Research On Diagenesis And Pore Evolution Of Chang 3 Low Permeability Reservoir Of Yanchang Formation In Longdong Area

The Chang3 oil reservoir in Longdong area belongs to the typical low-porosity and low-permeability tight sandstone reservoir,with abundant oil and gas resource,and it is of great significance to study the diagenesis and the characteristics of pore evolution.Based on the datas from 18 key wells,a lauge number of cores,cast slice,and scanning electron microscopy of the study area,by means of the X-ray diffraction,high pressure mercury injection,constant pressure mercury injection,and micro-CT scanning and other techniques,we have conducts the comprehensive research of the petrological characteristics,physical property and pore structure,the diagenesis and the characteristics of pore evolution,also we discussed the effects of diagenesis on physical properties.The analysis of the petrological characteristics of Chang 3 reservoir in Longdong area exhibites a northeast-southwest difference.The rock types in the northeast source area are mainly lithic feldspar sandstone,while the rock type of southwest source area are mainly feldspar lithic sandstone.The physical properties of Chang 3 reservoir is characterized by the statistical datas:the average porosity of Chang 3 reservoir is 13.35%and the permeability is 4.90×10-1 ?m2.The reservoir properties of Chang 31 sand group are the best.In terms of pore structure,the Chang 3 oil group is dominated by residual intergranular pores,followed by feldspar and lithic dissolution pores.The main type of throat are schistose,shrinkage and narrow neck type.The reservoir properties and pore structure of the main source area in southwest are better than those in the northeast,and the source area in south is the worst.The Chang 3 oil group of the stury area has a high degree of compaction,the plastic debris and mica are squeezed to form the fake miscellaneous to filling pores.The cementing minerals are chlorite,kaolinite,illite,iron dolomite,iron calcite and siliceous,with strong cementation.The dissolution is developing of Chang 3 oil group,and the dissolution of feldspar and lithic is the most common type.In terms of diagenetic intensity,the apparent compaction,cementation and dissolution rates of Chang 3 reservoir are 45.5%,55.83%,31.17%respectively.In the distribution of plane,the compaction and dissolution of the main source areas in southwest are higher than those in the northeast,the cementing strength of northeastern main source area is higher than that in the southwest.The most favourable diagenetic facies band of Chang 3 reservoir in the study area is the facies that are characterized by strong compaction,medium cementation and strong dissolution,its mainly distributed in Taibailiang-Tongchuan area of the southwest,and Mayingfang-Danba-Majiabian area of the the northeast.Chang 3 reservoir group in the study area is in the A period of middle diagenetic evolution stage,and some latest to B period of middle diagenetic evolution stage.The diagenetic sequence in the study area is as follow:early compaction?plastic debris extrusion and deformation?chlorite film precipitation?dissolution of feldspar and cuttings?quartz secondary growth ?-? ?authigenic kaolinite,Illite precipitation?authigenic quartz grains?iron-containing carbonate cementation.The initial porosity of Chang 3 reservoir in Longdong area is 38.9%.After strong compaction and cementation,the average reduction of the porosity by compaction and cementing are 17.8%,9.5%respectively,and the average increment of porosity of dissolution is only 3.74%.The plane distribution features display that,the reduction of the porosity by compaction and the increment by dissolution in the southwest are higher than those in the northeast,while the cement reduction is lower than that of the northeast main souce area.The compaction is the most important factor for the decrease of porosity and the deterioration of physical properties of Chang 3 reservoir in Longdong area.The authigenic chlorite,illite clay minerals,late carbonate,quartz secondary growth and authigenic quartz cementation lead to a further decrease in porosity and permeability of the reservoir.,and kaolinite precipitation has two-side effect on reservoir properties.The dissolution of feldspar and lithic improve the physical property of reservoir.