The Characteristics And Distribution Prediction Of Fractures In Lower Palaeozoic Shale,Southeast Of Chongqing Area

Shale gas is a kind of unconventional gas resources which is very important to China.Fracture is a kind of important reservoir space in shale reservoirs,because of the characteristics of low porosity and low permeability of shale,The southeast area of Chongqing is the pilot experimental area in China for key strategic investigations of shale gas.The particularity and complexity of tectonic in this area increases the difficulty of shale fractures research.In this paper,intensive research had been done on both identification and prediction of Lower Palaeozoic shale reservoir.The quantitative characterization methods had been built,and the stress field had been simulated for predicting fractures in work area.The controlling factors of tectonic and non tectonic fractures,which were definited clearly,were analysed quantitatively and qualitatively.The relationship of fractures development and shale gas content was revealed ultimately.Our works will be produced during future shale gas exploration and development in this area.The main conclusions and results in this thesis were as follows:As a result of Yanshan movement,threre are trough like folds-battlements fold-ejective folds,distribution of which is regular,from southeast to northwest in study area.Tectonic deformation is stronger in east and southeast of study area than in northwest,therefore,the stress intensity is higher in east and southeast than in northwest.Fracture zones are mainly concentrated in fold axis,transition location of fault and fold,the regions where are adjacent to faults.The fragile mineral contents in shelf facies rock from south of study area are higher than that in bathyal facies rock and abyssal facies rock from center of study area.Shale from south of study area characterized by low Poisson's ratio and high Young's modulus crack more easily during Himalayan orogenic movement.The natural macro-fractures,which tectonic fractures were predominant,are abundant in Lower Palaeozoic shale.The micro-fractures could be characterized quantitati-vely by analysing results of isotherms and SME images.The results indicated that non tectonic micro-fractures were developed in Bianmachong Shale,while the nanometer-scale pores were predominant in Longmaxi Shale and Niutitang Shale,followed by the non tectonic micro-fractures.The controlling factors of natural tectonic fractures in Lower Palaeozoic shale are tectonic stress,fragile mineral and TOC content,all of these controlling factors,show positive correlation with the fractures quantity.Tectonic factors are the principal factors that control fracture development in the Paleozoic shale of this region,followed by the fragile mineral content of the shale.The controlling factors of natural non tectonic fractures in Lower Palaeozoic shale,which shrinkage fractures predominates,are clay mineral content,palaeoenvironment and diagenesis.The illite and illite smectite mixed layer clay content show positive correlation with the shrinkage fractures quantity.The clay combination type and paleosalinity value revealed that Lower Palaeozoic shale was developed in the basin with moderate paleosalinity.The smectite content would reduced meanwhile the illite content and shrinkage fractures would increased in longitudinal direction of Lower Palaeozoic shale,however,these changes were not as continuous as freshwater basins.The effective fractures of Shale made a small contribution to reservoir porosity(?2%)but a great contribution to reservoir permeability(?50?m2).The effective fracture not only contributed to the adsorption desorption of shale gas,increased the free gas content,but also became a channel of gas migration,mining The intergranular pore is connected by microfractures,thereby,the validity of porosity had been improved.