**1. Introduction**

Shale gas is produced from organic-rich black shale and self-generation and self-storage natural gas, and is continuously accumulated in nano-scale micropores in shale [1]. Shale gas is a thermogenic natural gas generated by organic matter pyrolysis. The shale gas relative elemental abundance patterns and isotopic compositions fluctuate continuously throughout the pyrolysis process due to the fractionation effect [2,3].

Shale gas is produced in a closed system and gas does not easily migrate. As a result, compared with conventional natural gas it has a greater genetic accumulation impact [4–6]. Hence, shale gas maintains more of the original information regarding the means of oil and gas generation from source rocks than conventional natural gas does, and its geochemical characteristics could be a reflection of the evolutionary process of closed-system fossil energy production. Natural gas formation theory has focused on the generation and evolution of shale gas, since differences the in geochemical characteristics between conventional natural gas and shale gas were discovered [7–11]. Shale gas geochemical irregularities include (1) the rollover of iso-alkane/normal alkane ratios [12]; (2) the rollover of ethane and propane isotopic compositions [13]; and (3) abnormally light ethane and propane δ13C values and isotope reversals among methane, ethane, and propane [11,12,14–16]. Together, these irregularities reflect the complicated history of shale gas generation and the isotopic fractionation associated with it, as well as the in situ "mixing and accumulation" of gases that are generated from different precursors at various thermal maturities [4]. In addition, shale gases from different areas around the world also have many different geochemical characteristics [7,12,17–20]. Even if these shale gases come from the same area and same strata, variations in molecular composition, carbon isotopic composition, and noble gas abundance and isotopic composition can be found [15,16]. Recently, we found that the gas geochemical characteristics of shale gases from the Longmaxi Formation, Sichuan Basin, China, show several apparent differences between the Weiyuan (WY) and Changning (CN) areas [14–17,21–23]. For instance, there is more CH4 in CN shale gas, and its carbon isotope composition is heavier than that of WY shale gas [14,16,17,21–23]. Meanwhile, the He and Ar abundance and isotope composition are higher in WY shale gas than in CN shale gas [15]. Although previous studies have found differential gas geochemical characteristics between WY and CN shale gas, few studies have explained the potential reason for these differences.

We collected and compared the geochemical data from our previous works [14–16] and other studies [16,21,22] and combined the geological background and oil/gas generation theory to clarify the causes and mechanics of variations in the geochemical characteristics of shale gas from the Longmaxi Formation, Sichuan Basin, China. These results should increase our understanding of the generation and evolution of shale gas.
