Abstract
The Changjiang (Yangtze) is one of the largest rivers in the world. It formed a huge incised valley at its mouth during the Last Glacial Maximum; the incised-valley fill, approximately 80–110 m thick, supplies an important foundation for the generation of shallow biogenic-gas reservoirs. Two cores and 13 cone penetration tests were used to elaborate the characteristics, formation mechanism, and distribution of the shallow biogenic-gas reservoirs in the study area. The natural gas is mainly composed of CH4 (generally >95%) with a δ13CCH4 and δ13CCO2 of −75.8 to −67.7‰ and −34.5 to −6.6‰, respectively, and a δDCH4 of −215 to −185‰, indicating a biogenic origin by the carbon dioxide reduction pathway. Commercial biogenic gas occurs primarily in the sand bodies of fluvial-channel, floodplain, and paleo-estuary facies with a burial depth of 50–80 m. Gas sources as well as cap beds are gray to yellowish-gray mud of floodplain, paleo-estuary, and offshore shallow marine facies. The organic matter in gas sources is dominated by immature type III kerogen (gas prone). The difference in permeability (about 4–6 orders of magnitude) between cap beds and reservoirs makes the cap beds effectively prevent the upward escape of gas in the reservoirs. This formation mechanism is consistent with that for the shallow biogenic gas in the late Quaternary Qiantang River incised valley to the south. Therefore, this study should provide further insight into understanding the formation and distribution of shallow biogenic gas in other similar postglacial incised-valley systems.
Highlights
Biogenic gas is of significant importance because it is clean energy and an abundant resource, accounting for *20% of the conventional natural gas reserves in the world (Rice and Claypool 1981)
The carbon isotope values of CH4 and CO2 are -75.8 to -67.7% and -34.5 to -6.6%, respectively, and the hydrogen isotope values of CH4 are -215 to-185%. These results indicate a biogenic origin for the shallow gas
The natural gas in the modern Changjiang delta area consists primarily of CH4 and has a biogenic origin with carbon isotope ratios of CH4 and CO2 of -75.8 to -67.7% and -34.5 to -6.6%, respectively, and hydrogen isotope ratios of CH4 of -215 to -185%. It is mainly distributed in the postglacial Changjiang incisedvalley fill, which consists principally of five sedimentary facies in ascending order, i.e., fluvial channel (Facies V), floodplain (Facies IV), paleo-estuary (Facies III), offshore shallow marine (Facies II), and modern delta (Facies I)
Summary
Biogenic gas is of significant importance because it is clean energy and an abundant resource, accounting for *20% of the conventional natural gas reserves in the world (Rice and Claypool 1981). Shallow biogenic-gas reservoirs are principally distributed along the eastern and southern coasts of China, especially the Jiangsu–Zhejiang coastal plain area, where the postglacial Changjiang and Qiantang River incised valleys are located (Wang 1982; Zheng 1998; Lin et al 2004; Liu et al 2008; Li et al 2010a; Zhang et al 2013, 2014). Previous research indicates that shallow biogenic gas is mainly distributed in the northern margin of the modern Changjiang delta (i.e., the M area in Fig. 1a, *2.5 9 104 km2; Wang 1982; Zheng 1998), and there are generally three sets of gas-bearing intervals with burial-depth ranges of 7–15, 25–35, and [50 m, respectively, involving prodelta–shallow marine, delta front, coastal plain, and floodplain facies (Zheng 1998)
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