Characteristics and accumulation model of the late Quaternary shallow biogenic gas in the modern Changjiang delta area, eastern China
Characteristics and accumulation model of the late Quaternary shallow biogenic gas in the modern Changjiang delta area, eastern China
Xia Zhang 0
Chun-Ming Lin 0
0 State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University , Nanjing 210023, Jiangsu , China
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 d13CCH4 and d13CCO2 of -75.8 to -67.7% and -34.5 to -6.6%, respectively, and a dDCH4 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.
Biogenic gas; Formation mechanism; Late; Quaternary; Modern Changjiang delta; Eastern China
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). Recently, researchers have drawn
considerable attention to the shallow biogenic gas in the
near-surface marine and coastal sediments such as bay or
estuarine deposits and late Quaternary incised-valley fills
(Garc´ıa-Garc´ıa et al. 2007; Xu et al. 2009; Lin et al. 2010;
Zhang et al. 2013; Jones et al. 2014; Okay and Aydemir
2016), in terms of the shallow-burial depth ranging from
several tens of meters to hundreds of meters and easy
exploitation with low investment and high benefit.
Commercial reservoirs of shallow biogenic gas have been
widely found in the world, including the North Sea
(Heggland 1997; Vielsta¨dte et al. 2015), Ria de Vigo incised
valley, Spain (Garcia-Gil et al. 2002), western Gulf of
Maine, USA (Rogers et al. 2006), and Lawrence Estuary,
Canada (Pinet et al. 2008).
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). Incised valleys generally have high
preservation potential for their fill deposits (Dalrymple et al.
1994) and provide a fundamental and important
background for the formation of the shallow biogenic-gas
reservoirs therein, i.e., gas-source beds, cap beds, and sand
reservoirs can occur together in close geographic and
stratigraphic proximity (Xu et al. 2009; Lin et al. 2004;
Zhang et al. 2014). Considerable biogenic-gas
accumulations with depth \120 m have been discovered and well
documented in the late Quaternary Qiantang River incised
valley with a predicted total gas amount of 244.5 9 109 m3
(Lin et al. 2004, 2010; Zhang et al. 2013). Also, the
produced gas was used as gas supply for local villages and
factories (Li and Lin 2010). Nevertheless, there are
relatively few examples in the scientific literature regarding the
shallow biogenic gas in the Changjiang incised valley area
after [60 years’ research and development (cf. Wang
1982; Zheng 1998). 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 km ; 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).
Obviously, a poor understanding of the geological
background, distribution, and formation of the shallow
biogenic-gas reservoirs in the study area has seriously
hampered the exploration and exploitation processes.
This paper is an extension of the previous work based
primarily on the detailed observations and analyses of
newly acquired cores ZK01 and ZK02, as well as 13
surrounding cone penetration tests (CPTs) in the Qidong and
Haimen areas (Fig. 1), and the further correlation with
more than 600 boreholes, most of which have been
reported by Li et al. (2000, 2002) and Hori et al. (2002).
The objective of this study is to discuss the conditions
required for the formation of the shallow biogenic-gas
reservoirs and to summarize the regularities of their
distribution in the modern Changjiang delta area. This study
will provide a useful insight into the exploration and
exploitation of the shallow biogenic gas in similar
incisedvalley systems, more importantly for the modern
Changjiang delta area, where there is dense population and
tremendous economic development.
2 Geological setting
The Changjiang originating from the Qinghai–Tibet
Plateau annually discharges water and sediment of 924 km3
and 4.8 9 1011 kg, respectively (Milliman and Syvitski
1992), and provides the primary sediment source for the
modern Changjiang delta. The present-day Changjiang
delta is situated at a coastal subsidence zone, with the
altitude generally \5 m above mean sea level (Stanley and
Chen 1996). It is bounded by hills in the west (Fig. 1a) and
slopes gently toward the east with *250 km in length from
the apex around Zhenjiang–Yangzhou area to the modern
river mouth. The modern Changjiang delta covers an area
of *5.2 9 104 km2, with 2.3 9 104 km2 subaerial and
2.9 9 104 km2 subaqueous (Li et al. 2002). The former can
be divided into three major units: the main delta body (M),
and the southern (Sf) and northern (Nf) flanks (Li et al.
