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Marine and Petroleum Geology 27 (2010) 909–922

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Marine and Petroleum Geology

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Features and sealing mechanism of shallow biogenic gas in incisedvalley fills (the Qiantang River, eastern China): A case study

Chun-Ming Lin a,*, Yan-Li Li a, Hong-Chun Zhuo b,a, George W. Shurr c, Jennie L. Ridgley d,Zhi-Ping Zhang a, Tao Xue a

a State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, Chinab Chinese Academy of Land and Resource Economics, Mailbox 259#, Beijing 101149, Chinac GeoShurr Resources, LLC, 1803 11th St. 1, Box 91A, Ellsworth, MN 56129, USAd 12860 W 75th Ave., Arvada 80005, USA

a r t i c l e i n f o

Article history:Received 19 November 2008Received in revised form12 November 2009Accepted 13 November 2009Available online 26 November 2009

Keywords:Biogenic gasSealing mechanismIncised valleyQiantang river

* Corresponding author. Tel.: þ86 25 83597334; faxE-mail addresses: cmlin@nju.edu.cn (C.-M. Lin), liy

0264-8172/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.marpetgeo.2009.11.006

a b s t r a c t

Late Quaternary shallow biogenic gas reservoirs have been discovered and exploited in the QiantangRiver (QR) estuary area, eastern China. The fall of global sea level during the Last Glacial Maximumresulted in the formation of the QR incised valley. From bottom to top, the incised valley successions canbe grouped into four sedimentary facies: river channel facies, floodplain–estuarine facies, estuarine-shallow marine facies, and estuarine sand bar facies.

All commercial biogenic gas pools occur in floodplain–estuarine sand bodies of the QR incised valleyand its branches. The deeply incised valleys provided favorable conditions for the generation andaccumulation of shallow biogenic gas.

The clay beds that serve as the direct cap beds of the gas pools are mostly restricted within the QRincised valley, with burial depths ranging from 30 to 80 m, remnant thicknesses of 10–30 m, andporosities of 42.2–62.6%. In contrast, the mud beds cover the whole incised valley and occur as indirectcap beds, with burial depths varying from 5 to 35 m, thicknesses of 10–20 m, and porosities of 50.6–53.9%. The pore-water pressures of clay and mud beds are higher than that of sand bodies, and thedifference can be as much as 0.48 MPa. The pore-water pressures of clay or mud beds can exceed thetotal pore-water pressure and gas pressure of underlying sand reservoirs. Shallow biogenic gas can becompletely sealed by the clay and mud beds, which have higher pore-water pressure. The direct cap bedshave better sealing ability than the indirect cap beds.

Generally, the pore-water pressure dissipation time of clay and mud beds is conspicuously longer thanthat of sand beds. This indicates that the clay and mud beds have worse permeability and better sealingability than the sand beds. However, once the gas enters the sand lenses, the pore-water pressure cannotrelease efficiently.

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1. Introduction

The presence of shallow gas has been recognized for a long time(Vilks et al., 1974; Rice and Claypool, 1981; Shurr and Ridgley, 2002).Since the earliest report of gases in marine sediments (Emery andHoggan, 1958), some geologists have paid great attention to shallowgas in near-surface marine and coastal sediments such as bay orestuarine sediments (Rashid and Vilks, 1977; Van Weering et al.,1997; Albert et al., 1998; Orange et al., 2005; Garcıa-Garcıa et al.,

: þ86 25 83686016.anlishxi@126.com (Y.-L. Li).

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2007). Rice and Claypool (1981) summarized the geologic conditionsfavoring biogenic gas formation and accumulation. Different authors(Schoell, 1983; Whiticar et al., 1986; Floodgate and Judd, 1992; Bayliset al., 1997) have attempted to distinguish freshwater biogenicmethane from that formed in marine environments by using carbonand hydrogen stable isotopes of methane. Heggland (1997) usedthree-dimensional seismic data to detect shallow gas occurrence andseepage in specific parts of the North Sea. Okyar and Ediger (1999)interpreted the acoustic turbidity on high-resolution seismic profilesas representing gas accumulations in the Holocene sediments of theshelf off Trabzon, Turkey. Garcia-Gil et al. (2002) characterized thevarious appearances of nearshore shallow gas in an incised valley fill,taking as an example the Ria de Vigo, NW Spain. Rogers et al. (2006)

C.-M. Lin et al. / Marine and Petroleum Geology 27 (2010) 909–922910

have presented evidence that demonstrates that methane is abun-dant in the western Gulf of Maine, and that gas venting and migra-tion are processes associated with pockmark formation.

Such shallow buried gases are typically rich in methane and arenot associated with oils; methane is the only hydrocarbon actuallygenerated in significant volumes during the biogenic stage (Riceand Claypool, 1981; Schoell, 1983; Floodgate and Judd, 1992; Bayliset al., 1997).

Nowadays, the proved biogenic gas reserve of China is284.39 � 109 m3, which takes up 7.36% of the total geologicalreserves, indicating that there is a big biogenic gas resource potential(Lu et al., 2008). Along the eastern coast of China, shallow gasaccumulations have been found in several environments, such as theChangjiang River delta (Wang, 1982; Lu, 1998) and the QiantangRiver (QR) estuary (Zhang and Cheng, 1995; Jiang et al., 1997; Linet al., 2004). The only significant shallow gas accumulation in theeastern coast of China locates in the QR estuary region, ZhejiangProvince. Successful efforts have been made for exploration andexploitation of shallow gas at depths less than 120 m, especially inthe Taihu and QR incised valleys or its branches of the Last GlacialMaximum (LGM). Since 1991, six shallow gas fields, which includethose at Jiazao and Yisheng to the south of the QR and those at Lei-dian, Haiyan, Jiubao-Xiasha, and Huanggu to the north of the river,and Sanbei Shoal (SBS) shallow gas pool have been discovered in thisregion mainly by cone penetration test (CPT) boreholes (Fig. 1; Jianget al., 1997; Lin et al., 2006). For the purpose of orienting a bridgelocation, a drilling program was conducted by Hangzhou Bay Bridge-Building Company in 2001 and 2002. The analyses of 13 shallow gassamples show that the samples are dominated by methane (up to90%), with minor C2þ hydrocarbons (<0.2%), N2 and CO2, and thatcarbon isotope ratios of methane vary between �84.49& and

Fig. 1. Location of selected cores and transect in the present Qiantang River estuary region. Sand Fig. 2.

�65.8&. These results indicate a biogenic origin for the shallow gas(Lin et al., 2004).

This study of Late Quaternary biogenic gas found at less than120 m in the QR incised valley fills, eastern China, aims to: (1)describe the features and distribution of shallow biogenic gas, withan emphasis on the genesis of shallow biogenic gas sand bodies inthe QR incised valley; (2) document measured pore-water pressureand its dissipation time of Late Quaternary strata; and (3) discussthe sealing mechanism of shallow biogenic gas pools.

