Compositional and Isotopic Characteristics for The Longmaxi Shale Gas in The Northern Guizhou Area, South China


 The organic-rich marine shale of the Lower Silurian Longmaxi formation in the northern Guizhou area (NGA), China, is characterized by its high thermal maturity (Ro values range in 2.18%~3.12%), high TOC values (0.92%~4.87%), high gas contents (0.47~2.69 m3/t) and type II1 organic matter, and has recently been a precursor for shale gas exploration and development. Compositional and isotopic parameters of 7 gas samples from Longmaxi shale from DY-1 well were analyzed in this study. Dry coefficient of the gases is up to 30~200 making the northern Guizhou Longmaxi shale gas among the driest gaseous hydrocarbons in the world. The δ13CCH4 values range from -38.6‰ to -18.6‰ and the δ13CC2H6 values vary in -36.2‰~-30.8‰. These results indicate that the Longmaxi shale gas is of thermogenic origin and oil derived. This Longmaxi shale gas has high proportion of non-hydrocarbon gases especially including nitrogen in response to complicate tectonic movements and strong hydrodynamic flushing. Tectonic movement and hydrodynamic flushing not only destroy hydrocarbon gases reservoirs but also change the isotope distribution of gaseous hydrocarbons. Isotopic reversal is frequent in closed system, and under relatively bad preserving condition, the isotope distribution will back to normal even at overmature evolution stage.

Isotopic reversals and molecular compositions of shale gas are helpful for re ecting its origin and preserve condition. Dai (1993) and Sherwood et al. (2008) discovered that δ 13 C methane maintain increase but δ 13 C ethane and propane will decrease due to oil-primary cracking with increasing thermal maturity, which is one reason of isotopic reversals. Hao and Zou (2013) reported that isotopic reversals usually occur in closedsystem shales. Martini et al. (2003) and Zhao et al. (2016) concluded that the tectonic movement and hydrodynamic ushing represent poor preserve condition and make gaseous hydrocarbons desorbed and increase the contents of non-hydrocarbon gases.
The northern Guizhou area (NGA) (Fig. 2), which was selected as a precursor of shale gas by the state of Guizhou province and the Ministry of Land and Resources (MLR), is located to the south of the Sichuan basin. The marine black shales are widely distributed in the Lower Silurian Longmaxi formation in the NGA.
The Longmaxi organic-rich shale has experienced complicated tectonic movements, which bring di cult to analyze its hydrocarbon generation history and gas preservation condition (Dai et al., 2015). In recent years, signi cant attention has been paid to the Longmaxi organic-rich shale in the NGA, however, these researches mainly concerned its sedimentary, mineralogical, geochemical and geophysical properties. The isotopic and compositional characteristics of gas itself were discussed rarely. The objective of this study is to determine the origin of hydrocarbon gases and discuss the implications of carbon isotopic reversals, based on the molecular and stable carbon isotope compositions of gaseous hydrocarbons.

Samples And Experiments
A total of 7 Longmaxi shale core samples (their fundamental information and main organic geochemical parameters can be seen in Table 1) were collected from shale gas well DY-1 in the NGA, and these fresh samples were placed immediately inside a hermetically sealed canister immersed in a water bath at the reservoir temperature. The volumes of gas released inside the canister were measured using a graduated cylinder at atmospheric pressure, and the loss gas volumes were evaluated by the method in Tang et al.
(2011). The gas samples were collected from the released gas, which has been considered as mixture of adsorbed and free gases . Note: GC is gas content; Sa is sapropelite; In is inertinite; Vi is vitrinite and KT is kerogen type.
The molecular compositions of gas samples diluted in hydrogen were determined using an Agilent 6890N Thermo Delta V Advantage isotope mass spectrometer (IMS). The stable carbon isotopic data were presented in δ-notation (δ 13 C, ‰) relative to VPDB standard, and each sample was measured in triplicate with a precision of less than ± 0.5‰. The molecular compositions and stable carbon isotopic values of gas samples are documented in Table 2.

Results
As shown in Table 1, the organic matters of the shale samples used in this study are in the overmature stage with R o of 2.18%~3.12%, indicating that the organic matters evolved into a dry gas window. Visual assessment of the organic matter reveals that the kerogen type index (TI) ranges from 47 to 57, which con rms that humic-sapropelic (II 1 ) as the kerogen type. TOC contents of these shale samples vary from 0.92-4.87%, and have a signi cantly positive correlation with the burial depth. The gas contents range from 0.47 m 3 /t to 2.69 m 3 /t with an average of 1.76 m 3 /t.