2002; Fig. 1a). The main body is characterized by the
combination of distributary channels and three active
rivermouth sand bars, which are elongated and extend
southeastward (Li et al. 2002). The subaqueous part of the delta
can be classified into subtidal flats with water depths
\5–10 m, delta front ranging from 5–10 to 15–30 m, and
prodelta (water depth of [30 and \50 m).
3 Materials and methods
Cores ZK01 (112 m penetration depth, 0.1 m diameter)
and ZK02 (128 m penetration depth, 0.1 m diameter) were
taken from Qidong (31 50026.7400N, 121 33024.0800E) and
A A′ Profile
Fig. 1 a Schematic map of the modern Changjiang (Yangtze River)
delta showing the locations of cores ZK01 and ZK02, boreholes, cross
section, and gas-bearing area, as well as the basal topography of the
Changjiang incised valley during the Last Glacial Maximum
(modified from Wang 1982; Zheng 1998; Li et al. 2002; Zhang et al. 2015).
b Locations of core ZK01, cone penetration tests (CPTs), and
transects in the Qidong area from the northern margin of the modern
Changjiang delta (see location in Fig. 1a). M Main delta area, Sf
Southern flank, Nf Northern flank, QD Qidong area, HM Haimen area,
Haimen (31 52047.1200N, 121 09030.6900E) areas where
shallow gas is abundant, respectively, in 2014 by rotary
drilling and with almost 100% recovery (see locations in
Fig. 1a). In the laboratory, they were split, photographed,
described, and subsampled. The grain size of 271 samples
(0.5–1.0 m interval) was analyzed at 0.25 U spacing
according to a standard method (cf. Zhou and Gao 2004),
with grain-size parameters determined using the
GRADISTAT software (Blott and Pye 2001). Eighty-three samples
were collected for foraminifera analysis using the method
described by Zhang et al. (2014) and Wang et al. (1988).
The permeabilities of 16 samples were measured by
falling-head permeability test apparatus (cf. Zhang et al.
2013). The total organic carbon (TOC) and chloroform
bitumen contents of 41 samples were analyzed following
the method of Stax and Stein (1993). Eight mud samples
were obtained for pyrolysis analysis (cf. Zou et al. 2006).
Four 14C ages were determined on shells or organic
sediments by using accelerator-based mass spectroscopy in the
Beta Analytic Radiocarbon Dating Laboratory (Lab No.
Beta) in Miami, USA, and calibrated using the Calib Rev
7.1 (beta) program (Reimer et al. 2013; Table 1). Two 14C
ages on marine shells were calibrated utilizing the
Marine13 model with the 4R value of 135 ± 42 to deal with
the marine reservoir effect (cf. Yoneda et al. 2007; Wang
et al. 2012).
In addition, 13 CPTs were taken around the ZK01 core
(Fig. 1(b)), with the tested parameters being cone tip
resistance (qc) and sleeve friction (fs), in order to explore
the distribution pattern of shallow gas, i.e., potential sand
reservoirs (cf. Moran et al. 1989; Li and Lin 2010). During
exploration, almost every CPT inevitably encountered gas
with an original gas pressure of *0.5 MPa and a flame
height reaching up to 2 m when the gas was ignited. In this
study, a total of six gas samples were collected and
analyzed for chemical composition, stable carbon isotope
ratios of CH4 and CO2, and stable hydrogen isotope ratios
of CH4 according to a standard method (cf. Ni et al. 2013).
Furthermore, CPT-3 and the nearby ZK01 core were
compared to calibrate the qc and fs curves allowing for
distinguishing lithology, especially potential sandy
reservoirs (Fig. 1b). In general, qc and fs values increase as
grain size increases (cf. Li and Lin 2010; Lin et al. 2015).
Therefore, the qc and fs curves of sand and silt sediments
show high values and large separation of curves, and the qc
curve lies generally to the left of fs curve (cf. Li and Lin
2010; Lin et al. 2015).
