The mechanism and verification analysis of permafrost-associated gas hydrate formation in the Qilian Mountain, Northwest China

Accepted Manuscript The mechanism and verification analysis of permafrost-associated gas hydrate formation in the Qilian Mountain, Northwest China Bing Li, Youhong Sun, Wei Guo, Xuanlong Shan, Pingkang Wang, Shouji Pang, Rui Jia, Guobiao Zhang PII:

S0264-8172(17)30202-7

DOI:

10.1016/j.marpetgeo.2017.05.036

Reference:

JMPG 2929

To appear in:

Marine and Petroleum Geology

Received Date: 11 March 2017 Revised Date:

21 May 2017

Accepted Date: 25 May 2017

Please cite this article as: Li, B., Sun, Y., Guo, W., Shan, X., Wang, P., Pang, S., Jia, R., Zhang, G., The mechanism and verification analysis of permafrost-associated gas hydrate formation in the Qilian Mountain, Northwest China, Marine and Petroleum Geology (2017), doi: 10.1016/ j.marpetgeo.2017.05.036. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

The mechanism mechanism and verification analysis of permafrostpermafrost-associated gas

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hydrate formation in the Qilian Mountain, Mountain, Northwest China

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Bing Li a,b， Youhong Sun a,b,*，Wei Guo a,b,*，Xuanlong Shan c, Pingkang Wang d，Shouji Pangd， Rui Jia a,b，Guobiao

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Zhang a,b

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a

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b

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University, Changchun 130026, PR China

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c

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d

College of Construction Engineering, Jilin University, Changchun 130026, PR China

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College of Earth Sciences, Jilin University, Changchun 130061, PR China Oil and Gas Survey, China Geological Survey, Beijing 100029, PR China

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Key Laboratory of Drilling and Exploitation Technology in Complex Condition, Ministry of Land and Resources, Jilin

* Corresponding author (Youhong Sun, E-mail address: [email protected], Tel. and Fax: +86 431

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88502678; Wei Guo, E-mail address: [email protected] )

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Present address: No. 6 Ximinzhu Street, Changchun City, Jilin Province, PR China, 130026

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Abstract

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Muri Basin in the Qilian Mountain is the only permafrost area in China where gas

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hydrate samples have been obtained through scientific drilling. Fracture-filling hydrate is the

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main type of gas hydrate found in the Qilian Mountain permafrost. Most of gas hydrate

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samples had been found in a thin-layer-like, flake and block group in a fracture of Jurassic

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mudstone and oil shale, although some pore-filling hydrate was found in porous sandstone.

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The mechanism for gas hydrate formation in the Qilian Mountain permafrost is as follows:

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gas generation from source rock was controlled by tectonic subsidence and uplift--gas

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migration and accumulation was controlled by fault and tight formation--gas hydrate

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formation and accumulation was controlled by permafrost. Some control factors for gas

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hydrate formation in the Qilian Mountain permafrost were analyzed and validated through

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numerical analysis and laboratory experiments. CSMGem was used to estimate the gas

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hydrate stability zone in the Qilian permafrost at a depth of 100-400 m. This method was used

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ACCEPTED MANUSCRIPT to analyze the gas composition of gas hydrate to determine the gas composition before gas

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hydrate formation. When the overlying formation of gas accumulation zone had a

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permeability of 0.05 × 10-15 m2 and water saturation of more than 0.8, gas from deep source

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rocks was sealed up to form the gas accumulation zone. Fracture-filling hydrate was formed

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in the overlap area of gas hydrate stability zone and gas accumulation zone. The experimental

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results showed that the lithology of reservoir played a key role in controlling the occurrence

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and distribution of gas hydrate in the Qilian Mountain permafrost.

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Keywords

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Qilian Mountain permafrost; Mechanism of gas hydrate formation; Gas hydrate stability zone;

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Gas migration and accumulation; Verification analysis

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0 Introduction

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Gas hydrate is one kind of cage compound formed by natural gas and water that exists in

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enormous quantities in the permafrost and offshore environments (Collett, 2002). Recently,

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gas hydrate samples were obtained in the Sanlutian region of Muri Basin, Qilian permafrost

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(Zhu et al., 2010a; Wen et al., 2015). As a result, China became the fourth country to discover

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the permafrost-associated gas hydrate and the first country to discover permafrost-associated

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gas hydrate in the low-middle latitude permafrost of the world (Bily and Dick, 1974; Collett,

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1993; Yakushev and Chuvilin, 2000).

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Based on guiding concepts for conventional oil and gas exploration, Collett et al. (2011,

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2009) proposed the concept of a gas hydrate petroleum system, which was used to identify

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and assess the mechanism of gas hydrate formation in the Alaska North Slope. They proposed

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that the factors that contribute to formation of gas hydrate mainly included gas hydrate

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pressure-temperature stability conditions, gas source, gas migration, and suitable host

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sediment or “reservoir”. The mechanism of gas hydrate formation in the Alaska North Slope 2

ACCEPTED MANUSCRIPT can be summarized as follows: the thermo-genic gases from deep source rocks migrated to the

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shallow formation along permeable pathways such as Mount Elbert fault systems; the gases

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migrated into porous-permeable sediment layers and accumulated in the permeable sand layer

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that was controlled by a tight sand layer over a permeable sand layer; with the arrival of the

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Quaternary glaciation, the hydrate stability condition was reached, and the gas hydrate

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reservoir formed in the Alaska North Slope. The sedimentary environment of gas hydrate,

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which mainly occurred in porous-permeable sedimentary sand in Mackenzie Delta in Canada

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and Messoyakha Gas Field in Russia were similar to that of the Alaska North Slope, and the

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main mechanism for gas hydrate formation was similar as well (Yakushev and Chuvilin, 2000;

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Majorowicz and Osadetz, 2001). However, gas hydrate in the Qilian Mountain permafrost

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mainly occurred in the fracture of Jurassic mudstone and oil shale, while a small amount of

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gas hydrate was distributed in the fissures and pores of sandstones (Zhu et al., 2010a; Wang et

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al., 2011; Wang et al., 2015; Pang et al., 2013).

