Origin and depositional environments of source rocks and crude oils from Niger Delta Basin: Carbon isotopic evidence

Genesis Types and Migration of Middle and Lower Assemblages of Natural Gas in the Eastern Belt around the Penyijingxi Sag of the Junggar Basin, NW China

“…This partial reversal may be due to the mixing of natural gases of different genesis, migration, or secondary changes [11,12]. Primary alkane gases should have a positive carbon isotopic series (δ 13 C 1 < δC 13 2 < δ 13 C 3 < δ 13 C 4 ). In addition, while the isotopic composition of methane of inorganic origin is generally greater than −30‰, this value of the gases in the studied area is less than −30‰ (Figure 4).…”

“…There are two types of natural gas in sedimentary basins, inorganic and organic [1], and organic gas is further divided into oil-associated and coal-derived gas [2][3][4]. Inorganic gas is potentially associated with magmatic and deep-sea hydrothermal activity [5,6], with methane isotopes (δ 13 C 1 ) within the range of −50‰ (generally −30‰) to 10‰ [7,8], while organic gas is derived from the pyrolysis of kerogen in sedimentary rocks and the secondary cracking gas of crude oil, with methane isotopes in the range of −75‰ to −30‰ [7]. The inorganic alkane gas polymerized step-by-step to form long-chain alkanes through C-C bonding and the lower bond energy of 12 C- 12 C caused 12 C to join the polymerization reaction first, showing a negative carbon isotope series of δ 13 methane (C 1 ) > δ 13 ethane (C 2 ) > δ 13 propane (C 3 ) > δ 13 butane (C 4 ) [9].…”

“…Inorganic gas is potentially associated with magmatic and deep-sea hydrothermal activity [5,6], with methane isotopes (δ 13 C 1 ) within the range of −50‰ (generally −30‰) to 10‰ [7,8], while organic gas is derived from the pyrolysis of kerogen in sedimentary rocks and the secondary cracking gas of crude oil, with methane isotopes in the range of −75‰ to −30‰ [7]. The inorganic alkane gas polymerized step-by-step to form long-chain alkanes through C-C bonding and the lower bond energy of 12 C- 12 C caused 12 C to join the polymerization reaction first, showing a negative carbon isotope series of δ 13 methane (C 1 ) > δ 13 ethane (C 2 ) > δ 13 propane (C 3 ) > δ 13 butane (C 4 ) [9]. When alkane gas was generated from the degradation of kerogen, 12 C- 12 C with lower bond energy broke preferentially than 13 C- 13 C, leading to the gradual enrichment of δ 13 C in organic alkane gas with the increase of the carbon atom number, thus forming a positive carbon isotope series of δ 13 C 1 < δ 13 C 2 < δ 13 C 3 < δ 13 C 4 [10].…”

“…The inorganic alkane gas polymerized step-by-step to form long-chain alkanes through C-C bonding and the lower bond energy of 12 C- 12 C caused 12 C to join the polymerization reaction first, showing a negative carbon isotope series of δ 13 methane (C 1 ) > δ 13 ethane (C 2 ) > δ 13 propane (C 3 ) > δ 13 butane (C 4 ) [9]. When alkane gas was generated from the degradation of kerogen, 12 C- 12 C with lower bond energy broke preferentially than 13 C- 13 C, leading to the gradual enrichment of δ 13 C in organic alkane gas with the increase of the carbon atom number, thus forming a positive carbon isotope series of δ 13 C 1 < δ 13 C 2 < δ 13 C 3 < δ 13 C 4 [10]. In cases of mixing alkane gases of different genesis or sources and oxidation by microorganisms (propane bacteria), the arrangement of δ 13 C may be confused [10][11][12].…”

“…In cases of mixing alkane gases of different genesis or sources and oxidation by microorganisms (propane bacteria), the arrangement of δ 13 C may be confused [10][11][12]. The sedimentary environment controls the original carbon isotope composition of kerogen, and the carbon isotope of humic kerogen is greater than that of sapropelic kerogen [13]. Ethane has well inherited the difference of the original parent material, so that δ 13 C 2 is used as an important indicator to identify the genetic type of natural gas [14,15].…”