4 Stratigraphic architecture
More than 600 cores were drilled in the present-day
Changjiang delta area in the last five decades, laying a solid
foundation for understanding the stratigraphic architecture
of the coastal depositional system (Li et al. 2002; Hori
et al. 2001; Wang et al. 2012). There are at least three
stages for the formation of the Quaternary Changjiang
incised-valley fills (Hori et al. 2002; Li and Wang 1998; Li
et al. 2006; Figs. 1a, 2). Most of the incised-valley fills
generated during the preceding two stages are missing
resulted from the following strong down-cutting erosion
and in many cases are characterized by a superposition of
fluvial-channel sediments composed mainly of sandy and
gravelly sediments. However, the incised-valley fill formed
since the Last Glacial Maximum, the objective of this
paper, is relatively complete (Fig. 2). It can be classified
into five sedimentary facies that were deposited during the
sea-level rise and subsequent stillstand (Figs. 2, 3). In the
following text, we will use the newly drilled ZK01 core to
describe each facies (Fig. 3).
Facies V (Fluvial Channel Sediments) is bounded by an
erosional basal surface and shows a generally
upward-fining trend (Figs. 2, 3, 4a). Sediments at the bottom consist
mainly of gray or grayish-yellow gravelly sand interbedded
by fine sand, silty sand, and gravels (Figs. 3, 4a). The
gravels are angular to subangular, with a diameter of
2–50 mm (Fig. 4a), and the sand sediments are primarily
composed of coarse (av. 43.4%) and medium (av. 28.8%)
sands. The sediments at the top are characterized by an
alternation of gray or grayish-yellow silty sand and silty
fine sand (Fig. 3). The sediments are well-moderately
sorted with a sorting coefficient of 1.14–2.52. Massive,
age, yr B.P.
age, yr B.P.
In this paper, the 2d calibrated ages are adopted and labeled in Fig. 3
a The age is not used because it does not follow the general trend
Calibrated 14C age, cal. yr B.P.
The last glacial
Fig. 2 Stratigraphic transect (A–A00) in the modern Changjiang delta
region (modified from Zhang et al. 1998; see Fig. 1a for location).
SB: sequence boundary with the subscript indicating the distinct
stages for the formation of the Quaternary Changjiang incised valleys.
14C data were obtained from Li et al. (2002) and calibrated by using
graded, and parallel beddings, iron oxide spots, and shells
are common, and there is a lack of tide-influenced
sedimentary structures. Benthic foraminifera (BF) dominant by
Ammonia beccarii are identified at the top with nine
species and 15 individuals per 50 g (dry weight) sample size.
A 14C date at the burial depth of 75.6 m is
13,500 ± 180 cal. yr B.P. (Fig. 3; Table 1). This facies
represents part of the river system and may have been
deposited in a channel thalweg to bar environment (cf.
Nittrouer et al. 2011; Zhang et al. 2014).
Facies IV (Floodplain Sediments) consists mainly of an
alternation of gray mud and grayish-yellow sandy silt, silt,
and gravelly sand (Figs. 3 and 4b). The gravels occupying
5%–10% of the coarse sediments have a diameter of
2–5 mm, up to 20 mm. Massive to graded beddings are
common in coarse sediments, whereas silty blebs, and
massive and lenticular beddings are abundant in mud
sediments (Fig. 4b). Only one sample at 68.80 m depth
contains some BF with four species and eight individuals
per 50 g dry sample size.
Facies III (Paleo-estuary Sediments) is dominated by
gray or yellowish-gray mud interbedded by silt and coarse
sand with the lamina thickness ranging from 2 mm to 1 m
(Fig. 4c). The structureless sand beds are usually typified
by an erosional basal surface and present as a
fining-upward succession with numerous irregular mud pebbles
(Fig. 4d). They are well sorted with a sorting coefficient of
1.52–2.46 and mean grain size of 4.27–6.23 U. Wavy,
the Calib Rev 7.1 (beta) program (Reimer et al. 2013):
a6595 ± 320 cal. yr B.P., 11.70 m depth; b- 12,900 ± 190 cal. yr
B.P., 38.80 m depth; c- 39,200 ± 2200 cal. yr B.P., 94.50 m depth
horizontal, and massive beddings are common. In addition,
a set of tide-influenced sedimentary structures including
sand-mud couplet and lenticular bedding are common.