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Zhang et al. (2013) analyzed the gas hydrate accumulation pattern in the Qilian

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Mountain permafrost. They proposed that gas hydrate formed from deep pyrolysis gas and

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shallow coalbed methane under the tectonic uplift. However, they did not consider in detail

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the gas hydrate stability zone, gas hydrate formation time and the role of tectonic movement

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to control gas migration. Li et al. (2014, 2012) discussed the gas hydrate formation condition

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in the Qilian Mountain permafrost from four aspects: the material source, reservoir, cap rock

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and gas migration conditions. They postulated that coalbed methane was the main gas source

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for gas hydrate. With the Qinghai-Tibet Plateau uplifting, the formation temperature was

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reduced and the gas hydrate stability zone formed gradually. They proposed that the

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accumulation mode at Muri area was a self-born self-storage and short-range migration

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reservoir. Zhai et al. (2014) and Lu et al. (2013a) hypothesized that the gas source for gas

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hydrate in the Qilian Mountain permafrost was mainly composed of oil-typed thermo-genic

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gases that were mainly derived from lower or deeper Upper Triassic or Permian. Gas hydrate

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occurrence and gas migration in gas hydrate scientific wells DK-1, DK-2, DK-3, DK-7, DK-8,

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DK-9, and DK-12 were controlled by F1 and F2 faults. They described the gas hydrate

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geological

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hydrocarbon-generation, fluid-migration, and gas hydrate accumulation subsystems and the

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coupling relationship of these subsystems as determined by gas hydrate distribution. In

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addition, Lu et al. (2015a) also analyzed the control effect of F1, F2 and F30 faults on gas

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hydrate formation and distribution by testing drilling samples provided by Shenhua Qinghai

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Energy Co., Ltd. Li et al. (2013) noted that the well permeable sand formations and

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unconformity contact planes can also act as a gas migration system in gas hydrate

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accumulation pattern in the Qilian Mountain permafrost apart from faults.

in

the

Qilian

Mountain

permafrost

as

mainly

including

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Based on the literatures, we can only roughly infer the mechanism of gas hydrate

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formation in the Qilian Mountain permafrost. A more detailed and systematic mechanism,

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however, is needed. Discussion of the mechanism in the literature did not include information

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of all the gas hydrate scientific drilling data in the study area. These previous studies only

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combined geological data and theoretical analysis, while related laboratory experiment and

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numerical simulation were not conducted. To fill these gaps, our study systematically

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summarizes all of the existing gas hydrate scientific drilling data and discusses the

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mechanism of gas hydrate formation in detail. Then, some parts of factors for the mechanism

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of gas hydrate formation are verified through numerical analysis and laboratory tests.

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1 Gas hydrate in the Qilian Mountain permafrost

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As of October 2015, permafrost-associated gas hydrate in China had been found only in

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the Qilian Mountain permafrost. Twenty-nine wells were implemented by China Geological

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Survey and Shenhua Qinghai Energy Co., Ltd, as shown in Fig. 1 and Table 1 (Wang et al.,

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2011; Wang et al., 2015; Zhai et al., 2014; Lu et al., 2015a). 4

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Fig.1. The simplified tectonic map of Sanlutian of Muri Basin, Qilian permafrost and wells location (modified from

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Wang et al., 2015). 1-Upper Jiangcang; 2-Lower Jiangcang; 3-Upper Muli; 4-Lower Muli; 5-Lower Jurassic; 6-Upper

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Triassic; 7-Concordant stratigraphic boundary; 8-Discordant stratigraphic boundary; 9-Normal fault; 10- Supposed

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fault; 11- Reverse fault; 12-No hydrate hole; 13-Exploration line; 14-Research boundary; 15- Hydrate hole.

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Table 1 Gas hydrate scientific wells in the Qilian Mountain permafrost

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Agency

Well category

China Geological

Exploration wells

Gas hydrate sample

Gas hydrate abnormal*

Survey

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DK-1、DK-2、DK-3、

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Production wells

Shenhua Qinghai

DK-8、SK-0、SK-2

SK-1 DK2-25、DK2-26、DK4-23、DK4-24、

DK8-19、DK11-14、 DK5-22、DK6-21、DK7-20、DK10-16、

Exploration wells

Energy Co., Ltd

DK-4、DK-5、DK-6、DK-10、DK-11

DK-7、DK-9、DK-12

DK12-13、DK13-11 DK10-17、DK10-18

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* A series of hydrate-related anomalies were found, which mainly included unusual high pressure gas in the drilling

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process, gas blowouts near wellhead, incessant water seepage from rock fracture and pore, and abnormal low

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temperature of infrared imaging in the rock fracture surface.

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1.1 Geological setting Sanlutian area is located in the Juhugeng mine area, Muri Coalfield, Qilian Mountain.

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The basic structural pattern in Muri coalfield is a thrusting-folding type. Under the influence

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of regional fault zones and coal-bearing basement structures, fault structure is a complete

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evolution and thrust fault is dominant (Lu et al., 2015b). The Juhugeng mine area is located in

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the west Muri coalfield, and the overall tectonic morphology is shown as NW multiple

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syncline (Lu et al., 2015a). Juhugeng mine area is separated into three open pits and four mine

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fields. Sanlutian area is one pit and located in the south syncline tectonic unit of Juhugeng

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mine area multiple syncline. The exposure strata in Sanlutian area are mainly the Quaternary,

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Middle Jurassic and Upper Triassic. The upper Triassic is dominated by black mudstone,

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siltstone and thin coal seams (Wang et al., 2011; Wang et al., 2015). The Middle Jurassic

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includes Muri and Jiangcang formations. The Muri formation is dominated by white

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medium-coarse-grained sandstone, gray fine-grained sandstone and siltstone. The Jiangcang

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formation is mainly composed of dark gray mudstone and siltstone, dark mudstone, black oil

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shale and multiple thin coal seams. The elevation of Muri coalfield is 4000-4300m, where the

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annual average temperature is -5.1 ℃, and the depth of permafrost is generally 60-95m

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(Wang et al., 2011).