2. Geologic setting

2.1. Geomorphology

The Late Quaternary coastal plain of the present QR estuary liesin the areas of 120�–122� E and 29�500–30�500 N, with an area ofaround 20 000 km2 in the northern Zhejiang Province, easternChina (Fig. 1). The coastal plain, surrounded by hills to the west andsouth, slopes gently towards the east. The QR originates from thehilly areas in the vicinity of Hangzhou, flows through the easternplain, and discharges into Hangzhou Bay. Neotectonism in thestudy area is characterized by weak uplift of the hilly lands andslow subsidence in the coastal plain areas without intense folding.The subsidence rate is estimated to be 1–2 mm/yr (Hu et al., 1992).

The present QR estuary extends along the low-lying coastal plainof the passive continental margin of eastern China, towards the EastChina Sea, and is a tide-dominated estuary according to the classi-fication of Dalrymple et al. (1992). It is a funnel-shaped estuary. Thepresent QR estuary is approximately 270 km in length from Lucibuto Nanhui–Zhenhai transect and 100 km in width at the estuarymouth (Lin et al., 2005). The width of the estuary decreases rapidly

BS is Sanbei Shoal. Cores identified by letter and number are included in Tables 1 and 2

Fig. 2. Stratigraphic transect (A–A0) in the present Qiantang River (QR) estuary region showing major lithologies and facies distributions in boreholes (See Fig. 1 for location). Theentire QR incised valley includes river channel and floodplain–estuarine sediments that are now covered by marine and estuarine sediments. River channel sediments are limited todeposits in the incised valley and are composed of gravelly sands and sands with normal upward grading. Facies III occurs mainly in the incised valley, and consists of gray silty clays.Several sand lenses are developed in Facies III.

C.-M. Lin et al. / Marine and Petroleum Geology 27 (2010) 909–922 911

upstream, being 20 and 11 km at Zhapu and Haining, respectively,and 1 km at Hangzhou (Fig. 1; Sun et al., 1990).

2.2. The basement

Late Quaternary sediments are distinguished from the underlyingbasement by their poor compaction, high porosity (40.7–62.6%) andhigh water content (up to 70%). Late Quaternary sediments areseparated from the underlying basement by an undulating erosionsurface in the incised valley proper and by a depositional hiatus inthe interfluve (Fig. 2; Li et al., 2007). Two types of basement havebeen identified: (1) various-colored gravelly sandy clay, 1.6–5.6 m inthickness, which may represent Pleistocene talus or diluvial sedi-ments; and (2) Cretaceous purplish red gravelly sandstones, volcanicrocks, and the weathering products of these rocks, which are foundwithin the thalweg areas of the incised valley.

2.3. Stratigraphic sequence

Based on their lithology, geochemistry and radiocarbon age, thestratigraphic column of the incised valley can be divided, from thebottom upwards, into four facies (Fig. 2): Facies IV–river channelfacies, Facies III–floodplain–estuarine facies, Facies II–estuarine-shallow marine facies, and Facies I–estuarine sand bar facies.

2.3.1. Facies IVFacies IV occurs only in the incised valley and was deposited

during the deep-cutting stage and the early filling stage of the incisedvalley (Lin et al., 2004, 2005).

The fall of sea level before the LGM (18 000–15 000 yr BP) led tothe lowering of base level of the QR, and deep incision of basement,resulting in the formation of the QR and adjacent Taihu incisedvalleys (Fig. 3; Lin et al., 1999a, 2004, 2005). During the deep-cutting stage, most detritus was carried through the valley andtransported into the estuary mouth. Only fluvial gravels and coarsesands were left on the base of the river channel, forming thegravelly sand lag-deposit layers as the basal sediments of Facies IV

(Fig. 2). The majority of Facies IV, sand beds, formed during theearly filling stage, from 15 000 to 12 000 yr BP, when sea level roserapidly from 150 to 160 m up to 50 m below the present sea level,with a rate of sea-level rise 35 mm/yr (Lin et al., 1999a, 2004, 2005).

Facies IV lies on an uneven erosion surface at the bottom, withthe burial depths of 40–120 m, and has a gradational contact withthe overlying bed. The thicknesses of the facies range between 10and 20 m, but may be as much as 45 m (Figs. 2 and 3). The basalsediments show normal grading with a fining-upward trend andthe major portion of the facies reflects a transgressive channel-infilling sequence. Tide-influenced sedimentary structures such assand-mud couplets are absent. Neither foraminifers nor molluscanshells are found in the facies.

2.3.2. Facies IIIThis facies occurs mainly in the incised valley and formed during

the late filling stage of the incised valley corresponding to theperiod of 12 000–7500 yr BP. During this time sea level continuedto rise from�50 to�5 m below present sea level at an average rateof about 10 mm/yr (Lin et al., 1999a, 2004, 2005). With the rise ofsea level, the river mouth shifted gradually landward, and back-water and retrogressive aggradation was enhanced, resulting in theformation of the huge transgressive floodplain–estuarine sequencein the incised valley (Lin et al., 2005).

Facies III, presently buried 30–55 m deep, consists of dark grayclays and gray silty clays (Fig. 2). Ranging from 10 to 30 m with anaverage of 13 m, the thicknesses increase towards the river mouth.The dark gray clays, with organic carbon content generally morethan 0.4%, serve as not only one of the possible main gas sources, butalso supply good preservation conditions. The sediments are char-acterized by silt, sand and gravel lenses about 5–10 m in thicknessand 1–2 km in spacing; and they are surrounded by impermeableclay. The sand lenses can act as the gas reservoirs. Foraminifers andother marine fossils are rare in the middle and upper portion of thefacies. Shell beds, 1.0–2.4 m thick, are usually buried 28–31 m deepand appear to be mostly preserved in place; they can representoriginal sedimentary environment (Zhang and Li, 1996; Lin et al.,

Fig. 3. Biogenic gas distribution in the Late Quaternary Qiantang River and Taihu incised valleys around the time of the LGM. Thickness of the fluvial floor sediments reaches itsmaximum at the incised valleys thalweg. Both depth and width of the QR incised valley increase towards the east. SBS is Sanbei Shoal.

Table 114C age data for core sediments in various sedimentary facies (See Fig. 1 for corelocations).