Foraminiferal fossils are abundant and mainly composed of
BF which consists principally of Ammonia beccarii vars.,
Florilus decorus, Elphidium magellanicum, Cribrononion
vitreum Wang, and Elphidium advenum (Cushman). The
number of BF is 14–42 species and 61–10,304 individuals
per 50 g dry sample size. A 14C date of shells at the burial
depth of 60.0 m is 12,960 ± 120 cal. yr B.P. (Fig. 3;
Table 1). Facies II has been recorded as a macro-tidal
system like the Qiantang River estuary with the maximum
tidal range located in the Yangzhou area (Li et al. 2006),
which is further testified by numerical simulations (Yang
and Sun 1988; Uehara et al. 2002).
Facies II (Offshore Shallow Marine Sediments) consists
mainly of gray or yellowish-gray soft mud interbedded
with gray silt, fine sand, and clayey silt stripes
(0.001–0.03 m thick) and blebs (Fig. 4e). Massive,
horizontal and lenticular beddings, sand-filled burrows,
bioturbation, and seriously broken shells are common
(Fig. 4e). Foraminifera are also abundant in this facies and
are mainly composed of BF. There are [60 BF species
present, including Ammonia beccarii vars., Elphidium
magellanicum, and Cribrononion vitreum Wang. The
number of BF is 27–44 species and 139–5888 individuals,
respectively, based on 50 g (dry weight) sample size. The
foraminiferal fossils in Facies II resemble the living groups
Chloroform bitumen content ppm
Permeability mD TOC
Mean grain size Φ
Grain-size composition %
50 100150 0.1 10 1000 0.2 0.4 0.6 2 4 6 8
Fig. 3 Columnar section of core ZK01 in the modern Changjiang
delta area (see Fig. 1 for location). Black circles indicate the depths at
which various features were observed, and light stars show the
sediment samples of reservoirs. In the column of permeability, the
dashed line indicates horizontal permeability, while the solid line is
vertical permeability. SS: sedimentary structure
Fig. 4 Photographs of typical sedimentary characteristics in the
ZK01 core. Black scale bar = 10 cm. a Facies V, 82.30–82.80 m
depth: grayish-yellow gravelly sand (GS) with graded bedding.
b Facies IV, 68.45–68.80 m depth: alternation of gray mud (M) and
grayish-yellow silty fine sand (StFS), silty sand (StS), and gravelly
sand with the lithological unconformity surface indicated by a white
dotted line. Lenticular bedding and silty blebs (SB) are common in
the mud sediments, while graded and massive bedding and shells are
present in the sand sediments. c Facies III, 66.35–66.70 m depth: gray
in the offshore shallow water areas (\20–55 m) of the East
China Sea, South Yellow Sea, Changjiang delta, and Bohai
Bay (Wang et al. 1981; Li and Wang 1998; Zhuang et al.
2002; Li et al. 2002; Li et al. 2010b).
Facies I (Modern Delta Sediments) shows an
upwardcoarsening sequence. The sediments at the bottom are
characterized by an alternation of silty fine sand and mud
(Figs. 3, 4f). The sand beds (0.3–7 cm thick) consist
mainly of fine sand (52%–67%) and silt (29%–42%) with a
mean grain size of 3.8–4.5 U and are well sorted with a
sorting coefficient \2, while the mud sediments (0.1–3 cm
thick) dominated by silt and clay. The sediments at the top
consist predominantly of gray sand interbedded by gray or
brown mud stripes (0.5–3 cm thick) or muddy gravels
(Fig. 3). The sand sediments are mainly composed of fine
sand (50.3%–75.8%) and silt (13.6%–37.7%), with a mean
grain size of 3.3–4.4 U and sorting coefficient of 1.3–2.2.
