Key control factor for the gas-bearing properties difference of lower Paleozoic shale in southeast Chongqing

Southeast Chongqing is close to Fuling shale gasfield. Niutitang Formation and Wufeng- Longmaxi Formation are its two major explorative formations. Compared with Jiaoshiba area,the two formations experienced stronger refor- mation and had higher thermal maturity,and their explorative results differed obviously,which was caused by tectonic movement. The two formations in Southeast Chongqing have same material resource,but the gas-bearing properties has significant difference. Based on field desorption and isothermal adsorption experiment,according to the comparative study of static and dynamic indexes,the key control factors were discovered. Adsorption capacity of Niutitang Forma- tion was stronger,but its gas-bearing properties was poor that shown the characteristics of low hydrocarbon content and high nitrogen content. Adsorption capacity of Wufeng-Silurian Longmaxi Formation was weaker, but its gas-bearing properties was better than Niutitang Formation. There were several wells whose desorbed gas content reached industrial limit,and its gas composition was mainly hydrocarbon. It was proved that the organic pore structure and floor sealing ability of organic-rich shale were the main control factors for the gas-bearing properties difference of the two shale for- mations from the Lower Paleozoic in Southeast Chongqing. Organic pores of Niutitang Formation are underdeveloped, resulting in poor reservoir properties,and the unconformity of the bottom is accompanied by fluid activities,resulting in the destruction of gas reservoirs,which is the key to poor gas-bearing properties. Organic pores like honeycomb of Wufeng-Silurian Longmaxi Formation have greatly increased the space of shale gas accumulation,and the sealing of the above and under formation is better,which provides better accumulation conditions for shale gas. Then two typical models of shale gas accumulation for the Lower Paleozoic in the southeastern Chongqing were established including ac- cumulation-preservation model and washing-destruction model. In view of the differences in the key gas-controlling fac- tors of the two formations,it is suggested that the optimal indicators of favorable area should be further researched for Niutitang Formation, and the selection belt method for Wufeng-Silurian Longmaxi Formation should be further re- searched.