Core Depth(m)

Material Faciestype

14C age (yr BP) References

Xs3 34.2 Molluscan shell III 7770 � 320 Li et al., 199335.4 Organic clay III 8720 � 250 Li et al., 1993

Xs8 31.0 Organic mud III 9600 � 180 Li et al., 1993J4 23.0 Organic mud II 8220 � 1030 Lin et al., 1999a

38.5 Molluscan shell III 11 985 � 385 Lin et al., 1999aJ3 20.0 Organic mud III 11 270 � 260 Lin et al., 1999aXsh16 15.0 Gagatite I 4163 � 119 Lin, 1996

37.8 Molluscan shell III 8816 � 182 Lin, 1996D5 12.8 Organic mud II 5970 � 910 Lin, 1996

29.8 Organic clay III 11 490 � 210 Lin, 1996D4 30.8 Organic clay III 9445 � 955 Lin, 1996L1 35.3 Organic clay III 10 210 � 100 Lin, 1996SL 5.5 Molluscan shell II 7370 � 140 Yan and

Huang, 198715.5 Molluscan shell Interfluve 28 825 � 540 Yan and

Huang, 1987JLq 1.0 Molluscan shell I 5400 � 75 Yan and

Huang, 19876.0 Molluscan shell II 7400 � 80 Yan and

Huang, 1987D-1 9.4 Organic mud II 3140 � 155 Li et al., 1986

12.6 Organic mud II 5725 � 250 Li et al., 198616.5 Organic mud II 7950 � 550 Li et al., 198617.7 Organic mud II 10 025 � 250 Li et al., 198620.6 Organic mud III 10 165 � 625 Li et al., 198625.5 Stiff clay Interfluve 15 005 � 625 Li et al., 1986

C.-M. Lin et al. / Marine and Petroleum Geology 27 (2010) 909–922912

2005). A set of tide-influenced sedimentary structures such as sand–mud couplets, horizontal-, wavy-, lenticular- and bimodal-crossbeds, are common in the upper portion of the facies. 14C age of thefacies ranges from 7700 to 12 000 yr BP (Table 1; Lin et al., 2005).

2.3.3. Facies IIEstuarine-shallow marine facies, directly overlying Facies III,

was formed at the earlier half of the burial stage, 7500–4000 yr BP(Lin et al., 2004, 2005). Taihu and QR incised valleys began to beburied during the maximum transgression, when the sea floodedthe incised valleys and inundated adjacent interfluves. The coast-line migrated landward over the valley fill, and estuarine-shallowmarine sediments were deposited over Facies III.

The sediments of Facied II are buried at depths of 7–35 m andhave a remnant thickness normally of 10–20 m with an average of17.1 m (Lin et al., 2004, 2005). In the vertical transect, this faciestransition is abrupt and distinctive, and corresponds to an obviouslyundulating interface formed by the action of the river flow and tidalcurrents (Fig. 2; Lin et al., 2005). The bulk sediment consists mainlyof soft gray mud and a large percentage of water (up to 70%), but siltyand shelly laminae of 2–3 mm thickness and silt and sand lenses,1–2 m thickness with a maximum thickness of 8 m, are also present.The gray mud is the other possible main gas sources and can serve asthe cap beds. Abundant foraminiferal fossils are present in thesediment, with up to several hundred to more than 1000 individualsin a 50 g dry sample. 14C dating of the facies ranges between5970 � 910 and 8220 � 1030 yr BP (Table 1; Lin et al., 2005).

2.3.4. Facies IEstuarine sand bars formed during the later half of the burial

stage 4000 yr BP to present when sea level was relatively stable (Linet al., 2005). During this period, sedimentation rate exceeded that

C.-M. Lin et al. / Marine and Petroleum Geology 27 (2010) 909–922 913

of sea-level rise, resulting in the seaward shift of the coastline andthe formation of estuarine sand bars, which prograded over theestuarine-shallow marine sediments in the valley and interfluves.

Facies I is comprised mainly of well-sorted gray silt, sandy silt,and silty-fine sand; thicknesses range from 5 to 20 m and average of16.4 m (Fig. 2). The silty components account for more than 60–80%of the facies. At the base of this facies, there is a tidal ravinementsurface, and there are silty-fine sand layers of 1–3 m thick, con-taining shell fragments, marine authigenic glauconite and muddylaminae (Lin et al., 2005). The sediments of the facies demonstratea typical fining-upward succession. There are abundant foramini-fers and ostracoda, though not so abundant as in Facies II. The fossilshells are small-sized (i.e., mainly larvae shells) and broken, indi-cating that they had been transported by strong currents (Zhangand Li, 1996). 14C dating shows an age of 4163� 119 to 5400� 75 yrBP (Table 1; Lin et al., 2005).

3. The features and distribution of shallow biogenic gas

3.1. The distribution of shallow biogenic gas

Six shallow gas fields (i.e., Jiazao, Yisheng, Jiubao-Xiasha, Haiyan,Huanggu and Leidian) and one pool (i.e., SBS) have been discoveredin the north of Zhejiang Province since 1991. With the exception ofthe Leidian shallow gas field located in the Taihu incised valley, theothers occur in the QR incised valley or its branches (Fig. 3). In termsof reserves, producibility and duration of stable production, theYisheng and Jiazao shallow gas fields and the SBS shallow gas poolare better than the others (Lin et al., 2004). This may be attributed tothe larger size of the QR incised valley. The greater the depth andwidth of an incised valley, the thicker the sediment fill. The greaterthickness of sediment fill increases the potential thickness ofhydrocarbon source beds and reservoir development under theright depositional conditions. The small size of the Taihu incisedvalley and the branches of the QR incised valley (Fig. 3) do not favorthe development of large potential gas reservoirs.

The deeply Taihu and QR incised valleys in the study areaprovided favorable conditions for the generation and accumulationof shallow biogenic gas. The Taihu and QR incised valleys wererapidly and entirely filled by fluvial–estuarine sediments providedby the western and southern hills, thereby preserving organicmatter. The dark gray clays of Facies III and gray muds of Facies II arepossible main gas sources for the shallow gas. Sediments of bothfacies have organic carbon content, generally more than 0.4% (Linet al., 2004).

Four types of Late Quaternary gas potential reservoirs have beenrecognized in the QR estuary region (Fig. 2): (1) gravelly sand andsand beds in Facies IV; (2) sand lenses in Facies III; (3) sand lensesintercalated in sediments in Facies II; and (4) silts and sands inFacies I. According to the exploration data available for the QR area,all the commercial gas fields and pools are developed in sand lensesof Facies III (Lin, 1996; Jiang et al., 1997; Lin et al., 2004). Deter-mining the size, shape, porosity, and permeability of the sandbodies is essential for exploration prediction and exploitation ofthese shallow biogenic gas fields.

Thirty-nine boreholes unexpectedly penetrated the gas-bearingzones at SBS in the Hongzhou Bay area (Figs. 1, 2 and 4A). Nineboreholes penetrated the clay beds of Facies III and mud beds ofFacies II at depths of 18.5–53.5 m. In the nine boreholes, discharge ofgas bubbles from the mud of the boreholes lasted for 0.5–5 min afterthe drills were pulled out. The other boreholes encountered silts, finesands and medium sands of Facies III at depths of 45.0–61.0 m withgas, mud and sandy sediments ejecting from the bores when thedrills were withdrawn (Figs. 2 and 4). Some of the gas encounteredignited. Intense gas flows from some of these boreholes reached as

much as 10 m–20 m in height (Fig. 4B). Gas flow disturbed the seasurface and the welter of water lasted several days to several monthsafter the boring-casings were removed (Fig. 4C). Then a large numberof gas bubbles still emitted to the water surface and this conditionlasted up to three months.

