Investigations Of Nanopores Evaluation,Influence Factors And Impact Mechanisms On The Gas Content In Marine Shale,China

Shale gas resources are belonging to unconventional natural gas.Shale gas resources are rich and the development potential is large in our country.Compared with shale in America,the shale formation is older and higher maturity in our country,which causes the differential in microstructure and gas content.Consequently,it is important to study on the evoluation of microstructure and influence factors and mechanism of gas content.They will be of great importance in guiding the target zone selection of shale gas and reducing the risk in shale gas exploration and development.The gas cotent was mianly deteminded by total orangic carbon content（TOC）.According to shale methane isothermal adsorption experiments,the adsortption capacity was increased with the TOC.Comparing the specific surface area（SSA）between organci matter（OM）and shale,the SSA of OM is much higher than shale.As a result,the high TOC of shale cause large SSA,high adsorption capacity and gas content.The SSA and gas content of shale were mianly impacted by the development and pore distributrion of OM hosted pore.The major contributions of SSA were micropore and mesopore structure in the shale.The thermal decomposition of immaturity shale was carried out and the microstructure was characterized in different temperature.With the temperature rising,the SSA was decreased first and then increased and the turning point of temperature was 450℃.When the temperature is more than 450℃,the shale was generated to hydrocarbon,the pore and fracture were generated in the OM and the SSA was increased.For pore size distribution,the pore volume of micropore was obviously increased,the mesopore was slight increase and macropore remain about the same.Microscopic characterization experiments were performed before and after shale hydration.The size of OM hosted pore remained the same after shale hydration,but the fracture was produced in and edge of OM and between shale mineral grains.Because of dissolution of carbonate rock,many dissolution pores were produced and the pore size was range from several micrometers to tens micrometers.At the beginning of shale exploration in China,the Longmaxi formation（LF）and Qiongzhusi formation（QF）shale were considered as the target formation,but the LF shale was high gas content and the QF was low gas content from the practice development.In order the reveal the discrepancy of production between LF and QF shale,the reservoir characteristics were compared between these two formations.It is found that the TOC and mineral component were similar,but the thermal maturity was much higher in QF shale.The OM hosted pores were better development in LF shale and QF with TOC less than 1.0%,and poor development in high TOC of QF shale.Consequently,the gas content was enlarged with the increasing of TOC in LF shale,but not in QF shale,especially in the high TOC of QF shale.A new parameter V_TOC,TOC multiply with surface porosity of OM,presented to estimate the QF shale and should be consider as a new optimum index during the shale gas target selection in QF shale.