Influence Of Tectonic Compression On Shale Pore Structure Development:A Case Study And Simulation Experiments

Shale pore structure was extensively studied due to it is the reservoir space and transport tunnel of shale gas.Previous study showed the shale pore character was mostly contrlled by organic abundance,mineral composition and maturation level,but the impact of tectonic compression to shale pore structure is still not clear.In this work,samples from two Lower Cambrian Niutitang Formation shale profiles at different tectonic locations were selected for comprison of shale pore structure character by observing the field profiles,texting the mineral composition and geochemical data.The shale porosity,pore volume,specific surface area and pore size distribution were characterized by He pycnometry and low temperature N₂/CO₂ adsorption experiments.Also,thermal simulation experiments were conducted to low maturated Chang7 shale under 0/80MPa to characterize the variation of shale pore structure caused by different kind of compression condition.The conclusions are drawn:?1?The Lower Cambrian Niutitang Formation shales at different tectonic locations?the structural deformation zone Zunyi Songlin section and structural stable zone Youyang Danquan section?are over-mature stage,and the sedimentary environments at the two section showed slightly differences:Songlin section and the lower of Danquan section shales were sedimented in deep water anoxic environment,while the upper of Danquan section gradually transitioned to an oxygen-poor environment.The negative deviation of?¹³C_org showed weaken trend while the content of total organic carbon?TOC?and total sulfur?TS?gradually decreas upward from the bottom of the Niutitang shale profiles.The Niutitang shales are mostly composed by black carbonaceous and siliceous shale,and quartz,illite and feldspar are the main components of shale minerals.?2?According to the result of the low temperature N₂/CO₂ gas adsorption experiments,it showed that the OM pores of the Lower Cambrian Niutitang Formation shale were developed,and there is a good positive correlation between TOC and CO₂adsorption amount,micropore volume,specific surface area.The abundance of OM is the major factor affecting the development of shale micropores,but the controlling factor of mesopores and macropores is not obvious.Tectonic compression has little effect on the development of micropores,but the relative proportions of micro-,meso-and macropores showed variation at the two study sections.The proportion of micropores in shales at the deformed zone is relatively higher,and the result can be interpreted as:meso-and macropores transformed to micropores because of the compaction and collapse under the tectonic stress and structural deformation.Shales at the structural stability zone shale had notable higher BET specific surface area,N₂ adsorption amount and porosity than that of the deformed shales,but had lower micro-cracks formed through the tectonic deformation,while the latter has a larger BJH average pore width.?3?Semi-closed thermal simulation of chang7 shales illustrated the evolution of pore structure with increasing heat simulation temperature.The results indicated that the shale OMs'liquid hydrocarbons generation will cause blockages in some of the primary pores along with increasing temperature at the early stage of maturation to the peak stage of oil production?around 380°C?.At the stage,the pore specific surface area,pore volume,CO₂/N₂ adsorption amount all showed decreasing trends.A large number of secondary pores develop rapidly and the pores increase significantly at higher temperature.The volume of micropores and fine mesopores increase significantly after PE extraction.?4?Compression pressure has impaction to the pore structure of shales during thermal simulation experiments.Before the peak oil production period?less than 380??,the external 80MPa has little effect to shale pores structure.At 380°C⁴00°C,the existence of compression pressure could improve the extent of liquid hydrocarbons expulsion from the shale,which reduces the blockage of liquid hydrocarbons in the OM pores and mineral particle pores.As a result,some nanopores?<10nm?of the shales are released under 80MPa pressure than that conducted at no external pressure condition.After 400?,the existance of compression pressure may cause partial destruction of shale pore structure,expecially those large than 30nm diameter,and caused apparent lower pore volume compared with shale resudues conducted at no external compression pressure.