Generation Potential and Characteristics of Kerogen Cracking Gas of Over-Mature Shale

Abstract

To investigate the characteristics and generation potential of gas generated from over-mature shale, hydrous and anhydrous pyrolysis experiments were carried out on the Longmaxi Formation in the Anwen 1 well of the Sichuan Basin of China at temperatures of 400–598 °C and pressures of 50 Mpa, with (hydrous) and without (anhydrous) the addition of liquid water. The results show that in the presence of water, the total yield of carbon-containing gases (i.e., the sum of methane, ethane, and carbon dioxide) was increased by up to 1.8 times when compared to the total yield from the anhydrous pyrolysis experiments. The increased yield of carbon dioxide and methane accounted for 89% and 10.5% of the total increased yield of carbon-containing gases. This indicated that the participation of water could have promoted the release of carbon from over-mature shale, like we used in this study. The methane generated in the hydrous pyrolysis experiments was heavier, with a δ¹³C value of −21.27‰ (544 °C) compared to that generated in the anhydrous pyrolysis experiments, which showed a lighter δ¹³C of −33.70‰ (544 °C). It is noteworthy that the δ¹³C values of the methane from hydrous pyrolysis at >500 °C were even heavier than that of the kerogen from the over-mature shale, although the δ¹³C values of the methane show an overall increasing trend with increasing temperature both in hydrous and anhydrous pyrolysis. The carbon dioxide from hydrous pyrolysis was less enriched in ¹³C relative to that from anhydrous pyrolysis. Specifically, the δ ¹³C values of the carbon dioxide increased with the increasing temperature in anhydrous pyrolysis, whereas they remained nearly constant with increasing temperature in hydrous pyrolysis. The overall lighter δ¹³C values of the carbon dioxide generated in the hydrous pyrolysis experiments likely indicate that water tends to prompt the release of lighter carbon and/or suppress the release of heavier carbon from over-mature shale in the form of carbon dioxide, especially at higher temperatures, for example, of >510 °C.

1. Introduction

Due to increased temperature and pressure, kerogen is transformed into oil and natural gas, which results in a marked decrease in the H/C value during catagenesis and a slight decrease in the H/C value during methanogenesis [1]. The slow and slight elimination of hydrogen at the methanogenesis stage indicates the hydrocarbon generation potential of kerogen is nearly depleted. Hydrogen in kerogen is the dominant element that controls the yield and characteristics of hydrocarbons. Water and minerals could be exogenous sources of hydrogen for petroleum generation [1,2,3,4,5,6,7,8,9,10]. The participation of water in the kerogen maturation process could result in a variation in the yield, composition, biomarkers and isotopes of petroleum [5,11,12,13,14,15,16,17,18,19]. Immature kerogen is rich in hydrogen. The H/C value of the immature kerogen decreases dramatically during catagenesis, leading to the occurrence of an oil and gas window. The gas generation potential may be significantly underestimated by the petroleum generation model constrained without water [1]. Gas generated from experiments that involve the heating of immature kerogen to mature kerogen usually mix gas from oil cracking gas with mature or high-maturity kerogen cracking gas. Hence, it is not ideal to use immature–mature kerogen to study the characteristics of kerogen cracking gas generated during methanogenesis. As hydrogen was nearly depleted in over-mature kerogen, the participation of water should be taken into consideration.

Over-mature shale gas was discovered in Longmaxi shale in the Sichuan Basin [20,21,22,23]. Silurian Longmaxi shale in the Sichuan Basin is the most important shale gas reservoir in China and its maturity is 2.0–4.0% [23,24,25]. It was generally considered that the shale gas was oil cracking gas derived from its deep burial [26,27]. Taking the petroleum expulsive efficiency into consideration, the retained oil in the shale will decrease sharply with increasing temperature. At the methanogenesis stage, kerogen cracking gas may contribute a lot to shale gas. However, the contribution and its influence on shale gas geochemistry is unknown. Generally, the δ¹³C value of methane is lighter than that of kerogen. Recent research has shown that the δ¹³C value of desorption gas from Longmaxi shale in Jiaoshiba in the Sichuan Basin is heavier than that of kerogen [28]. The δ¹³C value of methane from anhydrous pyrolysis in highly over-mature shale was lighter than that of kerogen [29,30,31], but the δ¹³C value of methane from hydrous pyrolysis was different; the heavy δ¹³C in methane may be related to the water-facilitated pyrolysis of kerogen [32]. This indicates that the presence of methane with a very heavy δ¹³C value might be related to water. In this study, experimental pyrolysis was conducted on over-mature kerogen of Longmaxi shale with and without the addition of liquid water to understand the influence of water on kerogen cracking gas generation from over-mature Longmaxi shale. (Here, “hydrous” and “anhydrous” indicate that the experiments are conducted with or without the addition of extra water).

