Research paperGeochemical characteristics of natural gases related to Late Paleozoic coal measures in China
Introduction
Lithologically, coal measures are primarily composed of coal, black mudstone, and carbonaceous shales characterized by kerogen type III and gases generated from such humic source rocks are usually defined as coal-type gas. In contrast, gases generated from sapropelic source rocks characterized by kerogen type I–II are usually defined as oil-type gas. As early as the 1960s, German scholars had realized that coal measures could form commercial gas accumulations (Jüntgen and Karweil, 1966a; b; Lutz et al., 1975; Stach et al., 1982; Diamond, 1993; Behar et al., 1995). Research on coal measures and related natural gas started in China in 1979 (Dai, 1979) resulting in successful coal-type gas prospecting over the past 30 years. By 2011, the total proven reserves and yearly production of coal-type gas reached 5813.5 × 109 m3 and 64.8 × 109 m3, respectively, accounting for 69.7% and 63.2% of their respective totals in China (Dai et al., 2016).
From the Early Cambrian to the Tertiary, there are eight major coal-forming periods in China, of which the Late Carboniferous–Early Permian and the Late Permian are the most significant (Han and Yang, 1980). The Late Paleozoic coal measures primarily occur in the Upper Carboniferous Benxi Formation (C2b Fm), Lower Permian Shanxi (P1s) and Taiyuan (P1t) Fms in East and Central China (Han and Yang, 1980; Dai et al., 2005), the Lower Carboniferous Dishuiquan (C1d) and Upper Carboniferous Bashan (C2bs) Fms in Northwest China (Wang et al., 2013; Dai et al., 2016), and the Upper Permian Longtan (P2l) Fm in South China (Hu et al., 2014) (Fig. 1, Fig. 2). In addition, a set of humic source rocks interbedded with coal seams and fragments of charred plants is encountered in the Middle Permian Wuerhe (P2w) Fm with a significant thickness in the Junggar Basin (Wang et al., 2013) (Fig. 1, Fig. 2d).
By 2013, more than 20 gas fields related to the Late Paleozoic coal measures have been discovered in China, distributed in the Bohai Bay, Ordos, Sichuan, and Junggar Basins (Fig. 1). The total proven reserves of these gas fields is more than 3200 × 109 m3, accounting for 30.1% of the proven gas reserves in China (Fang et al., 2016). Of these gas fields, 12 have proven reserves of more than 30 × 109 m3, including the Sulige gas field, the largest gas field in China, whose gas production reached 21.2 × 109 m3 in 2013 and accounts for 17.6% of the total gas production in China (Fang et al., 2016; Dai et al., 2017). Clearly, the gas fields related to the Late Paleozoic coal measures play an important role in the Chinese natural gas industry.
Since these gas fields experienced complex geological processes, the gas origins (Dai et al., 2005; Ma et al., 2008; Liu et al., 2014a,b; 2016; Tu et al., 2016) and relevant secondary alterations (Cai et al., 2004; Hu et al., 2013a, 2014) are still much debated. Previous studies primarily focused on geochemical characteristics of natural gases from some specific gas fields (i.e., Dai et al., 2005; Li et al., 2008a; Hu et al., 2010a, 2013a; Chen et al., 2014), which lacked the comparison between gases from different gas fields or different proliferous basins. After 30 years of exploration, more and more wells have been drilled in these fields, providing us with a more comprehensive understanding of the geochemical characteristics of the natural gas. Our study examines 375 gas samples (96 gas samples were newly collected while the remaining data are from previously published results) gathered from the Bohai Bay, Ordos, Sichuan and Junggar basins (Fig. 1). After analyzing their molecular compositions [(C1–C5, carbon dioxide (CO2) and nitrogen (N2)] and stable carbon isotopes (C1–C3 and CO2), in conjunction with evidence from helium isotopes, the genetic types and origins of the natural gas, as well as the secondary alterations it experienced, are discussed. All gases in this study are produced from conventional reservoirs. Unconventional gases, such as shale gas and coal bed methane, are not included.
In the 21st century, oil and gas exploration is becoming increasingly difficult as the target strata turn older and deeper. Our study is not only of great significance for the future gas exploration in China but can also provide insights on gas exploration in other areas worldwide, which share a similar geological background.