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1.2 The characteristic of gas hydrate in the Qilian Mountain permafrost

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The gas hydrate samples from thirteen wells were obtained below permafrost in the

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range of 124.1m~396.0m. The distribution and characteristic of gas hydrate in the Qilian

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Mountain permafrost are shown in Table 2 (except DK-12) (Wang et al., 2011; Wang et al.,

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2014; Wang et al., 2015; Pang et al., 2013; Zhu et al. 2010b; Huang et al. 2011; Lu et al.

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2010a; Pang, 2012; Hou et al. 2015; Chen et al. 2015a; Lu et al. 2010b; Yang et al. 2015; Li

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et al. 2015; Meng et al. 2015; Tang et al. 2015a; He et al. 2015; Xu et al. 2015; Tang et al. 6

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2015b; Jiang et al. 2015; Lu et al. 2013b; Chen et al. 2015b; Lu et al. 2015b). Compared with

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gas hydrate in polar permafrost, gas hydrate in the Qilian Mountain permafrost has the

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characteristics of thin layer, shallow burial, fractured-filling type, complex spatial distribution,

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gas composition and gas source. Fracture-filling as the main occurrence type of gas hydrate, which is white or pale yellow

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white ice or jelly in the Qilian Mountain permafrost, occurs as the thin-layer-like, flake, block

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group in the fracture of low porosity and low permeability rocks such as mudstone, oil shale

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and siltstone. Pore-filling hydrate disseminated occurs in the porous of sandstone and is

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difficult to observe by the naked eye, but can be indirectly speculated by continuously

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emerged bubbles and the water drops, and dispersion-like abnormal low temperature of

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infrared imaging from the core. As shown in Table 2, the fracture-filling gas hydrate reservoir

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and fracture-filling + pore-filling gas hydrate reservoir account for about 80% of the hydrate

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reservoir, and the pore-filling gas hydrate reservoir and the pore-filling + fracture-filling gas

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hydrate reservoir account for about 20% of the hydrate reservoir (Fig. 2). The statistical

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results of cores indicate that the gas hydrate reservoirs are in good agreement with the

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reservoir fracture zone (Wang et al. 2011). From Table 2, we can know that gas hydrate in the

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Qilian Mountain permafrost has the characteristics of shallow burial, thin layer, large span in

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vertical distribution and significant difference in horizontal distribution.

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The gas composition of gas hydrate in the Qilian Mountain permafrost is relatively

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complex and varies in different proportions. However, the overall trend is basically identical,

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C1 61.3%~71.58%，C2 6.8%~10.9%，C3 9.8%~25.2%，i-C4 1.1%~3.3%，n-C4 0.8%~5.88%，

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C5 0.5%~2.04%，C6+ 0.1%~4.09%，CO2 0.28%~1.56%，individual samples can be as high as

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45.1%. The average δ13C values of C1, C2 and C3 of adsorbed gas from hydrate-bearing core

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are -50.07 ‰, -35.90 ‰ and -32.25 ‰, and the average δD values are -251 ‰, -267 ‰ and

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-233 ‰, all of which were δ13CC1 <δ13CC2 <δ13CC3, so hydrocarbon gas is typical organic

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ACCEPTED MANUSCRIPT origin (Cheng et al. 2017). It is shown that gas source in the study area is of mixed geneses,

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which are dominated by thermo-genic gases, and some biogenic gas may be mixed in some

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drilling holes (He et al. 2015).

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Fig.2. Proportion distribution of gas hydrate occurrence in the Qilian Mountain permafrost.

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Table 2 Distributions and characteristics of gas hydrate in the Qilian Mountain permafrost