2. Materials and Methods

2.1. Materials Description

The sample used in this study was isolated from the Longmaxi shale of the well Anwen 1. The well Anwen 1 is located in the Sichuan Basin in southern China. The total organic carbon (TOC) of the shale was 3.6%. The maturity (Ro) of the kerogen was 2.3% with an H/C value of 0.6. The carbon isotope of the isolated kerogen was −29‰. The hydrogen isotope of water used in this study was −433.3‰.

2.2. Experimental Methods

Experiments conducted in this study were performed in sealed gold capsules at the Guangzhou Institute of Geochemistry, Chinese Academy of Science. Aliquots of the isolated kerogen (60–80 mg) with and without addition of extra water were loaded into gold capsules (80 mm × 5 mm i.d.), which were then sealed under argon gas. The gold capsules were heated from about 23 °C to 400 °C in two hours and from 400 °C to 598 °C with a heating rate of 1.5 °C/h under 50 MPa pressure. Experimental temperatures were 400 °C, 436 °C, 472 °C, 508 °C, 544 °C, 560 °C, 580 °C, and 598 °C. Gas yield, carbon and hydrogen isotope of methane, carbon isotope of kerogen and CO₂, and H/C value of kerogen were analyzed. Gases generated during experiments were analyzed on HP 5890 GC equipped with a Poraplot Q column (30 m × 0.25 mm × 0.25 μm). The temperature of the GC oven was programmed from an initial temperature of 40 °C for 2 min, followed by heating at 4 °C/min to 180 °C, with a 10 min hold. Carbon isotope analysis was carried out on Delta plus XL GC-IRMS [33].

3. Results

3.1. Yields and Composition of Gas Products

The yields of methane and carbon dioxide are the highest in the gases generated from anhydrous pyrolysis (Figure 1a and Figure 2). The yields of H₂ and ethane are lower than 0.2 mL/g TOC (

Table 1. Yields, carbon, and hydrogen isotope of kerogen cracking gas in anhydrous and hydrous pyrolysis.

Pyrolysis Type	Sampling Temperature (°C)	Kerogen (mg)	Water (μL)	Yields (mL/g TOC)				Carbon Isotopes (‰ vs. VPDB)		Hydrogen Isotope (‰ vs. VSMOW)
				C1	C2	CO2	H2	δ13CCH4	δ13Cco2	δ2HCH4
Anhydrous	400	80	0	0.05	0.011	2.18	0.00	nd	−27.59	−230.09
	436	80	0	0.54	0.010	3.12	0.01	−40.49	−26.49	−143.55
	472	80	0	1.04	0.007	2.60	0.02	−35.22	−25.74	−99.43
	508	70	0	3.34	0.011	5.23	0.13	−31.45	−24.31	−78.84
	544	70	0	5.71	0.009	8.32	0.18	−33.70	−20.32	−52.98
	562	70	0	7.02	0.010	10.21	0.20	−32.78	−19.36	−70.55
	580	60	0	8.03	0.010	9.24	0.17	−25.30	−16.67	−51.20
	598	60	0	9.02	0.005	9.81	0.20	−27.53	−14.99	−36.31
Hydrous	400	80	80	0.05	0.013	2.69	0.01	nd	−27.78	−506.88
	436	80	80	0.26	0.009	4.11	0.08	−38.06	−26.29	−490.46
	472	80	80	0.65	0.006	5.47	0.13	−35.79	−25.56	−455.47
	508	70	70	0.80	0.005	10.22	0.40	−24.51	−25.33	−455.59
	544	70	70	1.31	0.020	19.45	1.69	−21.27	−25.55	−455.03
	562	70	70	2.32	0.043	22.13	2.83	−24.20	−25.71	−454.47
	580	60	60	6.59	0.104	30.90	6.51	−21.70	−25.67	−449.37
	598	60	60	12.63	0.146	40.47	8.86	−26.11	−26.61	−446.25

nd = not detected.

