Abstract
The molecular composition, stable carbon and hydrogen isotopes, and light hydrocarbons of the Lower Paleozoic natural gas in the Daniudi gas field in the Ordos Basin were investigated to study the geochemical characteristics. The Lower Paleozoic gas in the Daniudi gas field displays methane contents of 87.41–93.34%, dryness coefficients (C1/C1–5) ranging from 0.886 to 0.978, δ13C1 and δ13C2 values ranging from −40.3 to −36.4‰, with an average of −38.3‰, and from −33.6 to −24.2‰, with an average of −28.4‰, respectively, and δD1 values ranging from −197 to −160‰. The alkane gas generally displays positive carbon and hydrogen isotopic series, and the C7 and C5–7 light hydrocarbons of the Lower Paleozoic gas are dominated by methylcyclohexane and iso-alkanes, respectively. The Lower Paleozoic gas in the Daniudi gas field is mixed from coal-derived and oil-associated gases, similar to that observed in the Jingbian gas field. The oil-associated gas in the Lower Paleozoic gas is secondary oil cracking gas and displays a lower cracking extent than that in the Jingbian gas field. The coal-derived gas in the Lower Paleozoic gas in the Daniudi gas field migrated from the Upper Paleozoic gas through the window area where the iron–aluminum mudstone caprocks in the Upper Carboniferous Benxi Formation were missing. The oil-associated gas in the Lower Paleozoic gas in the Daniudi gas field was probably derived from presalt source rocks in the Lower Ordovician Majiagou Formation rather than the limestone in the Upper Carboniferous Taiyuan Formation. It seems unlikely that the marlstone in the Upper Ordovician Beiguoshan Formation and shale in the Middle Ordovician Pingliang Formation on the western and southwestern margins of the Ordos Basin contributed to the oil-associated gas in the Lower Paleozoic gas in the Daniudi gas field.
Highlights
The Ordos Basin, a petroliferous basin located in central China, is the most stable basin in China (Xu et al, 1995)
In addition to the Jingbian gas field, several gas reservoirs have been discovered in the Ordovician weathering crust in the northwestern Ordos Basin (Zhao et al, 2015), the Daniudi gas field in the northeastern Ordos Basin (Ding et al, 2016), and the presalt Majiagou Formation in the eastern Ordos Basin (Yang et al, 2009), which suggests that the Lower Paleozoic strata in the Ordos Basin have favorable exploration potential
There is no consensus on whether the Lower Paleozoic natural gas is dominated by coal-derived gas from the Carboniferous-Permian coal-measure source rocks (Dai et al, 2005; Hu and Zhang, 2011; Mi et al, 2012; Xia et al, 1999a; Yang and Liu, 2014) or oil-associated gas (Chen, 2002; Hao et al, 1997; Huang et al, 1996), and Li et al (2003) considered that the main genetic type of the Lower Paleozoic natural gas is different in different areas of the basin
Summary
The Ordos Basin, a petroliferous basin located in central China, is the most stable basin in China (Xu et al, 1995). There is no consensus on whether the Lower Paleozoic natural gas is dominated by coal-derived gas from the Carboniferous-Permian coal-measure source rocks (Dai et al, 2005; Hu and Zhang, 2011; Mi et al, 2012; Xia et al, 1999a; Yang and Liu, 2014) or oil-associated gas (Chen, 2002; Hao et al, 1997; Huang et al, 1996), and Li et al (2003) considered that the main genetic type of the Lower Paleozoic natural gas is different in different areas of the basin. The source of the controversy on the gas origin is a divergence on understanding the gas geochemical characteristics, and that on the source of oil-associated gas is derived from whether the carbonate rocks with low TOC contents in the Lower Ordovician Majiagou Formation can be considered as the effective source rocks
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