Parallel, massive, and convolute beddings, as well as
seriously broken shells, are common (Fig. 3).
mud interbedded by thin silt (St) and coarse sand (CS) layers. d Facies
III, 58.40–58.85 m depth: gray silty sand with an erosional basal
surface (white dashed line) and numerous irregular mud pebbles
(MP), present as a fining-upward succession. e Facies II,
57.30–57.80 m depth: alternation of gray mud and silty sand at the
bottom and then massive gray soft mud at the top. f Facies I,
17.30–17.80 m depth: alternation of gray silty fine sand and mud. G:
Foraminiferal fossils dominated by BF ([58 species;
[42,688 in a 50-g dry sample) are also abundant in this
facies. Most BF individuals are juveniles with small and/or
seriously abraded shells. The BF assemblage of this facies,
including Ammonia beccarii vars., Elphidium naraensis,
Florilus decorus, Protelphidium tuberculatum (d’Orbigny),
and Elphidium magellanicum, is similar to that of the
modern Changjiang delta (cf. Li and Wang 1998).
5 Characteristics of shallow biogenic-gas reservoirs
5.1 Biogenic origin of shallow gas
Table 2 presents the chemical and isotopic compositions of
the shallow gas in the study area. Results show that most of
the gas samples are dominated by CH4 (generally [95%),
with minor N2 (0.75%–9.07%) and CO2 (0.98%–3.23%).
The gas sample from CPT-1 (see Fig. 1b for location) is
special in consisting mainly of N2 (64.0%) and CH4
(23.5%). 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
(cf. Whiticar et al. 1986; Whiticar 1999; Humez et al.
2016; Tao et al. 2016; Sun et al. 2016). The nitrogen-rich
biogenic gas may be derived from the degradation of
nitrogen-rich organic matter, indicating that the organic
matter of the source sediments in the study area is
heterogeneous (cf. Wang 1982). In addition, all the gases
plot within ‘‘Bacterial Carbonate Reduction’’ zone in
Fig. 5, indicating that the methanogenesis is predominant
from carbon dioxide reduction (cf. Whiticar et al. 1986).
5.2 Gas-source sediments
There are three potential kinds of gas-source sediments in
this area, including the gray and light-brown mud of Facies
IV, gray and yellowish-gray mud of Facies III, gray and
yellowish-gray soft mud of Facies II (Figs. 2, 3). Mud beds
of Facies I can be excluded because they are thin (\0.5 m)
and close to the surface (Figs. 2, 3).
Systematic analysis of core ZK01 indicates that TOC
content increases with depth, and the argillic sediments of
Facies III and IV have higher TOC contents than those of
Facies II (Fig. 3). The TOC contents of Facies III and IV
exceed the lower limits for both terrestrial (0.18%, Zhou
et al. 1994) and marine gas sources (0.5%; Rice and
Claypool 1981), whereas those of Facies II are below the
lower limit for potential marine gas sources (Table 3). The
chloroform bitumen content of source sediments shows a
similar variation trend among distinct sedimentary facies
(Fig. 3; Table 3).
Pyrolysis results show that the Tmax values are
generally \435 C, the gas generation potential index ranges
from 0.09 to 0.19 mg/g sediment, and the hydrogen index
values vary from 15.97 to 42.48 mg/g TOC (Table 4),
implying that the organic materials are substantial at the
immature stage and the biogas is now still being formed at
a massive generation stage (cf. Lin et al. 2004; Zhang et al.
2013). H/C and O/C ratios of kerogen vary in a range of
0.89–1.25 and 0.27–0.38, respectively, and plot in the type
III kerogen area (gas prone; cf. Peters et al. 1986; Fig. 6),
which are supported by the correlation of Tmax and HI
In summary, the argillic sediments of Facies III and IV
are more likely to act as effective gas-source sediments (cf.
Zhang and Chen 1983; Lu and Hai 1991; Wu et al. 2014).
