Energy Conversion and Management 73 (2013) 186–194

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Energy Conversion and Management

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Gas hydrate formation and accumulation potential in the QiangtangBasin, northern Tibet, China

0196-8904/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.enconman.2013.04.020

⇑ Corresponding author at: Chengdu Institute of Geology and Mineral Resources,Chengdu 610081, China. Tel.: +86 28 83231651; fax: +86 28 83222657.

E-mail addresses: fuxiugen@126.com, wangjian@cgs.cn (J. Wang).

Xiugen Fu a,b, Jian Wang a,b,⇑, Fuwen Tan a,b, Xinglei Feng a, Dong Wang a, Jianglin He a

a Chengdu Institute of Geology and Mineral Resources, Chengdu 610081, Chinab Key Laboratory for Sedimentary Basin and Oil and Gas Resources, Ministry of Land and Resources, Chengdu 610081, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 13 December 2012Accepted 20 April 2013Available online 20 May 2013

Keywords:Gas hydratePetroleum systemTemperature–pressure conditionMud volcanoQiangtang Basin

The Qiangtang Basin is the biggest residual petroleum-bearing basin in the Qinghai–Tibet Plateau, and isalso an area of continuous permafrost in southwest China with strong similarities to other known gas-hydrate-bearing regions. Permafrost thickness is typically 60–180 m; average surface temperature rangesfrom �0.2 to �4.0 �C, and the geothermal gradient is about 2.64 �C/100 m. In the basin, the Late TriassicTumen Gela Formation is the most important gas source rock for gas, and there are 34.3 � 108 t of gasresources in the Tumen Gela Formation hydrocarbon system. Seventy-one potential anticline structuraltraps have been found nowadays covering an area of more than 30 km2 for each individual one, five ofthem are connected with the gas source by faults. Recently, a large number of mud volcanoes were dis-covered in the central Qiangtang Basin, which could be indicative of the formation of potential gashydrate. The North Qiangtang depression should be delineated as the main targets for the purpose ofgas hydrate exploration.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Gas hydrate, also known as methane hydrate for its methane-dominated composition, is an ice-like crystalloid solid compoundthat forms mainly from water and methane molecules under lowtemperature and high pressure [1]. Recently, gas hydrate has at-tracted great interest from the scientists [2–9] as an unconven-tional energy resource. Estimates of world gas hydrate reservesare very high ranging from 14 to 34,000 trillion m3 for permafrostareas and from 3100 to 7,600,000 trillion m3 for oceanic sediments[10]. Commercial production of just 15% of these gas reserveswould provide the world with energy for 200 years at the currentlevel of energy consumption [11].

Although marine settings house the majority of global gas hy-drate resources, permafrost is also an important area for the forma-tion and occurrence of gas hydrate [10]. The Qinghai–Tibet Plateaupermafrost is the largest permafrost area in China, covering an areaof about 150 � 104 km2 [12]. Recently, gas hydrate was collectedsuccessfully in the Muli region of the Qilian Mountain permafrostarea (Fig. 1a) in the northeast Qinghai–Tibet Plateau, which con-firmed the existence of gas hydrate accumulations in the high-mountain permafrost areas [12,13].

The Qiangtang Basin, situated in the northern part of the Qing-hai–Tibet Plateau, has the lowest average surface temperature inChina and widespread development of permafrost, and is also thebiggest residual petroleum-bearing basin in the Qinghai–Tibet Pla-teau. According to the regional survey for oil and gas, more than200 oil and gas shows have been found from Mesozoic formations[14]. There is a good hydrocarbon exploration prospect for thewell-developed thick Mesozoic marine sediments with the maxi-mum 13,000 m in the basin [14]. In the past years, great impor-tance has been attached to trap in the activity of gas hydrateexploration and research [6,7,15–18]. Gas hydrate deposits aregathered in a large anticline crest, or uplift site [19,20]. Seventy-one potential anticline traps have also been found in the QiangtangBasin to this date. Recently, a large number of mud volcanoes werediscovered in the central Qiangtang Basin [21]. Gas hydrates are of-ten associated with deep-water mud volcanoes [22]. These dataindicate that the Qiangtang Basin could hold potential gas hydrateresource.