Section snippets
Bohai Bay Basin
The Bohai Bay Basin, located in eastern China, is a rift basin on the Paleozoic North China Craton with an area of 600 × 103 km2 (Cao et al., 2001; Qi and Yang, 2010). It can be subdivided into several secondary tectonic units such as the Xialiaohe, Huanghua, Jizhong, Jiyang, Linqing, Dongpu, and Bozhong depressions (Yang and Xu, 2004; Zhao et al., 2015, Fig. 1a). In general, two petroleum systems, the Cenozoic–Mesozoic and Paleozoic, are developed in the Bohai Bay Basin (Cao et al., 2001; Zhao
Sample
This study examines data for 375 gas samples gathered from the Bohai Bay (Table 2), Ordos (Table 3), Sichuan (Table 4) and Junggar basins (Table 5). 96 gas samples were newly collected at the well heads, stored in 1 L gas-tight cylinders, and analyzed for this study. The remaining data are from previously published results (Xu et al., 1994; Dai et al., 2005, 2016; Jiang et al., 2005; Wen et al., 2005; Lin et al., 2007; Wang et al., 2007, 2013; Hao et al., 2008; Li et al., 2008c, 2009; Jin et
Molecular compositions of the natural gas
The natural gases in our study have alkane gas contents varying from 22.1% to 100.0% (90.5% on average). Even though the methane content varies greatly from 22.1% to 99.6% (86.2% on average), 96.1% of the samples are between 70.0% and 100.0% (Table 2, Table 3, Table 4, Table 5). Natural gas with low methane content is primarily distributed in the Bohai Bay and Ordos Basins, where gases usually have relatively high contents of hydrogen sulfide (H2S) and CO2 (Table 2, Table 3). The total content
Genesis of natural gas
Within intervals of similar thermal maturity, natural gases generated from coal measures generally have less negative carbon isotope compositions than those from sapropelic source rocks (Bernard et al., 1978; Schoell, 1980). In the plot of δ13C-CH4 versus C1/C2+3 (Bernard et al., 1978; Whiticar, 1994), most gas samples from the Bohai Bay, Ordos, and Junggar Basins are located along the thermal evolution trend of gas generated from kerogen type III (Fig. 5), which correlates well to the coal
Conclusions
Natural gases related to the Late Paleozoic coal measures in China are primarily distributed in the Bohai Bay, Ordos, Sichuan, and Junggar Basins, and are dominated by coal-type gases. Most of the gases are characterized by the usual positive carbon isotopes series. The observed carbon isotope reversal between the δ13C-C2H6 and δ13C-C3H8 values, encountered in samples from the Bohai Bay and Ordos Basins, may most probably result from mixing of coal-type gases generated at different maturities.
Acknowledgements
This work is funded by the Fundamental Research Programs of PetroChina (No. 2014B–0608 and No. 2016B–0301).
References (140)
- et al.
Geochemistry and origin of the world's largest gas field from Persian Gulf, Iran
J. Petrol. Sci. Eng.
(2006) The isotopic composition of radiogenic helium and its use to study groundwater movement in confined aquifers
Chem. Geol.
(1985)- et al.
Experimental simulation of gas generation from coals and a marine kerogen
Chem. Geol.
(1995) - et al.
Maturity related mixing model for methane, ethane and propane, based on carbon isotopes
Org. Geochem.
(1988) - et al.
Empirical carbon isotope/maturity relationships for gases from algal kerogens and terrigenous organic matter, based on dry, open-system pyrolysis
Org. Geochem.
(1996) - et al.
Primary cracking of a1gal and landplant kerogens: kinetic models of isotope variations in methane, ethane and propane
- et al.
Marine hydrocarbon source rocks of the Upper Permian Longtan Formation and their contribution to gas accumulation in the northeastern Sichuan Basin, southwest China
Mar. Petrol. Geol.
(2014) - et al.
Carbon and hydrogen isotopic reversals in deep basin gas: evidence for limits to the stability of hydrocarbons
Org. Geochem.
(2010) - et al.
Methane-dominated thermochemical sulphate reduction in the triassic feixianguan formation, east Sichuan Basin, China: towards prediction of fatal H2S concentrations
Mar. Petrol. Geol.
(2004) - et al.
Geochemistry and origins of natural gases in the central Junggar Basin, northwest China
Org. Geochem.
(2012)