Gas hydrate Well

Formation

layer

Depth/m

Lithology and characteristics of reservoir

Characteristics of cores

Average gas composition

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occurrence Middle Ⅰ

133.5~135.5

Gray fine grained sandstone

Jurassic Middle

Fracture 142.9~147.7

Gray-dark grey silty mudstone

Jurassic

F-F C3 9.10%、i–C4 1.12%

Local fracture development. core

Ⅲ

165.3~165.5

Dark gray - gray mud-siltstone

Jurassic

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Middle

n–C4 5.58%、C5 0.84% F-F, P- F

crushing

Middle

Fracture

Ⅳ

169.0~170.5

Gray siltstone

Jurassic

C6 4.06%、CO2 0.98% development,

core P- F, F- F

crushing

Ⅰ

Light gray-dust color medium- fine grained

Fracture development, local core

sandstone

crushing

144.4~152.0

Jurassic Middle 156.3~156.6

Jurassic DK-2 Middle Ⅲ

235.0~291.3

Jurassic Middle Ⅳ

377.3~387.5

Dark brown oil shale

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Ⅱ

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Middle

F- F, P- F

More fracture development

F- F

C1 71.584 %、C2 10.92% C3 13.07%、i–C4 1.20%

Local

fracture

development,

Brown-gray black oil shale, mudstone

F- F calcite filling

n–C4 2.58%、C5 0.84%

Local fracture development, core

Dark-medium grained sandstone

P- F

Jurassic Middle

C1 68.75 %、C2 8.65%

core

crushing

DK-1

DK-3

development,

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Ⅱ

F-F1, P-F2

Local core crushing

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crushing Ⅰ

133.0~156.0

Dust color- dark brown oil shale, mudstone 9

More fracture development, local

F-F

NA

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Jurassic

core crushing

Middle

Fracture development, local core Ⅱ

225.1~240.0

Gray-dark gray oil shale, mudstone

F-F crushing

Middle Ⅲ

Gray- brown grey fine grained sandstone,

Local

siltstone

more core crushing

367.7~396.0

Jurassic

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Jurassic

Middle

fracture

development, P- F, F-F

High angle fracture development, 136.6~138.0

Gray-dark gray siltstone

P- F, F-F

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Ⅰ Jurassic

core integrity well

Middle

Fracture development, local core

Ⅱ

143.4~146.7

NA

Dark gray mudstone, dust color- oil shale

Jurassic

F-F

crushing

Fracture

Middle Ⅰ

147.8~155.9

Dark gray fine grained sandstone

Jurassic

development ， core P- F, F-F

integrity well

Middle Ⅱ

171.6~175.0

Jurassic Middle

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DK-8

Gray-black mudstone, dust color- oil shale

Ⅳ

265.9~291.2

Dark brown oil shale, gray-black mudstone

Middle Ⅴ

301.8~304.2

Little fracture development, local F-F core crushing

Calcite filling、, core crushing

Fracture

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Jurassic

EP

226.3~236.5

Middle

NA

Gray-black mudstone, dust color- oil shale

Ⅲ Jurassic

DK-8

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DK-7

development,

F-F

core F-F, P- F

crushing Fracture

development ， core

Gray-black muddy siltstone

P- F, F-F

Jurassic

integrity well

10

NA

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Middle

Fracture development, local core Ⅰ

188.2~202.4

Gray mudstone, fine grained sandstone

F-F, P- F

Jurassic

crushing

Middle

Fracture development, local core 259.8~271.9

mudstone, siltstone

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Ⅱ Jurassic

crushing

Middle

Fracture

DK-9 Ⅲ

300.1~302.2

Gray mudstone

F-F

crushing

Middle

Fracture development, local core Ⅳ

357.5~367.6

mudstone, fine grained sandstone

F-F, P- F

crushing

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Jurassic

Fracture

NA

NA core

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Jurassic

development,

F-F, P- F

Ⅰ

184.5~185.5

Gray-brown oil shale

development,

core F-F

crushing

SK-0

Fracture

SK-2

NA

NA

DK

Ⅱ

Ⅲ

250.0~262.0

350.5~354.7

Gray mudstone

F-F

crushing Fracture development, local core F-F crushing Fracture development, local core F-F

NA

crushing Fracture development, local core

Fine grained sandstone

P- F, F-F crushing Fracture

124.1~129.7

core

Mudstone, silty mudstone

Middle Ⅰ

8-19

178.0~199.0

Gray-brown oil shale

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Ⅰ

195.5~196.5

EP

NA

Ⅱ

AC C

NA

NA development,

Gray brown-dark gray mudstone, oil shale

Jurassic

core F-F, P- F

crushing 11

development,

C1 69.04 %、C2 8.83%

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Middle

Fracture development, local core Ⅱ

133.4~136.1

Silty mudstone

C3 17.11%、i–C4 1.83% F-F

crushing

Jurassic

n–C41.03%、C5 0.5%

Middle 141.5~144.2

Gray muddy siltstone

Local fracture development,

F-F

Ⅰ

293.2~295.5

Dark gray mudstone

Fracture development, soft

F-F

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Ⅲ Jurassic Middle

Middle 320.6~322.4

Dark gray mudstone, gray-black oil shale

Jurassic

Fracture

Ⅰ

202.8~205.3

Light gray siltstone

Jurassic Middle

Fracture

Jurassic

Ⅲ

311.6~313.4

core P- F, F-F

Ⅰ

267.5~269.4

C1 65.65 %、C2 7.89% C3 19.96%、i–C4 2.96%

core F-F

crushing Fracture

n–C4 1.19%、C5 1.37% development,

core F-F

C6 0.65%、CO2 0.33%

crushing C1 66.43 %、C2 6.53%

AC C

Middle

development,

Gray - dark gray mudstone, oil shale

EP

Jurassic

Jurassic

development,

Brown black - gray black mudstone, oil shale

TE D

262.8~263.7

Middle

13-11

C6 0.28%、CO2 0.38%

crushing, scratches visible

Ⅱ

DK

n–C4 0.87%、C5 2.04% P- F, F-F

leafy

Middle

12-13

C3 25.2%、i–C4 3.2%

Fracture development, massive – Ⅱ

DK

C1 61.3 %、C2 6.8%

SC

11-14

Jurassic

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DK

C6 0.1%、CO2 1.56%

Fracture

development

well, C3 21.33%、i–C42.49%

Light gray - dark gray silty mudstone

calcite filling, scratches and fault

F-F n–C4 0.89%、C5 1.43%

breccia visible C6 0.16%、CO2 0.74%

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1. Fracture-Filling gas hydrate reservoir; 2. Pore-Filling gas hydrate reservoir; 12

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2 Mechanism of gas hydrate formation in the Qilian Mountain permafrost Gas hydrate in the Qilian Mountain permafrost was formed under a series of complicated

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processes, including sedimentation and tectonic movement. As shown in Fig. 3, mechanism of gas

168

hydrate formation in the Qilian Mountain permafrost was related to source rock, hydrocarbon gas,

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migration pathway, cap rock, reservoir, permafrost, gas hydrate stability zone and others. It mainly

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included three processes: gas generation, gas migration and accumulation and gas hydrate formation.

