4.1. Origin of alkane gases
Three genetic types of hydrocarbon gas (biogenic, thermogenic and mixed gases) can be identified using its molecular composition and carbon isotopes (Dai, 1993). Low δ13C1 value (usually, < -55‰) and high methane content (usually, > 1 mol/L) are two recognizing characteristics of biogenic gas (Martini et al., 2003). Compared with biogenic gas, thermogenic gas has a much higher δ13C1 value and a notable relationship of δ13C1 value and thermal maturity (Dai, 2011). Figure 4 shows a cross plot of gas molecular composition and carbon isotope of gaseous alkanes from the northern Guizhou Longmaxi shale gases, and reflects that the shale gas samples in the NGA, as well as the adjacent Sichuan basin (Feng et al., 2016) and Barnett and Fayetteville shale gases (Zumberge et al., 2012), are oil-derived thermogenic gas.
Wu et al. (2011) reported that coal-derived gas in the Sichuan basin has a negative correlation of δ13C1 value and thermal maturity. Even though all Barnett, Fayetteville and Longmaxi shale gases in the Sichuan basin and NGA are oil-derived thermogenic gas, the Longmaxi shale gases have both higher thermal maturity and δ13C1 value and belong to cracking gas, and a large apart of Barnett and Fayetteville shale gases are associated gas. As can be seen in Fig. 1, oil-cracking gas has higher thermal maturity than associated gas, and is composed of primary (condensate gas) and secondary (dry gas) oil-cracking gases. In this study, the organic matters are in the overmature stage with Ro of 2.18%~3.12% (Table 1), indicating that the organic matters evolved into a dry gas window. As a result, the gaseous hydrocarbon of northern Guizhou Longmaxi shale contains very low heavy hydrocarbon gases and large proportion of methane.
4.2. Isotopic rollovers and reversals
With the recent exploration and development successes in shale gas, it has been demonstrated that isotopic rollover and reversal are more commonplace in shale gas than in conventional reservoir (Zhao et al., 2016). The “rollover” means that the isotopic compositions change with increasing thermal maturity, and the “reversal” refers to that carbon isotopic sequence do not follow the normal carbon isotopic sequence (δ13C1 < δ13C2 < δ13C3) (Strapoc et al., 2010). A complete and a partial carbon isotopic reversals of n-alkane gases are probably due to (1) mixing of different origins of gases, (2) mixing of different types of gases, (3) the influence of oil and associated gas cracking (Martini et al., 2008; Dai et al., 2014).
Figure 5 shows a cross plot of δ13C1 and δ13C2 values, which reflects that all gases from the Antrim shale (Ro value is 0.4%~0.6%), New Albany shale (Ro value is 0.4%~1.0%) and Jurassic shale in Jafurah basin (Ro is 1.00%~1.55%), and most gases from the Barnett shale (Ro value is 0.5%~2.0%) have normal carbon isotope distribution (δ13C1 < δ13C2), whereas all gases from the Fayetteville shale (Ro value is 1.25%~4.00%), Lower Cambrain shale (Ro value is 2.2%~3.5%) and Wufeng-Longmaxi shale (Ro value is 2.1%~3.2%) in the Sichuan basin display isotopic reversals (δ13C1 > δ13C2). That is to say the thermal maturity has dominating effect on carbon isotopic reversal, and higher thermal evolution gas has more possibility to exhibit reversal. Owing to the reservoir openness and the effects from migration fractionation, the conventional natural gases in the Sichuan basin follow the normal distribution even though its high thermal maturity (Ro value is 2.2%~3.5%). Closed shale system is another prerequisite for isotopic reversal (Golding et al., 2013). As shown in Fig. 6, with the increasing dry coefficient (C1/(C2 + C3)), which can reflect thermal maturity, both ethane and propane δ13C values have two carbon isotopic rollovers (Burruss and Laughrey, 2010). All methane, ethane and propane δ13C values increase with increasing C1/(C2 + C3) when C1/(C2 + C3) lower than about 20, and they have normal isotopic distribution (δ13C1 < δ13C2 < δ13C3). The ethane and propane δ13C values begin to decrease (become isotopically lighter, the first rollover) at C1/(C2 + C3) around 20 due to the effect of oil and condensate cracking (Jarvie et al., 2007), and methane δ13C value maintains increase since it mainly generated by biological degradation (Martini et al., 1998). The second rollover occurred due to the decomposition of ethane and propane (Hill et al., 2003). Ethane and propane begin to decompose into methane at high thermal maturity (maybe corresponding Ro value is higher than 2.0%), and 12C ethane and 12C propane are much easier decomposed than 13C ethane and 13C propane due to their weaker polarity (Zumberge et al., 2012).
