Enrichment Laws Of Shale-Gas In The Wufeng-Longmaxi Formation,Changning Area,Southern Sichuan Basin

The Changning area in South Sichuan is the most representative and demonstrative area of marine shale gas exploration and development in South China.The total proved reserves exceeded 600 billion cubic meters,and the daily gas production exceeded 10 million cubic meters.However,the recent years of exploration shows that it has been found that the shale gas wells that have been completed and put into production are mostly distributed near the changning anticline,while the exploration and development in the Ningxi area has failed repeatedly.Therefore,based on structural geology,low-temperature thermal chronology,fluid inclusions,and basin thermal history simulation,this paper carries out a comprehensive comparative study of Changning shale gas field area and Ningxi area,taking the hydrocarbon generation kinetics of the Wufeng-Longmaxi Formation and the characteristics of pressure system changes as the carrier during the multi-stage structural deformation process to realize the study of the process and characteristics of shale gas accumulation,revealing he coupling process and characteristics of Cenozoic strong uplift and shale gas enrichment and destruction between Changning shale gas field and Wufeng-Longmaxi formation in Ningxi area.The low temperature thermochronology and thermal simulation show that the amount of surface uplift and denudation in the late Meso Cenozoic in Changning area is 2500 m,and the amount of surface denudation in the Cenozoic in Ningxi area is about 2000 m.The thermal history of the Wufeng-Longmaxi Formation shale shows that the Changning shale gas field in the Middle Silurian-Middle Permian,Late Permian-Late Triassic,Early Jurassic-Early Cretaceous and Early Cretaceous Source rocks have entered low maturity,medium maturity,high maturity and over-maturity stages since the epoch;the four stages of low-medium maturity,high maturity,and over-maturity thermal evolution of source rocks in the Ningxi area were: Early Silurian-Late Triassic,Early-Middle Jurassic,and Late Jurassic-Cretaceous.The study of fluid inclusions in the Wufeng-Longmaxi Formation reveals that the fluid uniform temperature is characterized by a bimodal distribution of 130-140 ? and 160-170 ? in the Changning shale gas field and 120-140 ? and 160-180 ? in the Ningxi region,respectively.The test results in the Ningxi area shows that the drastic changes in the salinity of the late fluids indicates that they were damaged by the contaminated fluid and the preservation conditions.The shale system of the Wufeng-Longmaxi Formation in the Changning shale gas field area belonged to the atmospheric system for a long time,and the burial depth and temperature increase in the late Jurassic led to the generation rate peak and the formation pressure increased sharply,gradually forming overpressure,early-middle In the Cretaceous,its burial depth reached the maximum and the formation pressure coefficient reached over 2.0.Although the Cenozoic suffered uplift and erosion,its overpressure was still preserved,and the current pressure coefficient is still 2.03.Therefore,the shale gas reservoirs of Wufeng-Longmaxi Formation in Changning shale gas field area had the characteristics of early limited accumulation and middle-late enrichment-preservation overpressure accumulation.The black shale strata of the Wufeng-Longmaxi Formation in the Ningxi area have a long-term overpressure during the hydrocarbon generation period.The continuous deep burial in the Late Jurassic-Cretaceous resulted in the peak gas generation rate and the formation of abnormal overpressure(the formation pressure coefficient reached 2.2),But the significant uplift and erosion of the Cenozoic resulted in the destruction of the overpressure system,which is now a normal pressure system.Therefore,the shale gas accumulation process of Wufeng-Longmaxi Formation in Ningxi area has obvious multi-phase characteristics of early imited aggregation-deep buried ultrahigh pressure maintenance-late tectonic uplift adjustment/destruction.