Sand bodies encountered in the boreholes vary in thicknessbetween 5 and 15 m, up to a maximum of 26 m (Fig. 2). Boththicknesses and burial depths of these sand bodies are greater thanthose in the Jiazao and Yisheng shallow gas fields, but they are stilllenticular in shape, and represent Facies III of the incised valley.Along the ancient river flowpath (eastward), the lenticular sandbodies are distributed in a long and narrow zone. Biogenic gas atSBS is stored in the upper 5 m of a number of sand bodies, andminor shallow biogenic gas is found in the clay and mud bedsencasing the sand lenses.

The mud proportion was increased in the subsequent drilling toavoid the ejection of the gas (Xiang et al., 2005). Therefore, some ofthe sand bodies in Facies III have no gas ejection (Fig. 2), whichsuggests that biogenic gas may occur, but its escape is hindered bythe mud coating of permeability pathways.

Based on borehole analysis, the sand lenses in Facies III are themajor gas reservoirs. The sand lenses are found both vertically orhorizontally within the Taihu and QR incised valleys, and theirthicknesses and burial depths gradually increase eastward.

3.2. Genesis of shallow biogenic gas sand bodies

Sand lenses that were formed in the floodplain–estuarine envi-ronment (Facies III) vary significantly in the thickness (from 0.5 toover 10 m) and burial depth (from 28 to 80 m, even up to 90 m inTransect A–A0 of Fig. 2). In adjacent boreholes, the depth differenceof sand lenses reaches over 10 m (Fig. 2). In some cases, in a singleborehole up to 3–4 sand lenses, with a total thickness of 7–8 m, maybe encountered (Li et al., 1993; Lin et al., 2004). In the presentestuary mouth, the total thickness of sand lenses may exceed 20 m(Transect A–A0 in Fig. 2), but sand layers may be absent in a neigh-boring borehole. All the sand bodies are surrounded entirely byclays in Facies III, and are separate lenses having flat tops and convexbottoms (Fig. 2). The sand bodies also vary in size from small to largescale. A single sand lens can extend for several kilometers, anda complex body which is composed of many superposed individualsands can extend for a few kilometers in width and more than 10 kmin length. This sand complex has been previously interpreted to beformed in the floodplain environment influenced by tidal current (Liet al., 1993; Lin et al., 2004).

The particle size of the sand lenses in Facies III varies widely andlithologies are described as silt, sandy silt, silty-fine sand, fine sandand medium sand (Table 2). Within a sand lens, particle size alsovaries vertically. In some cases there is a fining-upward trend, butin other cases fine-grained material is at the base (e.g. Jiazao GasField, Lin et al., 2004). Complicated coarse-fine-coarse trends, andhomogeneous trends are also found. All of the grain size distribu-tions are caused by the change in discharge and intensity of thewater flow. Generally, median diameter (Mz) of the sands rangesfrom 2.60 to 4.85F, and standard deviation (s) varies between 0.65and 3.86F. In two cores examined, most of the cumulative grainsize distributions can be described by two or three distinct curves,although a linear trend was observed in core J3 (Fig. 5). According toVisher (1969), distinct distribution curves represent differentmodes of transport. The grain size data indicate that the sandysediments were transported mainly as bed load.

The sand bodies emplaced along the paleocurrent direction ofthe QR incised valley. The size of the sand bodies varies, as indicatedby the transect A–A0 . As shown in Fig. 2, the sand bodies are in theform of lenses, around 10 m in thickness and 1–2 km in width. These

Fig. 4. A. Biogenic gas distribution in boreholes in the eastern part of the Qiantang River incised valley around the time of the LGM along transect A–A0 (See Fig. 1 for location).B. Vertical stream of flowing gas, mud and sandy sediments. C. Gas flow disturbing the sea surface.

C.-M. Lin et al. / Marine and Petroleum Geology 27 (2010) 909–922914

Table 2Size-grade of the floodplain–estuary sand bodies in the Qiantang River incised valley (See Fig. 1 for core locations).

Core Sampledepth (m)

Size-grade distribution (%) Lithology Mz (F) s (F)

Coarsesand �1w1F

Medium sand1w2F

Fine sand2w4F

Coarse silt4w6F

Fine silt6w8F

Clay >8F

Xs3 31.8 nda nd 53.04 28.83 5.37 12.76 silty-fine sand 4.85 1.7235.0 nd nd 71.94 7.35 7.95 11.59 fine sand 4.19 2.32

Xs13 26.9 0.41 19.70 55.92 6.33 6.71 10.93 3.65 2.4032.5 nd 3.43 75.47 6.82 6.60 7.67 3.59 1.9238.5 nd 4.98 87.96 2.30 2.14 2.61 2.60 0.8144.1 nd 5.98 88.43 2.27 1.56 1.75 2.70 0.65

Xs14 29.6 nd 1.56 68.56 7.98 8.95 13.24 4.32 2.5642.3 nd 4.68 85.49 3.72 2.15 3.96 2.88 1.06

Xs16 35.8 0.21 4.50 49.64 12.69 14.89 18.07 Silty-fine sand 4.77 3.05Xs17 40.1 8.34 29.18 18.98 14.23 11.82 17.46 Fine-medium sand 4.35 3.50J3 3.90 nd 2.91 65.28 9.45 12.18 10.16 Silty sand 4.34 2.31

41.0 nd 6.79 72.04 11.02 7.28 2.84 3.29 1.5243.0 nd 1.12 68.16 15.81 7.89 6.99 4.15 1.76

J4 43.0 nd 35.68 40.78 4.26 0.91 18.35 Medium-fine sand 4.70 3.8643.6 nd 29.84 46.60 12.43 2.77 8.33 3.07 2.2044.0 nd 33.82 42.45 5.82 3.24 14.65 3.69 3.00

a nd ¼ no detected.