In recent years, the Chengdu Institute of Geology and MineralResources underook a detailed study of gas hydrates in the Qiang-tang Basin, including geological, geophysical, drilling, and geo-chemical investigations. Using data from these investigations, aswell as previous research, this article summarizes the petroleum-geology and temperature–pressure conditions of gas hydrate for-mation in the Qiangtang Basin, and proposes favorable targets forgas hydrate exploration. Meanwhile, it can provide a template forexploring gas hydrate in other similar permafrost areas.

Fig. 1. (a) Map of the Tibetan Plateau showing major Terranes (after Ref. [33]). (b) Simplified tectonic map of Qiangtang Basin (modified from Ref. [33]). (c) Simplifiedgeological map of the Wanghuling area showing the distribution of mud volcanoes (modified from Ref. [21]). TR, Tarim basin; QL, Qilian Mountain; KL, Kunlun terrane; AKMS,Anyimaqen–Kunlun–Muztagh suture; HJS, Hoh Xil-Jinsha River suture; SP = Songpan–Ganzi flysch complex; HXP, Hoh Xili piedmont zone; QT, Qiagtang basin; BNS, BangongLake-Nujiang River suture; LS, Lhasa terrane; YTS, Yarlung Tsangpo suture; HMLY, Himalayas; SWM, South Wanghuling mud volcano group; NWM, North Wanghuling mudvolcano group; SHM, Suonahu mud volcano group.

X. Fu et al. / Energy Conversion and Management 73 (2013) 186–194 187

2. Geological setting

From north to south, the Tibetan Plateau is composed of theKunlun–Qaidam terrane, the Songpan–Ganzi flysch complex, andthe Qiangtang and Lhasa terranes, which are separated by theeast-striking Anyimaqen–Kunlun–Muztagh, Hoh Xil-Jinsha Riverand Bangong Lake-Nujiang River suture zones, respectively(Fig. 1a). It is generally accepted that the paleo-Tethys representedby the present Hoh Xil-Jinsha River suture was probably open inthe Early Carboniferous time and closed by Permian to Late Triassictime [23–29]. The mid-Tethys branch between the Lhasa andQiangtang terranes was open by the Late Triassic to Early Jurassictime and closed along the Bangong Lake-Nujiang River suture dur-ing the Late Jurassic time [14,30–32].

The Qiangtang Basin, marked by the Hoh Xil-Jinsha River suturezone to the north and the Bangong Lake-Nujiang River suture zoneto the south, consists of the North Qiangtang depression, the cen-tral uplift and the South Qiangtang depression (Fig. 1b). DuringPermo-Triassic time, the Paleo-Tethys Ocean closed by northwardsubduction beneath the Kunlun terrence and southward subduc-tion beneath the Qiangtang terrence [23–27], resulting in a large-scale regression in the Qiangtang Basin. During this interval, mostparts of the Qiangtang Basin were uplifted and exposed to erosion.Meanwhile paleo-weathering crusts occurred widely in the

Qiangtang Basin [33–35]. Subsequently, these weathering crustswere overlain unconformably by a succession of volcanic–volcani-clastic strata that mark the onset of the Mesozoic Qiangtang Basin[35–37]. As a result, the sediments are almost exclusively Mesozoicmarine deposits, including the Late Triassic Tumen Gela Formation(or Nadi Kangri Formation), Early to Middle Jurassic Quemo Co For-mation (or Sewa/Quse Formation), Middle Jurassic Buqqu and XialiFormations, Late Jurassic Suowa Formation, and Early CretaceousBailong Binghe (or Xueshan Formation) (Table 1). These strata cropout in the South Qiangtang depression and in the North Qiangtangdepression, while Paleozoic marine sedimentary sequences are lo-cally preserved in the central uplift.

3. Gas hydrate petroleum system

3.1. Gas source Rocks

Gas source rocks are certainly of great important to the forma-tion of gas hydrate. The total gas resources in the hydrocarbon sys-tems of the Qiangtang Basin amount to 42.8 � 108 t based onbasin-modeling methods, of which 34.3 � 108 t are in the Late Tri-assic Tumen Gela Formation hydrocarbon system [14]. Therefore,the Tumen Gela Formation is the most important gas source rockin the Qiangtang Basin.

Table 1Mainly Mesozoic strata fills in the Qiangtang Basin.

Strata Thickness(m)

Dominant lithology

JurassicEarly

CretaceousBailong Binghe Fm. (orXueshan Fm.)

