Natural Gas Generation Potential of the Wufeng-Longmaxi and Niutitang Shale Formations, Sichuan Basin, China: Insight from Hydrous and Anhydrous Pyrolysis
Xiao, Christopher 1992-
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A comparative study was conducted on the residual shale gas generation potential of the Silurian Wufeng-Longmaxi and Cambrian Niutitang formations of the Sichuan Basin, China. Previous studies focused on geological variables affecting gas adsorption and retention in the Silurian and Cambrian shale systems. However, relatively few have investigated the gas generation potential of the source rocks using artificial thermal maturation, e.g., pyrolysis, to investigate hydrocarbon generation potential. In this study, hydrous and anhydrous pyrolysis experiments were conducted on black shale core and outcrop samples to evaluate and compare the characteristics and gas generation potential of the Silurian and Cambrian source rocks and the potential role water may play in late stage gas generation. The results show that the critical factors on residual gas generation potential included thermal maturity, TOC, and mineralogy, which are intercorrelated. Silurian and Cambrian shales have very high %Ro (2.1 – 3.3%), suggesting main stage gas was generated early on during burial history. The majority of source rocks showed very low S1 + S2 peaks from Rock-Eval pyrolysis, suggesting no residual gas generation potential based on conventional thermal cracking mechanism. Source rocks which underwent hydrous pyrolysis generated 5 – 77 times greater yields of gas compared to anhydrous Rock-Eval pyrolysis. The Silurian consistently produced 2 - 10 times greater yields of gas through hydrous pyrolysis compared to equivalent Cambrian samples. However, Cambrian core and outcrop samples generated higher wet gas (C2 – C5) ratios than the Silurian, a pattern observed by other Sichuan Basin studies. Hydrous pyrolysis experiments combined with other conventional approaches suggested the Silurian is the primary target for Sichuan shale gas exploration, but the Cambrian could have potential if other favorable conditions are met. Both formations have comparable source rock and reservoir characteristics to the Barnett, Eagleford, and Woodford shales in N. America, with the exceptions of very high maturity. Conventional thinking based on thermal cracking would suggest the Longmaxi and Niutitang shales no longer have any residual gas generation potential based on Rock-Eval, however, the results from hydrous pyrolysis suggest the participation of water may play an important role in late-stage overmature gas generation.