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Source rocks associated with gas hydrate formation in Sanlutian area in the Qilian Mountain

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permafrost are mudstone from Upper Triassic Galedesi formation and coal-bearing mudstone from

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Middle Jurassic Yaojie Formation (Xu et al. 2015; Tang et al. 2015b). During the Early Jurassic to

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Late Cretaceous periods, Sanlutian area was in the subsidence stage. At the end of the Early

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Cretaceous, the burial depth of Upper Triassic mudstone and Middle Jurassic coal-bearing mudstone

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was approximately 3000 m, where the formation temperature was up to 120 ℃ and source rock was

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in the mature-high mature stage. Such conditions were beneficial for generating gas (Jiang et al.

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2015). Gas is mainly composed of crude oil-associated gas and crude oil cracking gas, and includes a

179

small amount of condensate-associated gas, kerogen cracking gas, coalbed gas, and others. In

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addition to methane, hydrocarbon gases were also mixed with ethane, propane and butane, as well as

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a small amounts of C5 + and CO2 (Lu et al. 2010a; Lu et al. 2013b). Since the Late Cretaceous, the

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regional began to uplift, and the formation was eroded. The maximum temperature of coal-bearing

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mudstone dropped, which led to sharply declining hydrocarbon generation until no more was

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generated (Jiang et al. 2015).

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13

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Fig.3. Mechanism of gas hydrate formation in the Qilian Mountain permafrost.

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During the Yanshan movement in the Late Middle Jurassic to the Early Cretaceous, NW-SE

188

thrust faults were formed in this area (Fig. 1). In the middle and western Sanlutian region, the deep

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hydrocarbon source rock and shallow formation were connected with F25 and F27 faults, and the deep

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thermo-genic gases migrated from deep to shallow depths. The thrust faults of F1, F2 and F30 that

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formed at a relatively late stage of this period were characterized by continuous compression and 14

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193

movement in this area had an inheritance effect on F1 and F2 faults, especially in the middle and

194

western regions, which is favorable for gas migration and accumulation. The faults, including F25,

195

F27, F1, F2 and F30, played a role in the migration pathway and effective plugging for gas in the

196

process of gas hydrate formation in Sanlutian area. The gas entered into the broken zone or rock

197

fracture at different depths in the process of gas migration along the main faults. The tight and

198

complete mudstone and oil shale above the fracture zone acted as cap rocks. Under its effective

199

plugging, the gas accumulation zone was formed in fractured reservoirs, such as mudstone, oil shale

200

and siltstone, in the upper part of the Middle Jurassic Jiangcang Formation. A small amount of

201

microbial biogenic gas or coalbed gas may be added locally to the shallow gas accumulation (He et

202

al. 2015; Lu et al. 2015b).

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The Qinghai-Tibet Plateau reached its present altitude after the Kunhuang movement and

204

Gonghe movement. With the interaction of Quaternary glacial, the permafrost in the Qilian

205

Mountains has extensively and steadily developed since the Early Middle Pleistocene (Jiang et al.

206

2015; Wang et al. 1989). The inclusion of ethane, propane and CO2 reduced the temperature and

207

pressure condition for gas hydrate formation. Therefore, although the permafrost in the Qilian

208

Mountains was thinner than it in Arctic, there was a gas hydrate stability zone. Gas hydrate samples

209

were found at the depth range of 124.4 to 396 m, which indicated that the temperature and pressure

210

conditions of this range met the need for gas hydrate formation. According to the current drilling

211

data, we consider the depth range of 124.4 to 396 m as a gas hydrate stability zone. Under joint

212

control of the fractured system and lithology, gas hydrate reservoirs formed in the overlap of the gas

213

hydrate stability zone and gas accumulation zone in Sanlutian area, with the fracture-filling type

214

being the main gas hydrate reservoir.

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215

Gas hydrate in Sanlutian area formed under joint control of sedimentation, paleoclimate, and, in

216

particular, tectonic movement. According to the previous description, the mechanism of gas hydrate 15

ACCEPTED MANUSCRIPT formation in this area can be summarized as follows: gas generation from source rock was controlled

218

by tectonic subsidence and uplift--gas migration, accumulation was controlled by fault and tight

219

formation--gas hydrate formation, and accumulation was controlled by permafrost. Due to different

220

gas sources, gas accumulation zone, permafrost thickness and their matching relationships, the

221

distribution of gas hydrate in the vertical section and horizontal plane were heterogeneous in the

222

Sanlutian area.

223

3 Verification analysis on mechanism of gas hydrate formation

224

3.1 Analysis of the gas hydrate stability zone

SC

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217

The gas hydrate stability zone was mainly determined by the annual mean surface temperature,

226

geothermal gradient, water salinity and gas composition. The annual mean surface temperature and

227

geothermal gradient in permafrost determine the depth of permafrost, as well as control the

228

temperature and pressure conditions for formation. Since the salinity value of groundwater in this

229

area is relatively small, it has little influence on the gas hydrate phase equilibrium condition and

230

liquidus temperature of water. So we can assume that the salinity was 0 in our study (Collett and

231

Dallimore, 2000). As shown in Table 2, the gas components of gas hydrate samples in different wells

232

are not the same, but the proportions of each component are roughly the same. For this reason, gas in

233

DK 12-13 was taken as an example to analyze the gas hydrate stability zone in the Qilian Mountain

234

permafrost.