532–2080 Bailong Binghe Formation includes Marl, mudstone, calcarenite, and shale intercalating with oil shale andevaporitic rocksXueshan Formation includes Sandstone, mudstone, and minor conglomerate

Late Jurassic Suowa Fm. 283–1825 Mmicritic limestone, marl, and shaleMiddle Jurassic Xiali Fm. 400–800 Siltstone, mudstone, sandstone, gypsum salt, and micritic limestone

Buqu Fm. 142–1446 Micritic limestone, marl, bioclastic limestone, oolitic limestone, oncolitic limestone, reef limestone, anddolomite intercalating with mudstone and shale

Early to MiddleJurassic

Quemo Co Fm. (Sewa/Quse Fm.)

498–2867 Quemo Co Formation includes Conglomerate, sandstone, siltstone, mudstone intercalated with micriticlimestone, marl, and evaporitic rocksSewa and Quse Formations includes shale, siltstone, mudstone, and marl intercalating with fine sandstone

TriassicLate Triassic Nadi Gangri Fm. 217–1571 Acid tuff, dacite, rhyolite and minor basic volcanic rocks

Tumen Gela Fm. 342–1000 Shale, mudstone, sandstone, and marl intercalating with coal

188 X. Fu et al. / Energy Conversion and Management 73 (2013) 186–194

The Tumen Gela strata are widespread in the North Qiangtangdepression (e.g., Woruo Mountain area, Wanwanliang area, andQuemo Co area), and locally in the northern part of the SouthQiangtang depression (e.g., Xiaochaka area) (Fig. 2) with a thick-ness ranging from several hundred meters to 1000 m (Table 1).The lithofacies of the stratum is mainly composed of black mud-stone (or shale), carbonate, coal, and sandstone. Delta and coastalsea were developed.

The mudstones (or shales) of the Tumen Gela Formation in theQiangtang Basin range from 42.0 to 645.8 m in thickness (Table 2).The total organic carbon (TOC) content of mudstone from theTumen Gela Formation ranges from 1.25% to 3.45%, averaging

Fig. 2. Distribution of gas source rocks of the Tu

Table 2Thickness and geochemical features of the Tumen Gela Formation source rocks in the Qia

Stratigraphicunit

Lithology Thickness(m)

TOC (%) Averag

Max. Min. Avg. n

TriassicTumen Gela Fm Mudstone 42.0–645.8 3.45 1.25 2.20 108 930

Coal 0.0–10.0 62.3 10.6 38.4 36 7360Carbonate 29.0–404.0 0.58 0.10 0.21 43 189

CEB = Chloroform-extract bitumen; Max. = the maximum value; Min. = the minimum va

2.20% (Table 2). The average content of chloroform-extract bitu-men is about 930 ppm. Petrographically, the Tumen Gela mud-stones have a vitrinite content of 10–19%, an inertinite content of27–42%, and slightly high saprovitrinite content (51–68%) (Table 2).All these data indicate that the kerogen type in the Tumen Gelamudstones is mainly type II.

The carbonates of the Tumen Gela Formation in the QiangtangBasin range from 29.0 to 404.0 m in thickness, and exhibit lowTOC contents ranging from 0.10% to 0.58% (0.21% average) (Ta-ble 2). The average content of chloroform-extract bitumen is about189 ppm. Under the microscope, the vitrinite content is less than3%, whereas the inertinite and saprovitrinite contents are more

men Gela Formation in the Qiangtang Basin.

ngtang Basin.

e CEB (ppm) KerogenType

Ro (%) Kerogen component (%)

Saprovitrinite Vitrinite Inertinite

II 0.94–3.00

51–68 10–19 27–42

III >90 <5 <4II >65 <3 >22

lue; Avg. = the average value; n = sample number.

Table 3The percentage (%) of each gas in total hydrocarbon gases from the drilled cores in the Qiangtang Basin.

Sample number Methane Ethane Ethylene Propane Propylene Normal butane Isobutene Normal pentane IsopentaneC1 C2 C2H4 C3 C3H6 nC4 iC4 nC5 iC5

