectivity.The expulsion pressure was less than 3MPa,and the maximum mercury saturation was more than 70%.It had good reservoir performance and was a high quality,tight reservoir.The Heshui-Taerwan area and the Qingcheng-Qingyang area were secondary favorable areas.This thesis could provide reference for the next step of tight oil exploration and development work in the study area,and it had the important theoretical and practical significance. |
PDF Full Text Request