As shown in Fig. 5, only a half of our samples (S2 and S3) are reversed (δ13C1 > δ13C2), and the others samples (S5 and S7) follow normal isotopic distribution (δ13C1 < δ13C2) even though they are overmatured with Ro values (Table 1) are approximated to those of the Wufeng-Longmaxi shale in the Sichuan basin. The main reasons of those odd normal isotopic distributions are the inferior preserving condition of reservoir. Large faults zones and the flash of spring water in them have damaged or even destroyed the shale gas reservoir, and as a result, the contents of non-hydrocarbon gases are prodigiously high and the evolution trend of carbon isotopes of alkane gases are disorganized.
4.3. Geological implications
Shale gas usually consists of methane, small amounts of heavy hydrocarbon gases (C2-C6) and non-hydrocarbon gases (CO2, N2). The proportion of heavy hydrocarbon gases (C2-C6) tend to decrease with increasing thermal maturity (Zhang et al., 2014), and the proportion of non-hydrocarbon gases (CO2, N2) are mainly affected by tectonic movement and hydrodynamic condition (Dai et al., 2008). As shown in Fig. 7, Barnett shale gas, Fayetteville shale gas and Longmaxi-Wufeng shale gas in the Sichuan basin are rich in methane and have been successfully developed, in which the methane percentage contents are no less than 80% (Zumberge et al., 2012; Dai et al., 2016). Martini et al. (2008) reported that New Albany shale gas has large proportion of ethane and propane (the content of ethane + propane can exceed 45%) duo to its low thermal maturity (Ro value is 0.4%~1.0%). However, Martini et al. (1998, 2003) also found that Antrim shale gas has relative lower proportion of heavy hydrocarbon gases but rich in N2 and CO2 even though its thermal maturity (Ro value is 0.4%~0.6%) is little lower than that of New Albany shale gas, and they believed that this phenomenon has been caused by the effect of Pleistocene glaciation. Our samples also have very little heavy hydrocarbon gases and rich in N2 and CO2, however, the differences between our samples and Antrim shale are (1) thermal maturity is much higher than Antrim shale, (2) no glaciation but large faults zones and springs are widespread (Fig. 2). Heavy hydrocarbon gases are gradually decomposed into methane in overmatured stage, and the faults zones and springs generated by tectonic movement damage gas reservoir. As a result, northern Guizhou Longmaxi shale gas is rich in non-hydrocarbon gases, and is lack of heavy hydrocarbon gases. Owing to their same tectonic evolution history and geological background, the Lower Cambrain shale, which is another important organic-rich black shale in the NGA, has the same molecular composition with the Longmaxi shale.
As shown in Fig. 6, carbon isotopic distribution changes with increasing thermal maturity. At lower thermal maturity, kerogen cracking in a closed system resulted in increasing ethane 13C and propane (Zhao et al., 2016). At relatively high thermal maturity, simultaneous cracking of kerogen, retained oil and condensate resulted in rollover of ethane δ13C and propane δ13C, and the resultant conversion of isotopic distribution patterns from normal though partial reversal to complete reversal. Carbon isotopic distribution can be used to distinguish gas origins combined with molecular composition. The isotopic reversal demonstrates a closed shale system, which has a better preserving condition than opened system (Xia et al., 2018). The larger the reversal degree is, the higher the gas content is. Moreover, the carbon isotopic distribution is also useful to evaluate reservoir preserve condition. Isotopic reversal is frequent in closed system, and under relatively bad preserving condition, the isotopic distribution will back to normal even at overmature evolution stage.