C.-M. Lin et al. / Marine and Petroleum Geology 27 (2010) 909–922 915

features may indicate that the sand bodies are tidal ridges formed ina large, broad estuary. Tidal ridges are widely distributed on conti-nental shelves and coastal waters; these large-scale bed forms areusually 10–30 m in height and 1–5 km in spacing, and normally formin water depths of 20–40 m (Collins et al., 1995). For the formation ofa tidal ridge system, there must be strong rectilinear tidal currents,abundant sandy sediment supply and sufficient accommodationspace (Dyer and Huntley, 1999). In Facies III, the sand bodies weredeposited approximately 7500–12,000 yr BP, based on 14C dating ofmolluscan shells and encasing organic mud (Table 1). During thatperiod of time, sea-level rose from �50 to �5 m (Wang, 1982; Caiand Lin, 1984; Yan and Hong, 1987; Yan and Huang, 1987), such that

Fig. 5. Cumulative grain size distributions of sand bodies in Facies III in the cores J3 and JA. Grain size distributions in core J3. B. Grain size distributions in core J4.

the water depths were suitable for the formation of tidal ridges.Numerical simulations show that there were strong tidal currentsover the region in the same period (Zhu, 2000, 2002; Uehara andSaito, 2003). Such a tidal regime would cause strong, rectilinearcurrents within the QR incised valley. During the period of sea levelrise, the coarse-grained sediment on the shelf areas tends to bereworked and transported landward, providing the QR valley withabundant sandy materials. Furthermore, when the sea-level rose,the incised valley, that was formed during the low sea-level period,provided a large accommodation space for the deposition andinfilling of sediment. As a result, tidal ridges were formed duringtransgression, and they are present as sand lenses on a transect

4 of the Xiaoshan area (See Fig. 1 for locations. Arrows show the depths of samples).

Fig. 6. Transect across the Jiazao shallow gas field. A. Plan view of the Jiazao shallowgas field showing location of transect B–B0 (See Fig. 1 for location). B. Profile across theJiazao shallow gas field showing facies distribution and different kinds of cap bedsdistribution. The main gas reservoirs are buried at a depth of 42–58 m and arecomposed of several discontinuous sand lenses confined in impermeable clays.

C.-M. Lin et al. / Marine and Petroleum Geology 27 (2010) 909–922916

through the valley. According to a study by Zhao et al. (2004), theChangjiang River delta, which is close to the north of the study area,was also a huge paleo-estuary at that stage and a tidal ridge system,with the apical area located near Zhenjiang and Yangzhou city,formed within it. The formation and evolution of the ChangjiangRiver incised valley are similar to those of the QR incised valley(Li et al., 2000, 2002, 2008; Lin et al., 2005). Such sand bodies arerare in Facies I and II, for several reasons. First, after the sea-levelrose, the study area began to receive fine-grained sediments fromthe Changjiang River, and the supply of sandy materials becamelimited. Thus, Facies II consists mainly of muddy sediments. Second,with the sediment accretion in the estuarine waters, the accom-modation space decreased, enhancing the tidal currents. Conse-quently, the tidal current speed became extremely high, preventingthe fine-grained material from accumulating in the channels.Muddy sediments would have accumulated on intertidal flats. Thesandy sediments became very mobile under the very high currentspeeds, like the situation of today. This explains the formation ofa giant sand bar (i.e., a large sand sheet) and the stratigraphicstructure of Facies I.

4. Sealing mechanism of shallow biogenic gas pools

4.1. The cap beds of shallow biogenic gas pools

Any natural gas accumulation is the result of relative equilib-rium to some degree between constant diffusion from andsupplement of the gas to the reservoir. The more the natural gas isgenerated, the greater the diffusion (Hao et al., 1991; Krooss andLeythaeuser, 1997), especially in shallow biogenic gas accumula-tions (DeLaune et al., 1986; Whiticar, 1999; Lin et al., 1999b). Sealingconditions play an important part in the formation of shallowbiogenic gas pools and are critical to the gas pool formation.

In the QR area, all the commercial gas fields and pools aredeveloped in sand bodies of Facies III. Both clay beds of Facies IIIand mud beds of Facies II serve as the cap beds on the shallowbiogenic gas reservoirs. The clay beds, as direct cap beds, are mainlyrestricted within the incised valley (Figs. 2, 6 and 7). The clay bedshave burial depths ranging from 30 to 80 m, remnant thicknessesfrom 10 to 30 m, porosities of 35.6–54.5%, water saturation of 70.1–100.0%, grain densities of 2.71–2.76 � 103 kg/m3, compressibilitycoefficient of 0.25–1.08, plastic index of 17.5–31.7, and liquidityindex of 0.13–1.09 (Fig. 2; Table 3), and area w8400 km2 (Lin et al.,2004). The direct cap beds have the cone tip resistance (qc) rangingfrom 1.1 to 1.9 MPa, the sleeve friction (fs) ranging from 0.02 to0.05 MPa and, the frictional ratio (Rf ¼ fs/qc) from 1.8% to 3.3%(Table 3). In contrast, the mud beds covering the whole incisedvalley were deposited in the estuarine-shallow marine environ-ment, and occur as indirect cap beds vertically overlying thefloodplain–estuarine sediments (Figs. 2, 6 and 7) with a lateralextent of 13 825 km2 in the Hangzhou Bay area (Lin et al., 2004).Based on core data, burial depths and remnant thicknesses of mudbeds increase seaward; burial depths range from 5 to 35 m andremnant thicknesses range from 10 to 20 m in the mouth of the QRand up to 38.5 m in Haiyan-Huanggu shallow gas field (Fig. 2; Table3). The thickness of estuarine-shallow marine deposits was influ-enced by the lower palaeotopographic fluctuation and subsequentflow erosion at the top. The thickness will be less, even absent,where the erosion was most pronounced in Facies II. The zerothickness contour of estuarine-shallow marine deposits is foundalong the hills in the west and south and northward extends toHaining area. The indirect cap beds have porosities ranging from48.9% to 61.4%, water saturation ranging from 82.6% to 100.0%,grain density from 2.72 to 2.73 � 103 kg/m3, compressibility coef-ficient of 0.40–0.80, plastic index of 10.1–16.7, and liquidity index of

1.02–1.81. The qc, fs and Rf of the indirect cap beds respectively are0.6–1.3 MPa, 0.00–0.03 MPa and 0.5–1.9% (Fig. 2; Table 3). Withrespect to mechanical properties, the clay and mud beds havea higher compressibility coefficient, higher liquidity index, andgreater shear strength than silty beds, indicating that they arecharacterized by weaker deformation resistance, highercompressibility, and better sealing ability (Lin et al., 1999b).

The qc, fs and Rf of the clay beds in Facies III are all higher thanthose of mud beds in Facies II. This demonstrates that the clay bedshave greater stickiness and better sealing ability than the mud beds.

During the construction of borehole JS1 in Jiazao shallow gasfield, the emission of gas became visible in surface when the drillbit was pulled out up to 26 m after the total depth of 47.3 m hadbeen reached. When the drill bit was pulled out up to 16 m, theemission became more intense, and gas emission are invisiblewhen the drill bit was put down again to 26.5 m (Figs. 6 and 7). It issuggested that the sediment below 26 m has much better sealingability than the sediment above it. At the same time, 26 m is theboundary between Facies II and Facies III. This also suggests that theclay beds of Facies III (the direct cap beds) have better sealing abilitythan mud beds of Facies II (the indirect cap beds).