However, the TOC contents and chloroform bitumen
values for the argillic sediments in the study area are
significantly lower than those from the nearby Qiantang River
-350 -250 -150
δD-methane, ‰ (SMOW)
Fig. 5 Cross plot of d13C and dD of the methane for the shallow gas
in the modern Changjiang delta area, eastern China, implying gases
generated through the pathway of H2 reduction of CO2 (base diagram
is from Whiticar 1999)
Chemical composition, %
d13CCH4 PDB, %
d13CCO2 PDB, %
dDCH4 SMOW, %
Note: u.d. under detection limit
Table 3 Organic matter abundance of the mud sediments from the different sedimentary facies of cores ZK01 and ZK02 in the modern
Changjiang delta area, China
TOC content, wt.%
Chloroform bitumen content, ppm
Numbers in parentheses = sample number of analyses
Note: Min. minimum, Max. maximum, Av. average
Table 4 Pyrolysis results of the gas-source sediments from different sedimentary facies of cores ZK01 and ZK02 in the modern Changjiang
delta area, China
Gas generation potential (S1 ? S2), mg/g
Hydrogen index, mg/g
Gray soft mud
incised-valley fill (cf. Lin et al. 2004; Zhang et al. 2013),
which may be caused by the remarkably different amount
of terrigenous sediment inputs. The mean annual
suspended sediment load is *4.8 9 108 t/yr for the
Changjiang, generally two orders of magnitude higher than that of
the Qiantang River (*0.09 9 108 t/yr). This huge ter
rigenous sediment input dilutes the organic matter
abundance, as a result implying that the mud sediments in the
postglacial Changjiang incised-valley fill have a relatively
lower biogas generation potential than those in the
Qiantang River incised-valley fill.
The potential shallow biogenic-gas reservoirs in the study
area can be classified into five types (Figs. 3, 7): (a) beds of
sandy gravel, gravelly sand, medium-grained fine sand, and
silty sand of Facies V; (b) sand bodies of Facies IV
composed mainly of silty sand, sandy silt, and silt; (c) sand
bodies of Facies III characterized by silty sand and sandy
silt; (d) lenses of sand (silty fine sand, sandy silt, silt, and
clayey silt) intercalated in the soft mud of Facies II; and
(e) silty fine sand and sandy silt in Facies I. Based on the
available exploration data, commercial gas was
encountered mainly in the sand bodies of Facies III and IV
and secondarily in the top parts of the sand beds of Facies
V (Figs. 3, 7). The sand bodies of Facies II are thin with a
thickness of 0.005–0.95 m (generally \0.5 m; Figs. 3, 4e,
7). Although the sand sediments of Facies I are thick and
composed mainly of coarse grains, the attributes including
being close to the surface and lack of effective cap beds
(0.1–3 m thick) make them unsuitable to be effective
reservoirs in the study area (Fig. 3).
Therefore, determining the size, shape, and permeability
of the sand bodies in Facies III, IV, and V is essential for
exploration prediction and exploitation of shallow biogenic
gas in the study area. The sand bodies in Facies III and IV
vary significantly in thickness (0.5–4.0 m) and burial depth
(50–70 m). Even in neighboring boreholes, the depth
difference may be over 3–4 m (Fig. 7). In some cases, a
borehole can go through up to [10 layers of sand with a
total thickness of *20 m (Fig. 7), but no sand layers are
penetrated by the neighboring borehole. All the sand bodies
are encased entirely by mud and vary in size (Fig. 7). Small
bodies are distributed locally, but large ones can extend up
to several hundreds of meters. The vertical permeabilities
of the sand bodies in Facies III are of 122.9–211.5 mD,
generally lower than the horizontal permeabilities
Facies I (Modern delta sediments)
Facies II (Shallow marine sediments)
Facies III (Paleo-estuary sediments)
Facies V (Fluvial channel sediments)
Fig. 6 Plot of H/C vs. O/C ratios (according to Peters et al. 1986)
implying the kerogen character for the mud sediments in the late
Quaternary Changjiang incised valley
(273.3–352.5 mD, Table 5), which may be influenced by
the heterogeneity of sand bodies, for instance, the presence
of parallel bedding and mud clasts (Figs. 4d, 8a). Generally
speaking, the size, thickness, and permeability of the sand
bodies in the study area are considerably lower than those
of the sand bodies from the same facies in the late
Quaternary Qiantang River incised-valley fill (Zhang et al.