QZ1 90.63 3.31 1.52 1.64 1.57 0.59 0.29 0.21 0.23QZ2 93.16 3.62 0.45 1.29 0.35 0.45 0.28 0.17 0.21QZ3 93.02 3.59 0.54 1.32 0.43 0.45 0.28 0.16 0.21QZ4 89.76 4.35 1.11 1.99 0.91 0.76 0.39 0.36 0.37QZ5 84.01 4.65 3.59 2.57 3.28 0.96 0.32 0.32 0.29QZ6 69.61 7.74 7.73 4.93 6.69 1.75 0.52 0.59 0.45QZ7 89.73 3.82 1.97 1.69 1.58 0.61 0.23 0.17 0.20QZ8 91.46 3.16 1.67 1.35 1.41 0.48 0.20 0.12 0.16QZ9 87.10 4.11 2.85 2.12 2.42 0.75 0.25 0.21 0.20QZ10 80.63 5.53 4.95 2.98 4.03 1.07 0.26 0.32 0.24QZ11 84.50 4.14 4.01 2.38 3.47 0.85 0.21 0.25 0.18QZ12 92.68 2.90 1.29 1.19 1.18 0.41 0.17 0.04 0.13QZ13 70.85 7.55 7.86 4.64 6.37 1.58 0.36 0.45 0.33QZ14 71.62 7.44 7.38 4.58 6.11 1.53 0.37 0.55 0.41QZ15 72.18 7.18 7.09 4.57 6.02 1.62 0.52 0.50 0.30QZ16 84.91 4.16 3.72 2.28 3.49 0.84 0.16 0.23 0.21QZ17 86.85 4.23 2.76 2.22 2.51 0.68 0.36 0.18 0.20QZ18 89.42 3.64 2.08 1.80 1.88 0.59 0.24 0.16 0.19QZ19 89.16 3.89 2.06 1.73 1.89 0.54 0.33 0.18 0.22QZ20 75.11 6.59 6.75 3.72 5.55 1.28 0.30 0.38 0.31QZ21 90.10 4.27 1.36 1.85 1.19 0.52 0.39 0.14 0.19QZ22 89.18 2.95 2.67 1.66 2.34 0.63 0.18 0.22 0.16QZ23 91.28 3.10 1.75 1.33 1.59 0.50 0.19 0.13 0.13QZ24 86.07 4.68 2.84 2.29 2.53 0.78 0.30 0.28 0.24QZ25 66.02 8.66 8.63 5.73 7.25 2.05 0.66 0.62 0.38QZ26 81.59 4.74 4.71 2.88 4.12 1.06 0.37 0.31 0.21QZ27 66.41 9.16 9.07 5.41 6.89 1.77 0.39 0.54 0.35QZ28 81.50 4.96 4.94 2.69 4.26 0.89 0.26 0.29 0.21QZ29 71.92 6.80 7.51 4.30 6.62 1.68 0.32 0.49 0.36QZ30 76.79 6.14 5.81 3.80 5.05 1.44 0.29 0.40 0.29QZ31 91.38 4.79 0.73 1.73 0.57 0.40 0.20 0.08 0.13QZ32 90.68 3.36 1.96 1.32 1.74 0.44 0.22 0.14 0.13QZ33 92.50 3.17 1.62 1.29 0.65 0.32 0.24 0.02 0.19QZ34 68.21 8.53 8.39 5.34 6.46 1.79 0.40 0.53 0.34QZ35 65.69 9.19 8.91 5.87 6.98 1.93 0.44 0.60 0.38QZ36 66.73 8.85 8.62 5.71 6.88 1.84 0.43 0.56 0.38QZ37 69.39 7.67 7.92 5.09 6.77 1.80 0.41 0.55 0.40QZ38 91.26 4.30 0.97 1.63 0.79 0.61 0.19 0.08 0.17QZ39 70.77 8.23 7.90 4.48 5.97 1.50 0.35 0.43 0.36QZ40 77.43 5.74 5.96 3.34 5.33 1.33 0.27 0.34 0.26QZ41 91.03 4.07 1.39 1.53 1.14 0.44 0.17 0.11 0.11QZ42 87.90 4.52 2.46 1.87 1.99 0.61 0.25 0.14 0.27QZ43 73.38 7.11 6.90 4.29 5.78 1.46 0.33 0.39 0.36QZ44 89.88 3.78 2.03 1.60 1.78 0.49 0.19 0.13 0.12QZ45 66.71 9.06 9.09 5.40 6.90 1.70 0.39 0.45 0.31QZ46 67.51 9.70 9.23 4.81 6.14 1.51 0.56 0.32 0.23QZ47 67.51 9.63 8.94 5.29 6.08 1.54 0.36 0.38 0.28QZ48 66.32 9.28 9.20 5.56 6.70 1.72 0.40 0.48 0.33QZ49 98.66 0.84 0.05 0.25 0.04 0.08 0.05 0.00 0.03QZ50 99.76 0.18 0.01 0.03 0.00 0.01 0.00 0.00 0.00QZ51 99.59 0.31 0.01 0.06 0.01 0.01 0.01 0.00 0.00QZ52 99.21 0.60 0.01 0.12 0.01 0.03 0.02 0.00 0.00QZ53 99.31 0.51 0.01 0.10 0.01 0.02 0.02 0.00 0.00QZ54 99.62 0.30 0.01 0.05 0.00 0.01 0.01 0.00 0.00QZ55 88.41 5.08 1.65 2.09 1.36 0.62 0.39 0.13 0.27QZ56 88.89 4.99 1.43 1.91 1.29 0.63 0.45 0.14 0.27QZ57 84.78 4.60 3.58 2.40 3.26 0.75 0.25 0.21 0.18QZ58 86.59 4.58 3.11 2.05 2.66 0.67 0.19 0.02 0.14QZ59 78.91 5.54 5.66 3.19 5.06 1.14 0.29 0.05 0.16QZ60 89.78 3.22 2.34 1.55 1.93 0.55 0.27 0.15 0.20QZ61 99.63 0.28 0.01 0.05 0.01 0.01 0.01 0.00 0.00QZ62 98.93 0.61 0.07 0.16 0.09 0.05 0.04 0.00 0.05QZ63 99.55 0.31 0.02 0.07 0.02 0.02 0.01 0.00 0.00QZ64 99.70 0.21 0.01 0.04 0.01 0.02 0.01 0.00 0.00QZ65 99.68 0.18 0.01 0.05 0.00 0.03 0.01 0.00 0.04QZ66 99.61 0.28 0.02 0.05 0.01 0.01 0.03 0.00 0.00QZ67 99.68 0.22 0.02 0.05 0.01 0.01 0.02 0.00 0.00QZ68 99.51 0.21 0.10 0.05 0.08 0.02 0.02 0.00 0.00QZ69 98.07 0.62 0.38 0.23 0.33 0.05 0.02 0.03 0.28QZ70 99.05 0.59 0.09 0.11 0.07 0.03 0.03 0.00 0.02QZ71 98.84 0.48 0.20 0.18 0.16 0.06 0.04 0.03 0.01QZ72 99.57 0.22 0.06 0.06 0.03 0.02 0.01 0.00 0.02Min. 65.69 0.18 0.01 0.03 0 0.01 0.00 0.00 0.00