AC C

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TE D

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225

235

The gas composition in mixed gas hydrate was not exactly the same as the original gas

236

composition (Herri et al., 2011). Therefore, we first needed to infer the original gas composition

237

according to the gas composition in gas hydrate obtained in DK12-13. Then, the equilibrium

238

condition for gas hydrate in Muri basin was predicted. At last, the gas hydrate stability zone was

239

inferred according to the gas hydrate equilibrium condition, and temperature and pressure conditions

240

of formation. 16

ACCEPTED MANUSCRIPT

242

3.1.1 Temperature and pressure distribution of formation

We obtained the temperature distribution for formation by using the equation:

243

=

+

244

=

+

245

where

≤ −

(1)

>

is formation temperature,



(2)

RI PT

241

is annual average surface temperature,

246

temperature of the bottom of permafrost,

247

geothermal gradient below permafrost,

248

bottom of permafrost.

is geothermal gradient of permafrost,

is

is the depth of the

M AN U

SC

is the depth of formation, and

is the

249

The annual average surface temperature in the Muri area is approximately -2.0 ℃ (Li et al.,

250

2014; Zhu et al., 2006). Based on analysis of logging temperature data of DK-1, DK-2 and DK-3, the

251

depth of the bottom of permafrost in the middle and western of Sanlutian area is approximately 95 m

252

(Zhu et al., 2010b). We assumed that

253

Therefore,

254

the Muri basin, the average geothermal gradient of permafrost in the region was 0.026 ℃/m (Cao et

255

al., 2013). According to the temperature logging data of gas hydrate scientific wells DK-1 and DK-8

256

in the middle and western Sanlutian region, the geothermal gradient below permafrost in this area

257

was about 0.286 ~ 0.0404℃/m (Jin et al., 2011). Therefore, in our study, the geothermal gradient of

258

DK 12-13 was assumed to be 0.026℃/m, 0.028℃/m, 0.032℃/m, 0.036℃/m and 0.040℃/m

259

respectively. This temperature profile was shown in Fig. 4.

TE D

is 0 ℃, which was independent of the formation pressure.

AC C

EP

was 0.02105 ℃/m. According to the simple temperature logging data of coal holes in

260

The local gas pressure anomaly was not considered during the formation pressure analysis; only

261

the confining pressure of formation was considered. The relationship between formation pressure

262

and depth was dependent on hydrostatic pore pressure (Sloan and Koh, 2007); therefore, we can

263

obtain the formation pressure-depth relationship. 17

ACCEPTED MANUSCRIPT 264

265

3.1.2 The original gas composition of gas hydrate in DK 12-13

The average gas composition from gas hydrate samples in DK 12-13 is C1 65.65%, C2 7.89%,

266

C3 19.96%, i-C4 2.96%, n-C4 1.19%, C5 1.37%, C6

267

composition was predicted to be C1 94.091%, C2 4.13%, C3 0.845%, i-C4 0.0535%, n-C4 0.275%,

268

C5+ 0.0655% and CO2 0.54% according to an analysis using CSMGem software developed by Center

269

for Hydrate Research, Colorado School of Mines. The average gas composition of headspace gas of

270

DK 12-13 was C1 95.32%, C2 3.41%, C3 1.00%, i-C4 0.07%, n-C4 0.15% and C5 + 0.05% (Tang et al.,

271

2015c), was consistent with the prediction. Therefore, the reliability of the prediction was verified.

272

The minor differences in gas compositions may be associated with later gas migration or new

273

biogenic gas incorporation.

274

3.1.3 Gas hydrate equilibrium condition and gas hydrate stability zone

0.65% and CO2 0.33%. The original gas

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+

The equilibrium temperature of mixed gas hydrate in the formation was predicted by CSMGem

276

software based on the formation pressure, and the mixed gas hydrate equilibrium curve in the

277

formation was obtained, as shown in Fig. 4. The range between two intersection points for the

278

temperature profile and the mixed gas hydrate phase equilibrium curve was called the gas hydrate

279

stability zone.

EP

As shown in Fig. 4, the gas hydrate stability zones of the two gas components had a large

AC C

280

TE D

275

281

difference. For example, the range of gas hydrate stability zone for

282

m for gas component 1 and 25 - 785 m for gas component 2. However, the gas hydrate samples in

283

this area were only found in the range of 124.4 - 396 m, and there was no gas hydrate in permafrost,

284

which was close to the predicted value for gas component 1. The correctness of selecting the original

285

gas components for gas hydrate stability zone analysis was verified.

18

0.032 ℃/m was 108 - 485

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ACCEPTED MANUSCRIPT

286 287

Fig. 4. Gas hydrate stability zone in the Qilian Mountain permafrost. Gas component 1: the original gas component before gas

288

hydrate formation; Gas component 2: the gas component from gas hydrate sample.

Fig. 4 showed that

had little effect on the top boundary of the gas hydrate stability zone,

TE D

289 290

whereas gas component 1 was within 100 - 120 m. However, the bottom boundary of gas hydrate

291

stability zone had a large difference. When

292

/m and 0.026 ℃/m, the corresponding bottom boundary of gas hydrate stability zone was 332 m,

293

398 m, 485 m, 598 m and 667 m, respectively. In comparison with the distribution interval of gas

294

hydrate samples from DK 12-13, which were 202.8 - 313.4 m, and the actual distribution interval of

295

gas hydrate in Sanlutian，which was 124.4 - 396 m, it can be deduced that the

296

0.036℃/m. This value coincides with the

297

method for analyzing the gas hydrate stability zone.

298 299

AC C

EP

was 0.040 ℃/m, 0.036 ℃/m, 0.032 ℃/m, 0.028 ℃

was 0.040℃/m -

of DK 8, which was used to verify the validity of the

Based on the described analysis, it can be inferred that the gas hydrate stability zone in the Qilian Mountain permafrost was 100 m - 400 m depth.

19

ACCEPTED MANUSCRIPT 300

3.2 Numerical simulation of gas accumulation The gas from deep formation entered into the broken zone or rock fracture in the process of gas

302

migration along the main fault. Under effective plugging of the tight formation above the broken

303

zone, the gas accumulation zone was formed in the fractured reservoirs. Based on the analysis of the

304

hydrocarbon gas content in headspace gas, the content of hydrocarbon gases in the fracture zone was

305

high, and they in the formation above the fracture zone decreased as the distance from the fracture

306

zone increased. This showed the property of gas accumulation in the fracture zone and gas migration

307

along the fracture zone or fault, and also indicated that low porosity and low permeability formation

308

effectively seals against gas migration.