2013, 2014). The sand bodies in Facies III of the Qiantang
River incised-valley fill are characterized by a thickness of
3–20 m, burial depth of 28–78 m, width of 0.4–2 km, and
permeability of 577–4,590 mD (Zhang et al. 2013, 2014).
The potential shallow biogenic-gas reservoirs in Facies
V occur in the local highs, which are capped directly by the
mud of Facies IV, and have a burial depth of 58–70 m
(Fig. 7). The local highs, which are surrounded by
fluvialchannel sand sediments formed in the previous
incisedvalley period however, cannot become effective gas
reservoirs (Fig. 9). Permeabilities of these sand reservoirs
(66.99–15,037.03 mD) are generally higher than those of
sand bodies in Facies III and IV, which may be caused by
the coarser grain size and lower contents of mud (Fig. 8b);
nevertheless, they are characterized by a complex
difference between the vertical and horizontal permeabilities,
i.e., sometimes vertical permeability is higher than the
horizontal, and vice versa (Table 5).
5.4 Cap beds
In the study area, commercial gas-bearing pools mainly
occur as sand bodies capped directly by the mud beds of
Facies III and IV, which are restricted within the incised
valley (burial depth of 30–90 and thickness of 10–18 m)
and are called direct or local cap beds (Figs. 2, 3, 9). By
contrast, the soft mud covering the whole incised valley
and deposited in an offshore shallow marine environment
is called regional or indirect cap beds with a burial depth of
10–60 m and thickness of 5–20 m (Figs. 2, 3, 9). This is
similar to that of the Qiantang River incised valley. Zhang
et al. (2013) proposed that capillary sealing, pore-water
pressure sealing, and hydrocarbon concentration sealing
are considered as the main mechanisms for the
conservation of the shallow biogenic gas in the Qiantang River
incised-valley area. In this paper, we use vertical
permeability, which has a negative relationship with the capillary
sealing capacity of cap beds, to indirectly illustrate the
sealing mechanism of cap beds. The vertical permeabilities
of cap beds (0.05–0.52 mD) are significantly lower than
those of sand reservoirs (66.99–15037 mD) with a
difference of 4–6 orders of magnitude, which makes cap beds
effective in preventing gas escaping from reservoirs
The vertical permeabilities have a similar variation trend
with the contents of sand and silt, but an inverse correlation
with the mud content (Fig. 3). The cap beds composed
mainly of mud have the lowest permeability, and the
vertical permeability is generally equal to the horizontal one,
whereas those with sand bands or inclusions are marked by
higher permeabilities, and the horizontal permeabilities are
significantly higher than the vertical ones, by about 3–4
orders of magnitude (Figs. 3, 8c, d, e, f; Table 5). This
result indicates that mud content plays a significant role in
the capillary sealing ability of cap beds, namely the
massive mud has much stronger sealing ability.
5.5 Gas migration and accumulation model
Methane is predominantly dissolved in the water within
gas-source beds, with less being absorbed by clay minerals,
and free gas is present only when saturation is attained with
increasing depth of burial (cf. Lin et al. 2004; Gao et al.