(continued on next page)

X. Fu et al. / Energy Conversion and Management 73 (2013) 186–194 189

Table 3 (continued)

Sample number Methane Ethane Ethylene Propane Propylene Normal butane Isobutene Normal pentane IsopentaneC1 C2 C2H4 C3 C3H6 nC4 iC4 nC5 iC5

Max. 99.76 9.70 9.23 5.87 7.25 2.05 0.66 0.62 0.45Mean 85.93 4.26 3.36 2.28 2.75 0.77 0.24 0.21 0.19ANS 91.19–99.53MS 98.6 0.1 0.1QM 54–76 8–15 4–21

Min., the minimum value; Max., the maximum value; Mean, the mean value.ANS: Alaska North Slope, after from Ref. [40].MS: Messsoyakha, after from Ref. [41].QM: Qilian Mountain, after from Ref. [13].

190 X. Fu et al. / Energy Conversion and Management 73 (2013) 186–194

than 22% and 65%, respectively (Table 2). These features indicatethat the kerogen type in Tumen Gela carbonate is mainly type II.

The thickness of coal from the Tumen Gela Formation in theQiangtang Basin ranges from 0 to 10 m (Table 2). The Tumen Gelacoals have TOC contents ranging from 10.6% to 62.3% (38.4% aver-age), and the average content of chloroform-extract bitumen isabout 7360 ppm (Table 2). Under the microscope, the vitriniteand saprovitrinite contents are less than 5% and 4%, respectively,whereas the inertinite content is more than 90% (Table 2). The ker-ogen is characterized by lignin and grass. These features indicatethat the kerogen type in Tumen Gela coal is mainly type III.

The gas source rocks of the Tumen Gela Formation reached peakoil generation in the early part of the Callovian (about 164 Ma), andentered wet-gas window in the middle of the Late Jurassic (about157 Ma), and reached the dry-gas window at about 148 Ma [14].At the present time, most of the Tumen Gela Formation is withinboth wet-gas and dry-gas windows, with a corresponding Ro inthe range of 0.94–3.0% (Table 2).

