Study On Pore And Fracture Structure Of Typical Gas Shales

As the main storage and migration space,the pore and fracture in shale have been a research focus in shale gas exploration and development industry.In this study,the Lower Paleozoic marine shale from southeast Chongqing,the Jurassic continental shale from the Ordos Basin and the Permian continental-continental transitional shale from the South China North Basin were selected as the main research objects.A combination of direct observation and physical testing methods were used to characterize the pore and fracture system.The conceptual models of the pore-fracture structure of different types of shale were established.Under the control of organic matter(OM)type and maturity,the organic pores forms because of hydrocarbon generation in the Paleozoic marine shale are most developed.Due to the low maturity and high percentage of humic OM,OM pores caused by hydrocarbon generation in the Jurassic continental shale are rare.The organic pores in the Jurassic continental shale are mostly inherited from the original biological structure and most of the organic pores in the Permian transitional shale are pores between OM and clay particles or pores caused by clay shrinkage.The contrasting results between the three suites of shale indicate that the degree of primary pores decreased with increasing thermal maturity.No micron-sized pores were found in the over-mature Paleozoic marine shale,and the appearance of residual nanoscale intergranular pores,intragranular pores and dissolution pores indicate the pores have been strongly compacted.The compaction degree,consistent with thermal maturity,is an important controlling factor on the development of primary pores.Due to the fierce regional tectonic movement,the tectonic fractures in the Paleozoic marine are developed and the horizontal fractures are mostly distributed along weakly stripped surfaces of mineral or OM at the edge of the bedding.The fractures in the Jurassic continental shale are mainly stratigraphic joints between the shale and interbedding silty layer.As the Permian transitional shale is rich in clay minerals,the microscopic fractures therein are mainly shrinkage fractures or interlayer fractures within clay while the macroscopic fractures are mainly between lithologic interfaces.Shales' lithofacies and microstructures controlled by depositional environment are the main cause of the difference of depositional fractures in shale.This study indicates that the pore-fracture system of Paleozoic marine shale,which is composed of organic pores,horizontal bedding fractures and high angle tectonic fractures,is easy to form a network for shale gas strorage and migration.The pore-fracture system of Jurassic continental shale which is composed of organic pores/intragranular pores,intergranular pores and horizontal bedding fractures can form good horizontal channels for shale gas.There is a lack of medium-sized connected channel between the tiny granular pores in the Permian transitional shale and the large-scale lithologic bedding fractures,which is not conducive to the desorption and diffusion of shale gas.