2010, 2012). As a result, most gas in the study area is
regarded as being transported from gas-source beds to sand
beds by formation water with the differential compaction
between sand and mud beds. Also, the capillary pressure
difference between gas-source beds and sand reservoirs
indicated by the huge difference in vertical permeability
(Table 5) also drives gas migrating from mud beds to
sandy reservoirs (cf. Magara 1987). After gas is released
from mud sediments, it can migrate toward the overlying,
V Shallow marine V Weak gas display
underlying, or lateral sand bodies (Fig. 9). Within sand
reservoirs, the methane-filled space is initially restricted to
the top, but it expands downward when methane is
abundant, and the formation water is expelled along the
gas–water interface. In addition, gas migrates more
frequently and easily along the bedding surface than in the
) ) )
D (06 (66 (06
,tlilitraeeaonpbymm ../()3167–25124466 .,.,./–2923561223127 .,.,./–1742494111711 ../()643–049554246 ../()13–12075886 ../()403–930330377 ../()303–287273326 ../()7220807176 ../()535–804354296 ../()2611493 ../()876–442466376 .,.,./–7642783960198 ../()318–694300496 ../()70–6306168 ../()09–16894816
irzo .531 ,452 ,590 .625 .348 .673 .645 .82– .412 .43– .283 ,723 .227 .784 .076
H 1 2 1 4 5 2 2 8 2 1 2 5 2 5 8
Fig. 8 Photographs showing the typical sedimentary characteristics
of permeability samples. a 64.35–64.55 m depth: massive silty fine
sand (StFS) with mud blocks (MB), vertical permeability of 123 mD,
and horizontal permeability of 352 mD. b 77.20–77.40 m depth:
structureless gravelly sand (GS) with vertical and horizontal
permeabilities of 263 and 304 mD, respectively. c 69.05–69.25 m depth:
massive mud (M) with vertical permeability of 0.48 mD and
horizontal permeability of 1.49 mD. d 89.80–90.00 m depth:
alternation of mud and silty sand (StS) with vertical and horizontal
permeabilities of 0.52 and 395.6 mD, respectively. f and
e 50.80–51.00 m depth: mud with thin-bedded sandy silt (SSt),
vertical permeability of 0.20 mD, and horizontal permeability of 552
mD; the sand beds have been washed away after experiment
direction perpendicular to it within gas-source layers
indicated by the difference between vertical and horizontal
permeabilities (Table 5; Fig. 3).
As neotectonism in the study area is only characterized
by uplift of the hilly lands and subsidence (1–3 mm/yr) in
the coastal plain area (Chen and Stanley 1993), gas-bearing
strata in the Changjiang delta area remain horizontal;
therefore, gas migration and accumulation are mainly
controlled by the lithology of gas-source layers and
reservoirs, and there are no significant structural traps. As a
consequence, sand bodies of Facies III and IV provide
optimum conditions for in situ stratigraphic entrapment of
biogenic gas and secondly in local highs of Facies V.
The natural gas in the modern Changjiang delta area
consists primarily of CH4 (generally[95%) 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).
The sand bodies of Facies III and IV and local highs of
Facies V are primary potential gas reservoirs. The former
vary significantly in thickness (0.5–4.0 m) and burial depth
(50–70 m), and the latter with a thickness larger than 10 m
and burial depth of 58–70 m (Fig. 7). The main gas sources
are gray or yellowish-gray mud of Facies III and IV, and
the organic matter is dominated by type III kerogen (gas
prone) at an immature stage. Meanwhile, the gas sources
occur as cap beds, and the mud sediments of Facies III and
IV that encase sand reservoirs directly are referred as direct
cap beds, while the soft mud of Facies II called indirect
ones. The huge difference in vertical permeability, about
4–6 orders of magnitude, between cap beds and reservoirs
allows cap beds to effectively reduce the upward escape of
gas in reservoirs. Therefore, it is notable that the shallow
biogenic-gas reservoirs in the study area are of classic
‘‘self-generated and self-reserved’’ lithological entrapment
type, and the sand bodies of Facies III, IV, and V should be
considered as promising targets for exploration.
Acknowledgements This work was supported by the National
Natural Science Foundation of China under Grant Numbers 41402092
and 41572112, the Natural Science Foundation (Youth Science Fund
Project) of Jiangsu Province (BK20140604), and the Scholarship
under State Scholarship Fund sponsored by the China Scholarship
Council (File No. 201506195035). We thank C. Liu, Y.M., Jiang, H.
Wang, J. Yu, C.W. Deng, Q.C. Yin, C. Lu, and Q. Wang for their
helpful discussions, and assistance in field and core observations, and
sample analyses. Special thanks should be extended to Dr. X.W. Sun
of Sun Petroleum Geoservices, Australia, and Dr. J. Cao of Nanjing
University for checking the English presentation. We also thank Dr.
Y.H. Shuai of the PetroChina Research Institute of Petroleum
Exploration and Development, China, anonymous reviewers and
Petroleum Science editors for their constructive suggestions and
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