Fracturing Geological Characteristics Of The Upper Palaeozoic Low Permeable Reservoir The Jiaxian-Qinjian Area,Eastern Ordos Basin

Ordos Basin is one of the largest bases for exploring and developing petroleum and natural gas in China.At present,up to hundreds of millions of cubic meters of natural gas reserves have been prospected in the east of the basin,where Upper Paleozoic strata develop well with great exploration potential.Nonetheless,complicated diagenesis of reservoirs,mechanical properties of rocks in these reservoirs and unapparent distribution laws of structural cracks are major causes of poor fracture reconstruction in that area.To improve fracture reconstruction of gas reservoirs in the research area,this paper systematically studies petrographic/mechanical properties and fracture development characteristics of the eighth member of the Shihezi Formation on dominant gas-bearing layers of Jia County and Qingjian County.Main conclusions and understandings are as follows:1.By petrographic observations,upper-paleozoic low-permeability sandstones of the research area are divided into matrix filling and strong compaction altered tight facies,quartz-enlarging siliceous cementation-intergranular pore facies,carbonate cementation metasomatism tight facies,calcium dissolution pore facies,matrix dissolution pore facies and clastics dissolution pore facies.2.For sandstones with different diagenetic facies in reservoirs of the research area,there are certain differences in compressive strength,elastic modulus and poisson ratio.The higher the content of brittle minerals and siliceous cements in sandstones and the higher the degree of compaction and recrystalization,the higher the compressive strength will be and vice versa.The elastic modulus is smaller in diagenetic facies with higher porosity.The poisson ratio is lower in diagenetic facies with more brittle minerals,lower compaction degree and higher porosity,and vice versa.3.In reservoirs of the research area,certain differences exist in fracability of sandstones with different diagenetic facies.The fracability will be high in quartz-enlarging siliceous cementation-intergranular pore facies,calcium dissolution pore facies,matrix dissolution pore facies and matrix filling and strong compaction altered tight facies when their brittleness coefficient is high.The fracability of these diagenetic facies is relatively low.4.In the research area,high-angle shear cracks are major ones that develop at an angle above 60°,among which the cracks of the near EW direction are the most developed.According to their field cross-cutting relationships,cracks may be classified into crack systems in the near S-N direction,NW-SE and NNE-SSW directions.The crack systems in near E-W and S-N directions indicate the tectonic stress field of the Yanshan period,while the crack system in NW-SE and NNE-SSW directions denote the tectonic stress field of the Indosinian period.To fracture artificial fractures,it is necessary to evade relatively developed crack systems of the research area and avoid opening original natural fractures during fracturing operations,in order that new artificial fractures and corresponding network can form after the fracturing to significantly increase fracturing effects.