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301

To verify the process of gas migration and accumulation, and analyze the influencing factors,

310

the numerical simulation was conducted using TOUGH+HYDRATE software. The interval between

311

125.0 m and 138.5 m in DK 8-19 was chosen as the target layer. The depth of 135.50 - 138.28 m was

312

the fracture zone and the content of hydrocarbon gas was very high. In addition, gas hydrate was

313

found at 135.87 m. From the fracture zone to 128.70 m, the content of hydrocarbon gas gradually

314

decreased.

315

3.2.1 Numerical model and parameters

317 318 319 320 321 322

EP

To conduct this numerical simulation, we must make some simplifications and assumptions,

AC C

316

TE D

309

which were as follows:

(1) The formation was simplified to two types of formation: the fracture zone (135.5-138.5 m) and its overburden(125-135.5 m), as shown in Fig. 5; (2) Each formation was homogenized, and the fracture zone was described by increasing the porosity and permeability; (3) Methane is the only gas.

20

ACCEPTED MANUSCRIPT The basic physical parameters of the interval were obtained from literature, as shown in Table 3

324

(Li et al., 2012; Li et al., 2015; Lu et al., 2010c). In our study, the porosity of overburden was set at

325

0.04 and the fracture zone was set at 0.08.

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323

327 328

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Fig.5. A simplified diagram of the fracture zone and its overburden. Table 3 The physical properties of the fracture zone and its overburden

Formation

Porosity (including fracture)

Permeability /10-15 m2

Overburden

0.03-0.05

0.01-1

0.07-0.09

1-20

329

TE D

Fracture zone

3.2.2 The setup of numerical simulation

At present, there are few reports on the water saturation of the fracture zone and its overburden

331

in the Qilian Mountain permafrost. To explore the effect of initial water saturation of overburden on

332

the sealing performance of gas, six different initial water saturations (0.95, 0.9, 0.8, 0.7, 0.5 and 0.2)

333

were selected for numerical simulation. Similarly, four different permeability (0.01×10-15 m2、

334

0.05×10-15 m2、0.1×10-15 m2 and 1×10-15 m2) were used.

335

3.2.3 Results and Analysis

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330

336

Fig. 6 shows the final distribution of gas saturation at different initial water saturation. When

337

the initial water saturation was high, gas saturation in the fracture zone was approximately 0.7, which 21

ACCEPTED MANUSCRIPT exceeded the initial gas saturation and indicated that the entry of deep high-pressure gas caused the

339

movement of water in the fracture zone and its adjacent overburden. When the initial water

340

saturation was low (as in the case of 0.2), gas from deep reservoirs had enough space and relatively

341

large permeability in the fracture zone; therefore, there was almost no water migration. When the

342

initial water saturation was 0.9 and 0.95, the gas saturation decreased sharply from 128.5 to 129 m

343

and from 126.5 to 127 m, respectively, and there was no change between the final gas saturation

344

above this interval and the initial gas saturation. This indicated that the overburden played a role in

345

gas sealing, when the initial water saturation was higher than 0.8. When the initial water saturation

346

was less than 0.8, the gas saturation in the formation is gradually reduced, and there is no sharp

347

decrease, which indicates that the overburden had no role in gas sealing.

348 349

AC C

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TE D

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338

Fig.6. The distribution of gas saturation at different initial water saturations.

350

Fig. 7 shows the final distribution of gas saturation at different permeability of overburden.

351

When the permeability of overburden was 0.01 × 10-15 m2 and 0.05 × 10-15 m2, the gas saturation

352

decreased sharply at depths of 132 m and 128.7 m, respectively. This indicated that the overburden 22

ACCEPTED MANUSCRIPT played a role in gas sealing. However, when permeability was more than 0.1 ×10-15 m2, there was no

354

sharp decrease, which indicated that the sealing property of the overburden was poor.

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353

355

Fig.7. The distribution of gas saturation at the different permeability of overburden.

TE D

356

In summary, in addition to the effect of porosity, the sealing property of the overburden was

358

mainly controlled by rock permeability and the initial water saturation. When the initial water

359

saturation was 0.9 and the permeability was 0.05 ×10-15 m2, gas saturation rapidly decreased from

360

128.5 to 129 m, which was close to the depth of 128.7 m observed in drilling. This indicated that the

361

overburden was an effective sealant, which could achieve gas accumulation in the fracture zone, and

362

provided adequate gas for gas hydrate formation. The values of the permeability obtained by

363

numerical simulation were close to those measured in the laboratory, which proved that the

364

numerical simulation was valid. For the mudstone, which had less porosity and permeability, an

365

effective sealing effect for gas accumulation can be achieved.

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357

366

Numerical simulation was used to further verify that the low-porosity and low-permeability

367

formation with high water saturation can effectively seal gas and form gas accumulation zone. This

23

ACCEPTED MANUSCRIPT 368

supported the hypothesis that gas hydrate formation occurred after the formation of gas accumulation

369

zone.

370

3.3 Experimental Study on Synthesis of Gas Hydrate in Different Lithology Gas hydrates in the Qilian Mountain permafrost mainly occurred in the fracture of mudstone, oil

372

shale, siltstone and fine sandstone and in the pores of siltstone and fine sandstone. To investigate the

373

effect of lithology on synthesis of gas hydrate, three different lithologies were used.

374

3.3.1 Experimental preparation

SC

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371

Mudstone, sandstone and granite core with fractures were selected as the experimental medium

376

in this paper. Mudstone is one of the main lithology containing gas hydrate in the Qilian Mountain

377

permafrost. It is a type of clay rock with low permeability and porosity, and has good water

378

absorption and water retention. Sandstone is the main lithology for pore gas hydrate. Granite is also a

379

type of rock with the low permeability and porosity, but its water absorption and water retention is

380

very poor.