4.2. The data and methods

In the unconsolidated Late Quaternary sediments, the cap bedshave a high porosity, high permeability, small capillary pressureand poor capillary sealing ability. It is high pore-water pressure of

Fig. 7. Lithology, facies distribution, cone penetration curves, curve of measured pore-water pressure, and computed frictional ratio of Late Quaternary strata of borehole JS1 in theJiazao shallow gas field (See Fig. 6 for location). Results of test numbers 1–10 are shown in Table 4. The increase of qc, fs and Rf, from the mud beds of Facies II to the clay beds ofFacies III, indicates that the stickiness and sealing ability of sediments is enhanced.

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the cap beds that is likely the important mechanism for the pres-ervation of the shallow biogenic gas which is the combined actionof pore-water pressure, capillary pressure and hydrocarbonconcentration (Lin et al., 1999b, 2004). Consequently, themeasurement of the pore-water pressure in the cap beds is aneffective way to evaluate the sealing mechanism of the shallow gasfields. In this paper, we studied the pore-water pressure sealingpotential of the cap beds of Facies III by using experimental data of

pore-water pressure and its dissipation time obtained from conepenetration tests with pore pressure measurements (piezocone testor CPTU). CPTU is one of the in situ measurements in foundationsoils (Lin et al., 2004).

The CPTU test has been commonly used in foundation engi-neering and geotechnical subsuperficial studies because of itsexcellent potential for in situ continuous vertical profiling in non-cemented soil at a fast rate and low cost (Amorosi and Marchi, 1999;

Table 3Properties contrast between the direct cap beds and indirect cap beds.

Properties Cap beds

Direct cap beds Indirect cap beds

Distribution area Restricted in theincised valley,about 8400 km2

The whole incised valley,about 13 825 km2

Lithology Clay of Facies III Mud of Facies IIDepth (m) 30–80 5–35Thickness (m) 10–30 10–20Porosity (%) 35.6–54.5 48.9–61.4Water saturation (%) 70.1–100.0 82.6–100.0Grain density

(103 kg/m3)2.71–2.76 2.72–2.73

Compressibilitycoefficient (MPa�1)

0.25–1.08 0.40–0.80

Plastic index 17.5–31.7 10.1–16.7Liquidity index 0.13–1.09 1.02–1.81Qc (MPa) 1.1–1.9 0.6–1.3Fs (MPa) 0.02–0.05 0.00–0.03Rf (%) 1.8–3.3 0.5–1.9Sealing ability Better Good

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Lafuerza et al., 2005). The CPTU differs from a normal cone pene-tration test (CPT) by the presence of a porous filter that allows pore-water pressure measurement. The filter is immediately behind thecone tip (60� of cone opening angle, 3 cm of height and 10 cm2 ofbasal area), that is located at the lower end of a 150 cm2 successionof metallic cylinders. A conducting cable through the cylinder linksthe cone tip to the recording units so that the acquired data can bedisplayed on line. The field data acquisition system includesanalogue to digital converters so that the analog signals can bedirectly converted to digital form for data logging. When the cone ispushed down into the sediment at a constant speed of 2 cm/s by theforce of a push engine with thrust capacity of 20 tonnes, qc(measured in MPa), fs (measured in MPa), Rf, and the pore-waterpressure (u, measured in MPa) measurements are visualized on thecomputers and stored on floppy disks for processing and plotting inreal time. This procedure allows encountered formation conditionsto be analyzed in the field. The pore-water pressure (u) is describedby: u ¼ u0þDu, where u0 is the calculated static equilibrium porepressure and Du is the change in pore pressure that is created atcone penetration. During a stop in penetration, the excess pore-water pressure (Du) starts to dissipate, and the decay of the excesspore-water is monitored and then plotted against time (Amorosiand Marchi, 1999; Meng et al., 2001; Lafuerza et al., 2005).

The CPTU test doesn’t need sampling, so the result has goodreliability and repeatability. In addition, adoption of the electricalmeasurement method facilitates the automation and computerizedmanagement. During the CPTU test, the pore-water pressure ismeasured every 2 cm. Therefore, the pore-water pressure iscontinuous and the original pore-water pressure can be describedprecisely in the whole procedure. The CPTU is not applicable for thestony clay and gravel stratum, and we cannot observe and identifydirectly the soil by the CPTU, The relation between the engineeringproperties of soils and measuring data is still based on the plenty ofexperience.

The CPTU test was performed on borehole JS1 in the Jiazaoshallow gas field in Xiaoshan city, Zhejiang Province (Fig. 6). TheLMC-D130 type cone penetration microcomputer which was madeby Liyang Keer Instrument Company Limited of Jiangsu Province,China, was used in the test. Besides qc, fs, Rf and u, 10 pore-waterpressure dissipation tests were carried out on key horizons, such ascontinuous cap beds, reservoirs and their interface (Figs. 7 and 8).These data can be used to describe the mechanism of formationand main aspects of the pore-water pressure in the shallow gasfield.

4.3. Sealing mechanism by the pore-water pressure of shallowbiogenic gas cap beds

The CPTU test on borehole JS1 of Jiazao shallow gas field showsus the variation of pore-water pressure (Fig. 7; Table 4). Pore-waterpressures of sandy sediments above the water table, which kicks inat 5.9 m depth, are low, about 0.01 MPa. Pore-water pressures ofpelitic silts at depths of 6.0–9.1 m increase approximate linearly,with a maximum up to 0.13 MPa. The pore-water pressures of mudat depths of 9.2–26.6 m in Facies II increase from 0.14 to 0.58 MPa.Pore-water pressures of silty clay in Facies III, at depths of26.7–36.5 m, are irregular but higher than those observed inshallower sediments, with the maximum up to 0.79 MPa at thedepth of 36.5 m. Pore-water pressure decreased sharply to0.31 MPa at the depth of 36.6 m where the fine-grained sand bedsare first encountered with the difference up to 0.48 MPa. The pore-water pressure changes little below this depth. Fig. 7 shows that:(1) the pore-water pressures of mud and clay beds are obviouslyhigher than that of sand. Compared to the calculated hydrostaticpressure, the measuring pressures of mud and clay beds are higher,while those of sand beds are close; (2) the pore-water pressure ofthe same lithology increases with increasing depth, and theincrement of pore-water pressure in argillaceous sediments is morethan that in sandy sediments. These observations suggest that forthe cap beds, the greater the burial depth, the stronger the sealingability; and (3) the maximum pore-water pressure occurs at the topof the shallow gas reservoirs, just at the interface between gasreservoirs and argillaceous cap beds. The original gas pressure ofshallow gas fields in the study area ranges from 0.22 to 0.40 MPa,with a maximum of 0.46 MPa (Lin et al., 2004). Thus the pore-waterpressure of the clay or mud cap beds can exceed the total of pore-water pressure and original gas pressure in the underlying sandreservoirs. The difference in pressure between the cap beds andreservoirs is the fundamental cause for the existence of cap beds’sealing ability (Dai et al., 1986). Therefore, considering pore-waterpressure, the shallow biogenic gas can be completely sealed by theoverlying clay and mud beds, and the pore-water pressure plays animportant part in the preservation of biogenic gas reservoirs.

