Hydrocarbon generation and shale gas accumulation in the Longmaxi Formation, Southern Sichuan Basin, China

The gas-bearing Longmaxi Formation (Fm.) records a complex tectonic history in which variations in structural and burial history directly effect the hydrocarbon maturation process. Through analysis of the tectonic-burial history, hydrocarbon maturity, and EASY%Ro numerical simulations we examine three areas representing the full spectrum of geological variability in the southern Sichuan Basin, the Changning area, the Luzhou area and the Zigong area (Zishen well-1). Following deposition of the Longmaxi Fm., all three areas underwent five major tectonic events of subsidence and uplift, showing an overall long-term oscillation of the basin. The Longmaxi Fm. has undergone three major heating stages, Caledonian- Hercynian orogenesis time, the high geothermal gradient field of Hercynian and Indosinian time (approximately 200-300 Ma), and the Yanshanian-Himalayan stage. The high geothermal gradient stage is associated with extensive Emeishan flood basalts or concealed basalts, which could have played an important role in hydrocarbon maturation. The maturation process can be divided into five stages: Caledonian, Hercyninan, Indosinian, Yanshanian, and Himalayan. In the Yanshan period, the highest heating temperature of organic matter exceeded 200 °C and the maturation level was more than 2.4%, producing large amounts of natural gas, which is the main accumulation period of shale gas in the Longmaxi Fm. Due to the large amount of hydrocarbon generated, a large number of micropores were formed from organic matter evolution, especially nanoscale pores, during the middle diagenesis stage. The Longmaxi Fm. shale gas accumulation process is divided into four periods: the source-reservoir-cap deposition period, the initial accumulation period, the main accumulation period, and the adjustment period. In the main accumulation period, pyrolysis gas was dominant, which was stored within the pores, diagenetic fractures, and structural fractures in adsorbed and free state ways, forming the basic pattern of the Longmaxi Fm. shale gas. The current burial depth and structural stability of the Longmaxi Fm. are critical to gas reservoir protection.