3.2. Composition of hydrocarbon gases

Gas composition is one of the most important factors regardinggas hydrate stability. Generally, methane hydrates (Type I) aremore stable at shallower depths than Type II hydrates (which in-clude propane and ethane with methane) [19,38,39]. Seventy-two drilled-core samples (56 carbonates and 16 sandstones, Ta-ble 3) were collected from the Well QZ-1 in the North Qiangtang

Fig. 3. Geological (modified from Ref. [14]) and tectonic outline map

Qiangtang depression (Fig. 1), with a vertical sampling interval of8 m on average. Acid-degassed gases from drilled-core sampleswere analyzed by gas chromatography spectrometry. Gas chro-matograph analyses of the absorbed gas from 72 drilled cores inwell QZ-1 show that it is mainly composed of methane (C1) withsome wider range of heavier hydrocarbon (C2+), that is, it com-posed of about 65.69–99.76% methane (C1), 0.18–9.70% ethane(C2), 0.01–9.23% ethylene (C2H2), 0.03–5.87% propane (C3), 0.00–7.25% propylene (C3H3), 0.01–2.05% normal butane (nC4), 0.00–0.66% isobutene (iC4), 0.00–0.62% normal pentane (nC5), and0.00–0.45% isopentane (iC5) (Table 3).

Compared with other permafrost areas where gas hydrate werefound, the methane content of absorbed gases in the Qiangtang Ba-sin exhibits a wider range, which is less than both the Alaska NorthSlope [19] and Messoyakha [40], but slightly higher than QianlianMountain area in China [12,13]. Therefore, as far as gas composi-tion is concerned, the hydrocarbon gases in the Qiangtang Basinare fully able to meet the basic requirements for gas hydrateformation.

3.3. Reservoir rocks

The reservoir plays a significant role in gas hydrate accumula-tion, a fact that was recognized by most of the explorer andresearchers in gas hydrate area [16,40–43]. In the Qiangtang Ba-sin, the Tumen Gela Formation contains about 47.3–1419.9 m ofsandstones [14], occurring at the top of the Tumen Gale Forma-

of Qiangtang Basin showing the main anticline structural traps.

X. Fu et al. / Energy Conversion and Management 73 (2013) 186–194 191

tion strata, which may be important reservoirs for gas hydrateaccumulation. Measured porosity values from outcrop samplesrange from 0.09% to 6.40% (3.27% average), while permeabilityvaries mostly between 0.0004 � 10�3 and 13.73 � 10�3 lm2

(2.25 � 10�3 lm2 average) [14]. Furthermore, these sandstonesare extensively fractured, adding significant additional permeabil-ity and bulk porosity.

3.4. Gas migration and potential traps

A highly concentrated gas hydrate accumulation contains a sub-stantial volume of gas [44]. As a result, it is essential for the forma-tion of large-scale gas hydrate deposits that external hydrocarbongases be supplied continuously by faults, fractures, permeable stra-ta, etc. [6,20].

In the Qiangtang Basin, 71 potential anticline structural traps(Fig. 3) have been found to date covering an area of more than30 km2 for each individual one. Fifteen of them cover areas of morethan 100 km2. These structural traps formed before or during

Fig. 4. A seismic image showing the traps connected with the oil source by faults(T3t, Tumen Gela Formation).

Table 4The analytical results of geothermal gradient in different areas of

Sample location (T1 � T2) (�C) H (m)

Yeniu GouSection 1 7.0 266Section 2 5.0 189

Quemo CoSection 1 6.5 246Section 2 5.0 188

AnduoSection 1 6.0 227Section 2 6.0 226

LongeniSection 1 5.5 209Section 2 6.0 227

DonghuaisangSection 1 4.5 172Section 2 4.5 170

Laxiong CoSection 1 5.0 191Section 2 5.0 189

petroleum migration [14] as a result of the closure of Meso-Teth-yan Ocean [28], mainly during the Late Jurassic to Early Cretaceous.The traps formed before or during petroleum migration. From seis-mic data, some traps are connected with the oil source by faults,and five of them are connected with the gas source by faults(Fig. 4). Thus, it is a very favorable environment for gas hydrateformation.