In the study area, the pore of cap beds is more in quantity butsmaller in size than that of reservoir, because the cap beds havehigher porosity and lower permeability than the reservoirs. Thecapillary pressure needed for the gas to penetrate the pore of capbed is high, and the migration of gas is prevented. The capillarysealing exists in the study area, however, further research is neededto ascertain its effect on the conservation of the gas.

The result of the pore-water pressure dissipation experiment onborehole JS1 in Jiazao shallow gas field (Fig. 8; Table 4) shows thatthe pore-water pressure dissipates relatively fast and conspicu-ously in the mud and silty clay beds above 36.5 m during 1–1000 or2000 s. By way of contrast, in the fine-grained sand reservoirsbelow 36.5 m, the pore-water pressure doesn’t decrease and evenincreases locally with the time during 0–700 or 1500 s. This indi-cates that the pore-water pressure of fine-grained sand beds below36.5 m dissipates very slowly.

In an area without shallow biogenic gas accumulations, take theborehole T1 of Tongji University, Shanghai as an example (Fig. 9;Table 4), the pore-water pressure dissipates slower in the sandy siltat the depth of 5.7 m than silty clay at depths of 2.8 and 22.5 mduring 1–400 s. The pore-water pressure and dissipation time ofsandy silt are relatively low, respectively, 0.25–0.35 MPa and60–120 s (Table 5). The time needed for the pore-water pressure ofsandy silt to achieve a stable state is only a little more than 10 min,and it seems not to be related with the burial depth. Pore-waterpressures in the mud and clay beds are relatively higher than thosein silty sediments, 0.90–1.60 MPa, and the dissipation time is

Fig. 8. The pore-water pressure dissipation time of Late Quaternary strata of borehole JS1 in the Jiazao shallow gas field (See Fig. 6 for core location and Fig. 7 for the test sites).A. Cap beds show obvious dissipation and high pore-water pressures. B. Reservoir sands show essentially no dissipation and lower pore-water pressures (Arrows point to individuallithology locations. See results in Table 4).

C.-M. Lin et al. / Marine and Petroleum Geology 27 (2010) 909–922 919

longer, 350 to greater than 500 s (Table 5). At greater depths,dissipation time may be more than 10 h and sometimes the steadystate is even difficult to get. This indicates that the mud and clayhave worse permeability and better sealing ability than the sandysilts, and that with the increasing burial depth, compaction andsealing ability of the mud and clay are enhanced. The increase ofburial depth has little effect on the compaction of the sandy silts(Table 5; Fig. 9).

It is concluded that the pore-water pressure dissipation of sandysediment in the area with shallow biogenic gas accumulationsmarkedly slows down. This may be caused by the prompt supply ofbiogenic gas after the pore-water pressure dissipation of the fine-grained reservoirs. Once the gas enters the sand lenses, the pore-water pressure can’t be released easily.

There are several possible causes for the formation of the pore-water pressure sealing mechanism in the study area.

First, clay and mud beds have higher natural water content,grain density, plastic index, liquidity index, and adhesion than thesandy sediments, and have strong plasticity and certain self-sealingeffect. The presence of abundant organic matter dispersed in theclay and mud beds increases the disconnected pores space, and

Table 4The pore-water pressure dissipation time of Late Quaternary strata of borehole JS1 in th

Site Testnumber

Depth(m)

Lithology Dissipationtime (s)

Start Fin

Borehole JS1 in Jiazaoshallow gas field(with biogenic gas)

1 25.5 Mud of Facies II 1 1002 29.5 Silty clay of Facies III 1 2003 33.5 1 1504 36.5 1 1505 37.1 Fine sand of Facies III 1 1506 38.5 1 1507 40.5 1 1508 41.7 Clayey silt of Facies III 1 1509 43.5 Fine sand of Facies III 1 100

10 47.3 Clay of Facies III 1 70Borehole T1 of

Tongji University(no biogenic gas)

1 2.8 Silty clay of Facies I 1 402 5.7 Sandy silt of Facies I 1 403 20.0 Clay of Facies III 1 6004 22.5 Silty clay of Facies III 1 40

decreases the cross section of connected pores (Dai et al., 1986). Theresult is that pore-water and its pressure are difficult to dissipate.

Second, the Late Quaternary subsidence rate is estimated to be 1–2 mm/yr (Hu et al., 1992). Sea level initially ascended rapidly witha speed over 10 mm/yr, then slowed to become stable, and the averagesedimentary rate of Facies III in Hangzhou Bay area is 2.9 mm/yr (Linet al., 1999a, 2005). Within Facies III, the cap beds are differentiallycompacted. Cap beds near the permeable sandy reservoirs are morecompacted forming a sealing layer, but the thick middle clay beds maybe undercompacted because of the difficulty of removing pore-water.Uncompacted clay cannot seal only oil and gas in free phase, but alsooil and gas in water-dissolved phase (Fu et al., 1995).

Third, smectite clay has favorable properties of wetting,swelling, low permeability, and good self-sealing ability, therefore,it is widely used for making liner material (Bai et al., 2000; Chenet al., 2006), the base bottom liner in the garbage landfill (Chenet al., 2006), and sealing purposes in repositories for nuclear waste(Parker and Rae, 1998; Villar et al., 2006). Just for this special featureof smectite clay, groundwater has little influence on the supportingof soft rock roadway (Yao and Qiu, 1999), and preserve the ancientivory unearthed from Chengdu Jinsha site for more than 3000 years

e Jiazao shallow gas field, and of borehole T1 in Tongji University, Shanghai.

Pore-waterpressure (MPa)

Difference ofPore-waterpressure (MPa)

Rate of pore-waterpressure dissipation(10�6 MPa/s)

Variation ofpore-waterpressure

ish Start Finish

0 0.457 0.297 0.160 160.000 Obvious0 0.588 0.328 0.260 130.0000 0.648 0.452 0.197 131.3330 0.776 0.473 0.303 202.0000 0.320 0.322 0.002 1.333 Unobvious0 0.299 0.322 0.023 15.3330 0.285 0.342 0.057 38.0000 0.311 0.313 0.002 1.3330 0.310 0.338 0.028 28.0000 0.420 0.407 0.013 18.5710 0.470 0.160 0.310 775.000 Obvious0 0.331 0.105 0.226 565.0000 0.850 0.135 0.715 119.1670 1.120 0.705 0.415 1037.500

Fig. 9. Lithology, facies, and pore-water pressure in Late Quaternary strata in boreholeT1, Tongji University, Shanghai. A. Lithologic profile, facies distributions, and pore-water pressures. B. Curves showing measured pore-water pressure dissipation.