). The amount of methane increases with increasing temperature. The generation process of methane could be divided into three stages according to the accumulated yield trend. The yield of methane has a slow growth between 400 °C and 472 °C, a marked increase from 508 °C to 562 °C, and a gentle increase from 580 °C to 598 °C (Figure 3a). The yield of carbon dioxide is stable from 400 °C to 472 °C, then increases slightly from 508 °C to 562 °C, and finally becomes stable again from 580 °C to 598 °C (Figure 3b). The total released carbon from kerogen exhibits a similar trend with the accumulated yield curve of carbon dioxide (Figure 2).

The yield of carbon dioxide is the highest in the gases generated from hydrous pyrolysis. The amounts of methane and H₂ are much less than that of carbon dioxide (Table 1 and Figure 1b). Methane production in hydrous pyrolysis increases slightly from 400 °C to 544 °C and then increases sharply from 562 °C to 598 °C (Figure 3a). The yields of H₂ and ethane are stable from 400 °C to 544 °C, then increase dramatically from 562 °C to 598 °C (Figure 3b,d). Carbon dioxide production has a steady increase from 400 °C to 544 °C and a rapid increase from 562 °C to 598 °C (Figure 3b). The total released carbon from kerogen has a similar trend to that of carbon dioxide (Figure 2).

The yield of methane generated from hydrous pyrolysis is slightly lower than anhydrous pyrolysis from 400 °C to 472 °C. From 472 °C to 544 °C, the yield of methane in hydrous pyrolysis is dramatically lower than in anhydrous pyrolysis and the gap in the methane yield between hydrous and anhydrous pyrolysis becomes larger with increasing temperature. From 544 °C to 580 °C, this gap reduces quickly with increasing temperature. After 580 °C, the yield of methane in hydrous pyrolysis is larger than that in anhydrous pyrolysis (Figure 3a). The yield of carbon dioxide generated from both hydrous and anhydrous pyrolysis increases with increasing temperature. The difference in the carbon dioxide yield between hydrous and anhydrous pyrolysis becomes larger with increasing temperature (Figure 3b). The yields of ethane and H₂ are pretty low from anhydrous pyrolysis and do not increase with temperature increase. The yields of ethane and H₂ from hydrous pyrolysis did not increase much until 544 °C (Figure 3c,d).

3.2. Carbon Isotope of Methane and Carbon Dioxide

The carbon isotopes of methane show a similar trend between anhydrous and hydrous pyrolysis. The carbon isotope of methane becomes heavier when the temperature increases from 400 °C to 508 °C, lighter from 508 °C to 544 °C, heavier from 544 °C to 580 °C, and finally decreases to the carbon isotope of kerogen at 598 °C in anhydrous pyrolysis (Figure 4a). In hydrous pyrolysis, the carbon isotope of methane becomes heavier when the temperature increases from 400 °C to 544 °C, lighter from 508 °C to 562 °C, heavier from 562 °C to 580 °C, and finally decreases to the carbon isotope of kerogen at 598 °C (Figure 4a). At the same temperature, the carbon isotopes of methane in hydrous pyrolysis are heavier than that in anhydrous pyrolysis. The presence of water leads to a heavier carbon isotope of methane than that of kerogen. The heavier carbon isotopes of methane emerge at lower temperatures in hydrous pyrolysis than in anhydrous pyrolysis. The carbon isotopes of methane are lighter than the carbon isotopes of kerogen from 400 °C to 562 °C and heavier from 580 °C to 598 °C in anhydrous pyrolysis (Figure 4a). The carbon isotopes of methane are lighter than the carbon isotopes of kerogen from 400 °C to 472 °C and heavier from 508 °C to 598 °C in hydrous pyrolysis (Figure 4a).