4. Temperature–pressure condition and groundwater

4.1. Thickness of permafrost and geothermal gradients

Permafrost is continuously spread throughout Qiangtang Basin[45,46] and thickens to the north. Wu et al. deduced that the per-mafrost thickness in the Qiangtang Basin ranges from 100 to morethan 200 m [45]. Our drilling data from 2006 to 2012 reveal thatthe thickness of permafrost is about 60–180 m in the North Qiang-tang depression. These thicknesses are comparable to those re-ported in other gas-hydrate-bearing regions such as QilianMountain region, China [12,13] and Yamal Peninsula in Siberia[47].

The geothermal gradient is very important to the thickness ofpermafrost. Prior data indicate that the mean annual temperatureof the Qiangtang Basin is from �0.2 to �4.0 �C [45], and up to �6 �Clocally [46]. The geothermal gradient was obtained by inclusiontemperatures using the formula T = (T1 � T2)/H, where T is the geo-thermal gradient, T1 and T2 are the analytical temperatures ofinclusions in location 1 and location 2, respectively, and H is dis-tance between location 1 and location 2. The analytical resultsfor the geothermal gradients in different areas of the Qiangtang Ba-sin are listed in Table 4, indicating that the geothermal gradient inthe Qiangtang Basin is around 2.64 �C/100 m. Additionally, the dril-ling data show that the geothermal gradient within permafrost inthe Tuonamu area of the Qiangtang Basin is 2.5 �C/100 m [46].All these data indicate that permafrost in the Qiangtang Basinhas a favorable geothermal condition for gas hydrate formation.Our conclusion is consistent with the published data [45,46].

4.2. Temperature–pressure condition of gas hydrate formation

The thickness of the gas hydrate stability zone can be calculatedusing surface temperature, geothermal gradient and strata pres-sure [48]. Prior data indicate that the Qiangtang Basin has a annual

the Qiangtang Basin (after from Ref. [14]).

P (�C/100 m) P (average) (�C/100 m)

2.630 2.642.650

2.645 2.652.655

2.645 2.652.655

2.635 2.642.645

2.620 2.632.640

2.620 2.632.640

Fig. 5. Temperature–pressure conditions in the Qiangtang Basin. 1 – phase balanceboundary of CO2; 2 – phase balance boundary of pure methane hydrate; 3 – phasebalance boundary of hydrate of absorbed hydrocarbon gases from drilled cores; 4 –bottom boundary of 60 m thickness of permafrost; 5 – bottom boundary of 180 mthickness of permafrost.

192 X. Fu et al. / Energy Conversion and Management 73 (2013) 186–194

mean surface temperature of about �2.0 �C [45]; and its geother-mal gradient is about 2.64 �C/100 m as discussed above. The stratapressure in this area, which is hydrostatic pressure plus lithostaticpressure, is calculated according to the thickness and density ofstrata in the region using the formula: P = P0 + qg(H � H0), whereP is the hydrostatic pressure at the depth of study point (Pa); P0

is the pure water density (1000 kg/m3); g is the gravity (9.8 m/s2); H is the depth of the study point (m); and H0 is the depth ofthe bottom surface of the permafrost layer (m). The data we usehere are as follows: 60 m and 180 m of permafrost thickness, about1700 kg/m3, 2370 kg/m3, and 2510 kg/m3 of rock density (q) forregolith, mudstone, and sandstone, respectively [46].

The temperature–pressure condition for gas hydrate stability isdetermined using Sloan’s software [49]. The results (Fig. 5) indicatethat the thickness of permafrost in the Qiangtang Basin has animportant impact on the thickness of the gas hydrate stabilityzone. Given 60 m of permafrost thickness and about 2.64 �C/100 m of geothermal gradient, no gas hydrate can form assuming

Fig. 6. Mud volcanoes discovered i

pure methane gas. Given 180 m of permafrost thickness and thegeothermal gradient above, gas hydrates of both pure methaneand absorbed gas collected from the drilled cores may form.

4.3. Groundwater salinity

Recent studies have shown that, when the salinity is less than4 � 10�2, there is little effect on the formation and stability of gas hy-drates [50]. The salinity of groundwater in the Qiangtang Basin isgenerally less than 1 � 10�4 [14]. Additionally, our analyses of fivespring water samples from the Qiangtang Basin show groundwatersalinity (Cl-ion concentration) of �1.17–27.99 � 10�6, higher thanin Alaska’s North Slope (<1.0–19 � 10�12 [51]), but lower than Mes-soyakha, Siberia (61.5 � 10�2 [40]). Therefore, salinity in the Qiang-tang Basin has little effect on gas hydrate formation and can beignored.