Table 5The measured value and dissipation time of pore-water pressure in the LateQuaternary strata, Shanghai area (From Lin et al., 1999b).

Lithology Depth (m) Pore-waterpressure (MPa)

Dissipationtime (s)

Sealing ability

Gray sandy silt 3.00–18.70 0.25–0.35 60–120 Very badDrab silty clay 1.40–5.60 0.42–0.76 150–200 BadGray muddy

silty clay2.10–12.00 0.60–0.80 200–300

Dark greensilty clay

25.60–34.00 0.80–1.05 240–300 Medium

Gray clay 19.00–25.60 0.90–1.20 350–410 GoodGray mud 12.00–19.00 1.40–1.60 >500 Very good

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(Dan et al., 2006). When soils contained smectite clay suffer fromrainfall or sprinkler or surface irrigation, a depositional seal will beformed and accompany with dramatic reduce in infiltration (Mer-mut et al., 1995; RoSmkens et al., 1995; Parker and Rae, 1998; Ladoet al., 2007;). In the study area, illite/smectite mixed layer dominatein the argillaceous cap beds, accounting for 28–45% of the total clayminerals content. The water content of the cap beds is high (Table3). The existence of plentiful illite/smectite mixed layer suggeststhat smectite has wetted and swelled in the study area. This hasresulted in the smaller connected pores, lower permeability, andhigher pore-water pressure in the clay beds.

5. Conclusions

Late Quaternary shallow biogenic gas reservoirs have recentlybeen discovered and exploited in the QR estuary area, ZhejiangProvince, eastern China. The QR in this area strongly incised theunderlying older beds during the period of glacial maximum andsea-level fall, which resulted in the formation of the QR incisedvalley. The incised valley initially was filled with fluvial sedimentsand subsequently buried by marine sediments during the post-glacial period. Late Quaternary strata of the incised valley area arecomposed of four sedimentary facies in ascending order: riverchannel, Facies IV; floodplain–estuarine, Facies III; estuarine-shallow marine, Facies II; and estuarine sand bar, Facies I.

All commercial shallow biogenic gas is stored in the floodplain–estuarine (Facies III) sand lenses in the Taihu and QR incised valleysor its branches. The deeply incised valleys in the study areaprovided favorable conditions for the generation and accumulationof shallow biogenic gas. The main hydrocarbon sources are darkgray clays of Facies III and gray muds of Facies II. Sediments of bothfacies have organic carbon content generally more than 0.4%. Thesand lenses associated with Facies III may represent tidal ridgesthat formed in the incised valley. The conditions of sea-level rise,tidal regime, and sediment supply and accommodation spaceappear to favor the development of a tidal ridge system. The sandylenses of Facies III provide favorable conditions for in situ strati-graphic entrapment of biogenic methane.

The clay beds of the floodplain–estuarine environment (FaciesIII) form the direct cap beds for the shallow gas pools and aremainly restricted within the incised valley. In contrast, the mudbeds of estuarine-shallow marine environment (Facies II) coverthe entire incised valley and occur as indirect cap beds. The pore-water pressure of the clay and mud beds of Facies II and III canexceed the total pore-water pressure and gas pressure of under-lying sand reservoirs, and the maximum original gas pressure isdetermined to be 0.46 MPa. The maximum pore-water pressureoccurs at the top of the shallow gas reservoirs, just at the interfaceof gas reservoirs and argillaceous cap beds. The shallow gas poolsin the study area are characterized by pore-water pressure sealing,which plays an important role in the preservation of the biogenicgas accumulations.

C.-M. Lin et al. / Marine and Petroleum Geology 27 (2010) 909–922 921

Based on the experimental data, the pore-water pressures anddissipation times in sandy silt beds are relatively lower and thepenetrability is better than clay and mud beds. Consequentlythe sealing ability of sandy silt beds is significantly less than that inthe overlying clay and mud cap beds. On the other hand, the pore-water pressures and dissipation times in clay or mud beds arerelatively higher and the penetrability is less than the sandy silt beds.Therefore the sealing ability is better, especially in the clay beds.Because of these properties, the clay and mud beds of Facies II and IIIserve as caps for the preservation of shallow biogenic gas. Withincreasing burial depth, the degree of compaction and lithificationintensify and the sealing ability increases. Thus, the deeper clay bedsof Facies III have better sealing ability than the mud beds of Facies II.

Acknowledgements

This research is supported jointly by the National Natural ScienceFoundation of China (Grants No. 40872075 and No. 40272063). J. R.Tang and M. S. Fang of the Hangzhou Bay Bridge-Building Companyenthusiastically provided the authors with data. Thanks should begiven to Professor X. Z. Li of Nanjing University discussions withwhom greatly benefited the authors. The constructive suggestionsof Marine and Petroleum Geology editor John Holbrook and thereviewer Dave Dewhurst.

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C. M. Lin received a B.Sc. degree from Daqing Petroleum Institute in 1986. He obtainedhis M.Sc. degree from the Petroleum University of China in 1995, and his Ph.D. from theTongji University in 1997. Lin is currently a professor at the School of Earth Sciencesand Engineering at Nanjing University, where he studies sedimentology and petroleumgeology. E-mail: cmlin@nju.edu.cn

Y. L. Li received a B.Sc degree in geology from Jilin University in 2005 and is currentlya Ph.D. student in the School of Earth Sciences and Engineering at Nanjing University.Li’s current research deals with the formation mechanism of gas reservoirs. E-mail:liyanlishxi@126.com

H. C. Zhuo received a B.Sc. degree from Nanjing University of Technology in 2003, andobtained his M.Sc. degree from Nanjing University in 2006. Zhuo is currently a researchassistant at China Academy of Land and Resource Economics, where he studies envi-ronmental economics. E-mail: zhuohongchun@163.com

G. W. Shurr is an independent geologist and partner in GeoShurr Resources, LLC. HisB.A. degree is from the University of South Dakota, his M.S. degree is from North-western University, and his Ph.D. is from the University of Montana. His current focusis on shallow biogenic gas systems worldwide. E-mail: geoshurr@rockrapids.net

J. L. Ridgley received her B.S. degree in mathematics from Pennsylvania StateUniversity and M.Sc. degree in geology from the University of Wyoming. She worked inthe U.S. Geological Survey before her retirement, and now is an emeritus scientist. E-mail: ridgley@usgs.gov

Z. P. Zhang received a B.Sc degree in geology from Xi’an University of Science andTechnology in 2005 and is currently a M.Sc. student in the School of Earth Sciences andEngineering at Nanjing University. Zhang’s current research deals with the sedimen-tary geology. E-mail: zzpsucf720@yahoo.com.cn

T. Xue received a B.Sc. degree from Daqing Petroleum Institute in 1999 and is currentlya M.Sc. student in the School of Earth Sciences and Engineering at Nanjing University.Xue’s current research deals with the evaluation of reservoirs. E-mail: xuet@petrochina.com.cn