Molecular composition of shale gas
As can be seen in Table 2

Stable carbon isotopic composition of shale gas
As shown in Table 2, the δ 13 C 1 values of the gas samples in this study vary from − 36.2‰ to -18.6‰ with an average of -31.2‰, which is much wider and abnormal than those in the adjacent Sichuan basin (Feng et al.,   2016). The carbon isotopic composition of ethane in primary natural gas has been considered as having an outstanding parent material inheritance, and the δ 13 C 2 values of oil-type and coal-derived gases are < -29‰ and > -28‰, respectively (Gang et al., 1997). The δ 13 C 2 values of our gas samples range from − 36.2‰ to -30.8‰ (averaging − 34.1‰), which re ect an oil-derived characteristic. Dai et al. (2014) reported that the phenomenon of δ 13 C 1 > δ 13 C 2 (isotopic reversal) is common in the Sichuan basin, which has a close relationship with the high maturity of Longmaxi shale. Even though Longmaxi shales in the Sichuan basin and NGA have a similar R o distribution, the relationship of δ 13 C 1 and δ 13 C 2 is much complicate in the NGA than that in the Sichuan basin. As shown in Fig. 3, two of our gas samples (S5 and S7) have normal carbon isotope distribution (δ 13 C 1 < δ 13 C 2 ). Compared to S5 and S7, the others two samples (S2 and S3) have notably heavier carbon isotope and reversed carbon isotope distribution (δ 13 C 1 > δ 13 C 2 > δ 13 C 3 ).

Origin of alkane gases
Three genetic types of hydrocarbon gas (biogenic, thermogenic and mixed gases) can be identi ed using its molecular composition and carbon isotopes (Dai, 1993). Low δ 13 C 1 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 δ 13 C 1 value and a notable relationship of δ 13 C 1 value and thermal maturity (Dai, 2011). and NGA are oil-derived thermogenic gas, the Longmaxi shale gases have both higher thermal maturity and δ 13 C 1 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 R o 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.

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 (δ 13 C 1 < δ 13 C 2 < δ 13 C 3 ) (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 in uence of oil and associated gas cracking (Martini et al., 2008;Dai et al., 2014). Figure 5 shows a cross plot of δ 13 C 1 and δ 13  . 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 (R o 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 coe cient (C 1 /(C 2 + C 3 )), which can re ect thermal maturity, both ethane and propane δ 13 C values have two carbon isotopic rollovers (Burruss and Laughrey, 2010). All methane, ethane and propane δ 13 C values increase with increasing C 1 /(C 2 + C 3 ) when C 1 /(C 2 + C 3 ) lower than about 20, and they have normal isotopic distribution (δ 13 C 1 < δ 13 C 2 < δ 13 C 3 ). The ethane and propane δ 13 C values begin to decrease (become isotopically lighter, the rst rollover) at C 1 /(C 2 + C 3 ) around 20 due to the effect of oil and condensate cracking (Jarvie et al., 2007), and methane δ 13 C 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 R o value is higher than 2.0%), and 12 C ethane and 12 C propane are much easier decomposed than 13 C ethane and 13 C 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 (δ 13 C 1 > δ 13 C 2 ), and the others samples (S5 and S7) follow normal isotopic distribution (δ 13 C 1 < δ 13 C 2 ) even though they are overmatured with R o 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 ash 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.

Geological implications
Shale gas usually consists of methane, small amounts of heavy hydrocarbon gases (C 2 -C 6 ) and nonhydrocarbon gases (CO 2 , N 2 ). The proportion of heavy hydrocarbon gases (C 2 -C 6 ) tend to decrease with increasing thermal maturity (Zhang et al., 2014), and the proportion of non-hydrocarbon gases (CO 2 , N 2 ) 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 (R o value is 0.4%~1.0%). However, Martini et al. (1998Martini et al. ( , 2003 also found that Antrim shale gas has relative lower proportion of heavy hydrocarbon gases but rich in N 2 and CO 2 even though its thermal maturity (R o 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 N 2 and CO 2 , 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 13 C and propane (Zhao et al., 2016). At relatively high thermal maturity, simultaneous cracking of kerogen, retained oil and condensate resulted in rollover of ethane δ 13 C and propane δ 13 C, 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.

Conclusions
The northern Guizhou Longmaxi shale gas has abnormal high proportion of non-hydrocarbon gases especially including nitrogen as the result of complicate tectonic movements and strong hydrodynamic ushing, and is of thermogenic origin and oil-derived. The northern Guizhou Longmaxi shale gas evolved into dry gas window with R o value is ranging in 2.18%~3.12%, which is close to the thermal maturity of Longmaxi shale in the adjacent Sichuan basin (2.1%~3.2%). Even though they have a similar thermal maturity distribution, the northern Guizhou Longmaxi shale gas has abnormal high proportion of non-hydrocarbon gases (e.g. nitrogen) compared with the Sichuan basin Longmaxi shale gas due to the effects from complicate tectonic movements and strong hydrodynamic ushing. Tectonic movement and hydrodynamic ushing not only destroy hydrocarbon gases reservoirs but also change the isotope distribution of gaseous hydrocarbons. Isotopic reversal is frequent in closed system, and under relatively bad preserving condition, the isotope distribution will back to normal even at overmature evolution stage.      Ternary plot of shale gas components.