The carbon isotopes of carbon dioxide have different trends with temperature increases in anhydrous and hydrous pyrolysis. The carbon isotopes of carbon dioxide become heavier as the temperature increases in anhydrous pyrolysis, with the biggest carbon isotope variation being 12.6‰ (Figure 4b). While the variation in the carbon isotopes of carbon dioxide, about 2.45‰, is relatively slight in hydrous pyrolysis. The carbon isotopes of carbon dioxide become heavier as the temperature increases from 400 °C to 508 °C and then slightly lighter after 508 °C (Figure 4b). From 400 °C to 472 °C, the carbon isotopes of carbon dioxide remain nearly the same in hydrous and anhydrous pyrolysis. After 472 °C, the carbon isotopes of carbon dioxide in anhydrous pyrolysis are much heavier than those in hydrous pyrolysis and the variation becomes larger with increasing temperature. To investigate the carbon isotope of released carbon-containing gases from kerogen, this study proposes a parameter named δ¹³C_{Yield average}. In anhydrous pyrolysis, the δ¹³C_{Yield average} has temperature increases from 436 °C to 580 °C with the largest variation being about 7.9‰ and then drops slightly at 598 °C (Table 1, Figure 5). The largest variation in the δ¹³C_{Yield average} is about 1.7‰ in hydrous pyrolysis. Figure 5 shows that the δ¹³C _{Yield average} is heavier in hydrous pyrolysis at temperatures from 436 °C to 544 °C, while, at temperatures from 562 °C to 598 °C, the δ¹³C_{Yield average} in hydrous pyrolysis is lighter than that in anhydrous pyrolysis.

3.3. Hydrogen Isotope of Methane

Hydrogen isotopes of methane become heavier with increasing temperature in hydrous and anhydrous pyrolysis (Figure 6). In hydrous pyrolysis, the hydrogen isotope of methane is heavier than the hydrogen isotope of water used in this study. The hydrogen isotope of methane gradually becomes more similar to the hydrogen isotope of water as the temperature increases (Figure 6).

4. Discussion

Compared with anhydrous pyrolysis, the accumulated yield of methane generated at 598 °C from hydrous pyrolysis increased from 9.02 mL/g TOC to 12.63 mL/g TOC (Table 1). Previous studies on kerogen cracking gas have achieved a larger yield of methane, CO₂, and H₂ at high temperatures in hydrous pyrolysis (Wang et al., 2013) [33]. The instantaneous yield of methane showed that the peak of kerogen cracking methane was postponed when water was involved in kerogen pyrolysis (Figure 7). The generation peak of kerogen cracking methane occurs at about 530 °C, with a peak yield of about 3.2 mL/g TOC in anhydrous pyrolysis of over-mature kerogen, while the generation peak of kerogen cracking methane occurs at about 630 °C or higher, with a peak yield of 8 mL/g TOC or more in hydrous pyrolysis (Figure 7). The yield of kerogen cracking methane is about 0.46 m³/t in hydrous pyrolysis at 598 °C (Table 1). The gas content in Longmaxi shale from the Sichuan Basin is 1.9–8.4 m³/t (Zou et al., 2016) [24]. The yield of kerogen cracking gas generated during metagenesis could be up to 5.4–24.2% of the total shale gas content.

Methane produced from over-mature kerogen in hydrous pyrolysis has a heavier carbon isotope than kerogen. It was reported that the carbon isotope of desorbed gas in Longmaxi shale in the JY 2 well in Sichuan could be much heavier than that of kerogen [28]. The abnormally heavier carbon isotope of methane than kerogen might be caused by the cracking of kerogen with the presence of water during metagenesis. Hydrous pyrolysis of over-mature kerogen during metagenesis may further lead to carbon isotope reversal of shale gas in the Longmaxi Formation in the Sichuan Basin [26]. Anhydrous and hydrous pyrolysis shows that the effect of water on the hydrogen isotope of kerogen cracking methane is significant (Figure 6). The hydrous pyrolysis of immature kerogen indicated that inorganic hydrogen from water added to, and/or exchanged with, organic hydrogen with increasing temperature [12].

