Characteristics and accumulation model of the late Quaternary shallow biogenic gas in the modern Changjiang delta area, eastern China

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 CH 4 (generally[95%) with a d 13 C CH4 and d 13 C CO2 of -75.8 to -67.7% and -34.5 to -6.6%, respectively, and a dD CH4 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, paleoestuary, 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.

Refining the late-Holocene coastline and delta development of the northern Yangtze River delta: Combining historical archives and OSL dating

“…The top ~2 or 3 m of the cores comprises yellowish clayey silts with redoximorphic features. On the basis of stratigraphic correlation with neighbouring cores (EGQD14, CM97 and ZK01) (Figure 2), Unit A comprises shallow sea/prodelta facies, while Units B and C are delta front and delta plain facies, respectively (Gao et al, 2019; Goodbred and Saito, 2012; Hori et al, 2001; Zhang and Lin, 2017).…”

“…Although there have been a number of detailed dating studies on cores in the YRD since the 2000s, the chronologies of these cores are mainly based on 14 C dating (e.g. Hori et al, 2001; Li et al, 2014; Long et al, 2014; Wang et al, 2011, 2013; Yi et al, 2003; Zhang and Lin, 2017). However, so-called old carbon, caused by the reworking of sediment, and radiocarbon reservoir effects in the coastal environment lead to the reliability of 14 C ages being questioned; furthermore, the paucity of carbon material in delta front sandy deposits can preclude 14 C dating entirely (Stanley and Chen, 2000).…”

“…The rectangle in (b) is enlarged in (c) to (f), with the palaeoshoreline estimated by historical records (Chen and Chen, 2010; Tan, 1987; Zhou, 1999) shown in dashed lines (red). The location of cores BX, MQ and WB (yellow stars) is shown with cited cores CM97 (Hori et al, 2001, 2002), ZK01 (Zhang and Lin, 2017), NT (Nian et al, 2018a), SD (Nian et al, 2018b) and EGQD14 (Gao et al, 2019) from previous studies (blue stars). Please visit the journal website to view this figure in color.…”

“…Lithological description of cores BX, MQ and WB, and their stratigraphic correlation with neighbouring cores EGQD14 (Gao et al, 2018), CM97 (Hori et al, 2001), and ZK01 (Zhang and Lin, 2017). The age in red has been excluded.…”

Controls on shallow gas distribution, migration, and associated geohazards in the Yangtze subaqueous delta and the Hangzhou Bay

“…Shallow gas has been reported to be widely distributed in the post-LGM (last glacial maximum) strata in the Yangtze Delta and the Hangzhou Bay (Li et al, 2008;Hu et al, 2016;Xu et al, 2017;Wang et al, 2018). Thick muddy strata are indicated to be rich in organic matter and formed in an anoxic setting, favoring biogenic gas generation (Lin et al, 2015;Zhang and Lin, 2017). The top border of shallow gas is named the gas front, and its depth generally varies in a wide range from a few meters to tens of meters below the seafloor (Ye et al, 2003;Hu et al, 2016;Wang et al, 2018).…”