5. Mud volcanoes: probable indications of gas hydrateoccurrence

The accumulation of gas in mud-volcanic structures has been atarget of hydrocarbon exploration, and drilling in these structureshas resulted in successful hydrocarbon discoveries [52]. The asso-ciation of gas hydrates with mud volcanoes has also been observed[22,53,54]. Mud volcanoes can be an indicative of the formation ofpotential gas hydrate.

A large number of mud volcanoes (Fig. 6) were discovered in thecentral Qiangtang Basin, including the South Wanghuling mud vol-cano group, the North Wanghuling mud volcano group, and theSuonahu mud volcano group (Fig. 1c). Here detailed investigationswere carried out on the South Wanghuling mud volcano group,where a total of 117 mud volcanoes were identified. Fifty-two ofthem are cone-shaped, and 41 are pie-shaped (Fig. 6a), while 24are ‘‘twin’’ mud volcanoes (Fig. 6b). Cone-shaped mud volcanoesare widespread in the South Wanghuling mud volcano group,while pie-shaped volcanoes are distributed mainly in the NorthWanghuling mud volcano group and the Suonahu mud volcanogroup. The ‘‘twin’’ mud volcanoes are distributed locally in theeastern part of the South Wanghuling mud volcano group. Theheight of most mud volcanoes ranges from 1.0 to 7.0 m with thehighest mud volcano at 20 m. The crater sizes (the maximumdiameter) range from 9.0 to 150.0 m. Of 117 mud volcanoes, onlyfive craters are a truly circular, while others are elliptical in shape.

The distribution of mud volcanoes is compared to the locationof the principal structural features in the South Wanghuling area(Fig. 1c), where the mud volcanic craters are spread along aneast–west fault exposed for a distance of more than 50 km. Mostof the mud in the crater has become dried, and asphalt was ob-served in mud at several sites.

n the central Qiangtang Basin.

X. Fu et al. / Energy Conversion and Management 73 (2013) 186–194 193

The emplacement age of the mud volcano in the South Wangh-uling area is about 11 ka BP [21], whereas the permafrost form isolder, at about 0.71 Ma in the Qinghai–Tibet Plateau [55]. If thegas hydrate formed during the emplacement stage of the mud vol-cano, it is possibly preserved today.

6. Exploration potential

As discussed above, the total gas resource in the hydrocarbonsystems of the Qiangtang Basin amounts to 42.8 � 108 t, of which34.3 � 108 t are in the Late Triassic Tumen Gela Formation hydro-carbon system [14]. The Tumen Gela strata are distributed mainlyin the North Qiangtang depression, and locally in the northern partof the South Qiangtang depression [Fig. 2]. It can be seen that theNorth Qiangtang depression should be delineated as a target forthe purpose of gas hydrate exploration.

The central uplift, where a large number of mud volcanoes werediscovered, is also considered to have a significant exploration poten-tial. However, the permafrost here is less thick, about 38 m basined onthe measured temperature data of Well QZ-2 (Fig. 1b). Therefore, thehydrate layer is probably less thick in the central uplift.

7. Conclusions

(1) The Late Triassic Tumen Gela Formation is the most impor-tant gas source rock for the formation of the gas hydrate inthe Qiangtang Basin. The gas composition is mainly com-posed of methane with some heavier hydrocarbon (C2+).

(2) In the Qiangtang Basin, 71 potential anticline structuraltraps have been found each covering an area of more than30 km2, all are potential traps for the formation of gashydrate.

(3) Our drilling data reveal that the permafrost thickness inNorth Qiangtang depression is comparable to those reportedin other gas-hydrate-bearing regions, and the geothermalgradient calculated by inclusion indicates a favorable geo-thermal condition for gas hydrate formation.

(4) A large number of mud volcanoes were discovered in thecentral Qiangtang Basin, which could be indicative of gashydrate formation. However, the permafrost is less thick inthe central uplift suggesting that the hydrate layer is proba-bly less thick.

(5) The North Qiangtang depression should be considered as themain target for the purpose of gas hydrate exploration.

Acknowledgements

The authors would like to thank two anonymous reviewer fortheir critical and constructive reviews and comments. We are alsograteful to Betty Dusseault for improving the English of the text.This work was supported by the Qiangtang Basin gas hydrate re-source exploration (No. GZHL20110301) and the National NaturalScience Foundation of China (Nos. 41172098 and 40972087), andthe National Oil and Gas Special Project (No. XQ-2009-01).

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