The kerogen cracking process during metagenesis in hydrous pyrolysis could be divided into three stages. (1) From 400 °C to 472 °C, the gas yield, gas composition, as well as the carbon isotope of methane and carbon dioxide are nearly the same between hydrous and anhydrous pyrolysis. These indicate that water has little influence on kerogen cracking gas generation at this stage. (2) From 472 °C to 544 °C, water starts to play a significant role in kerogen cracking gas generation. The yield of methane increases rapidly in anhydrous pyrolysis, whereas there remains a slow increase in hydrous pyrolysis. Water suppresses methane generation but stimulates carbon dioxide generation at this stage. The yield of total carbon-containing gases increases with increasing temperature in both hydrous and anhydrous pyrolysis. Methane is more enriched in ¹³C compared to carbon dioxide-generated hydrous pyrolysis. The δ¹³C_{Yield average} is heavier in hydrous pyrolysis than in anhydrous pyrolysis. (3) From 544 °C to 598 °C, the water–kerogen interaction may facilitate the cracking of some particular functional groups in kerogen. The accumulated yield of carbon-containing gases reached a plateau in the anhydrous pyrolysis at this stage. The yield rate of methane becomes slower and slower with increasing temperature. This could be due to the depletion of hydrogen in kerogen. However, in hydrous pyrolysis, the yields of carbon-containing gases and methane increase dramatically at this stage. This means that water facilitates the cracking of some particular functional groups that could not crack without water to generate hydrocarbon gases and carbon dioxide. These particular functional groups in kerogen exist in relatively lighter carbon isotopes. This results in a heavier δ¹³C_{Yield average}.

5. Conclusions

Carbon dioxide and methane are major products of kerogen cracking during metagenesis. Water enhances the yields of carbon dioxide, methane, and H₂. The yield of kerogen cracking methane generated during metagenesis could reach up to 5.4–24.2% of the total shale gas content in Longmaxi shale. The instantaneous yield peak of kerogen cracking methane was postponed as water was involved in kerogen pyrolysis. Hydrous pyrolysis of over-mature kerogen results in methane with a heavier carbon isotope than that of kerogen. Hydrous pyrolysis of over-mature kerogen during metagenesis may further lead to carbon isotope reversal of shale gas in the Longmaxi Formation in the Sichuan Basin. The kerogen cracking process during metagenesis in hydrous pyrolysis could be divided into three stages. From 400 °C to 472 °C, water has little influence on kerogen cracking gases. From 472 °C to 544 °C, water starts to play a significant role in kerogen cracking gases. From 544 °C to 598 °C, the water–kerogen interaction may facilitate the cracking of some particular functional groups, which could not crack without water in kerogen to generate methane with a heavier carbon isotope and carbon dioxide with a lighter carbon isotope.

5.1. Formula

$${\mathsf{\delta }}^{13}{\mathrm{C}}_{\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d} \mathrm{a}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{g}\mathrm{e}}=\frac{{\mathsf{\delta }}^{13}{\mathrm{C}}_{\mathrm{C}\mathrm{H}4}\ast {\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}}_{\mathrm{C}\mathrm{H}4}+{\mathsf{\delta }}^{13}{\mathrm{C}}_{\mathrm{C}\mathrm{O}2}\ast {\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}}_{\mathrm{C}\mathrm{O}2}}{{\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}}_{\mathrm{C}\mathrm{H}4}+{\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}}_{\mathrm{C}\mathrm{O}2}}$$

Yield_CH4 and Yield_CO2 are the yields of methane and carbon dioxide, respectively.

5.2. Uncertainty Analysis

The thermal simulation experiment is a crucial step in the exploration of oil and gas resources. It simulates the formation process of hydrocarbons in the geological strata by heating and compressing the sample under certain conditions. This process allows researchers to understand the properties and characteristics of hydrocarbons and estimate the amount of oil and gas that can be extracted from a particular region. There are several factors that contribute to the uncertainty in the results of thermal simulation experiments. Some of these factors include the following:

Sample heterogeneity: The sample used in the experiment may vary in composition, texture, and other properties. This heterogeneity can affect the accuracy of the simulation results.

Temperature and pressure control: It is difficult to perfectly control the temperature and pressure during the experiment. Small variations in these parameters can lead to significant changes in the formation process of hydrocarbons.

Experimental setup: The experimental setup, including the type of equipment used and the way it is configured, can also affect the results. Different setups may lead to different outcomes even when using the same sample and conditions.

Interpretation of results: The interpretation of experimental results is subjective and can vary between different researchers. The choice of data analysis method and interpretation can introduce uncertainty into the final conclusions.