TE D

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375

The experimental apparatus for synthesis of gas hydrate mainly includes the reactor system, the

382

control system for temperature and pressure, and a detection and recording system, as shown in Fig.

383

8.

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381

384 385

Fig.8. The schematic diagram of experimental apparatus for synthesis of gas hydrate. 1 Methane cylinder; 2 Gas pressure

386

reducing valve; 3 Shut-off valve; 4 Reactor; 5 Temperature sensor; 6 Constant temperature water bath; 7 Pressure sensor; 8

387

Pressure gauge; 9 Vent valve; 10 Paperless recorder. 24

ACCEPTED MANUSCRIPT 388

3.3.2 Experimental result

Methane hydrate was formed in all three fissures, most of which occurred in mudstone fissures;

390

there were few methane hydrates in sandstone fissures and granite fissures. The pore-filling hydrates

391

were present only in the pore of sandstone cores. The experimental results are as follows:

RI PT

389

(1) Methane hydrate distribution in mudstone fracture

393

Most methane hydrates were found in mudstone fractures, and they were milky white and

394

occurred as layers or nodules. Both the horizontal and oblique fractures had large amounts of gas

395

hydrate. The hydrate layer in the horizontal fracture was thicker than the hydrate layer in the oblique

396

or vertical fracture. Methane hydrate in the vertical fracture or high-angle oblique fracture occurred

397

as nodular or wavy, as shown in Fig. 9-a. Methane hydrates in the mudstone fracture were flammable

398

and boiling during combustion, as shown in Fig. 9-b.

400

AC C

399

EP

TE D

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SC

392

Fig.9. Methane hydrate in mudstone fracture and ignition.

401

(2) Gas hydrate distribution in sandstone

402

Only a small amount of methane hydrates were formed in the sandstone fracture, and it

403

decomposed during the reactor disassembly. Because sandstone pore is small, pore-filling gas

404

hydrates could not be visually observed by the naked eye. The presence of gas hydrates in the pore

405

were determined by the strong bubbling in water, as shown in Fig. 10.

406

(3) Gas hydrate distribution on granite fracture 25

ACCEPTED MANUSCRIPT There was a very small amount of methane hydrate on the horizontal surface of the granite and

408

it appeared only in low-lying areas of horizontal surface or at the core edge. No methane hydrate was

409

formed in vertical fractures or high-angle fractures, as shown in Fig. 11.

410 Fig.10. Strong bubbling caused by hydrate dissolution in sandstone.

412

3.3.3 Analysis and discussion

Fig. 11. Little methane hydrate on the granite fracture.

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411

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407

Granite and mudstone cores were low-porosity and low-permeability rock, there was only a

414

very small amount of methane hydrate on the granite fracture. However, a large amount of methane

415

hydrate formed in the mudstone fracture, which was mainly controlled by effective water absorption

416

and water retention in the mudstone and water migration during hydrate formation. There was a

417

thickness of free water film in the mudstone fracture, which could provide full contact with gas and

418

transformed into methane hydrate under appropriate condition. When methane hydrate formed in the

419

mudstone fracture, a suction effect on the water in the micro-pore or micro-fissure and in the

420

metastable state and unstable state in clay minerals was induced. This resulted in a migration of

421

water to fracture surface that continued to form hydrate. According to the experimental results,

422

methane hydrate was mainly distributed in the fracture for low-porosity and low-permeability rock,

423

and its occurrence was mainly controlled by water distribution and migration.

AC C

EP

TE D

413

424

Since the mudstone pores were mainly microporous and mesoporous, the formation of methane

425

hydrate was difficult. Therefore, the pore-filling methane hydrate was not observed in mudstone in 26

ACCEPTED MANUSCRIPT the experiment. The pore of the sandstone was relatively large, and most pores had no effect on the

427

methane hydrate phase equilibrium condition. In addition, the free water saturation in the sandstone

428

pore was low relative to the mudstone; therefore, the gas can diffuse and migrate in the sandstone

429

pore and form methane hydrate with water. The water film on the sandstone fracture was relatively

430

thin; therefore, the amount of gas hydrate in the fracture was less. As porosity of sandstone increased,

431

the thickness of the water film decreased (Wang et al., 2012), and the gas relative permeability

432

increased. Therefore, the amount of gas hydrate in the fine or medium grained sandstone fracture

433

was little or zero, while the amount of gas hydrate in the pores increased. The formation of gas

434

hydrate in the sandstone was mainly controlled by pore size and clay content, which further

435

controlled water distribution and gas permeability.

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426

The occurrence of methane hydrate in the lab experiment was consistent with observations in

437

the field, which verified the hypothesis that the occurrence of gas hydrate was controlled by lithology

438

and the fracture system.

439

4 Conclusions

TE D

436

1. Gas hydrate in the Sanlutian region formed under the joint control of tectonic movement,

441

sedimentation and paleoclimate. The mechanism of gas hydrate formation in this area can be

442

summarized as follows: gas generation from source rock was controlled by tectonic subsidence and

443

uplift--gas migration and accumulation was controlled by fault and tight formation--gas hydrate

444

formation and accumulation was controlled by permafrost. At the end of the Early Cretaceous, the

445

Upper Triassic mudstone and Middle Jurassic coal-bearing mudstone began to generate gas. Gas

446

entered into the broken zone at different depths during the process of gas migration along the main

447

fault, such as F25, F27, F1, F2 and F30, which was formed in the Yanshan movement and the Xishan

448

movement. Under effective plugging of the tight and complete mudstone and oil shale formation

449

above the fracture zone and fault F1 and F2 with continuous compression, the gas accumulation zone

450

was formed in the fractured reservoirs. Under joint control of the Qinghai-Tibet Plateau uplift and

AC C

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440

27

ACCEPTED MANUSCRIPT 451

the Quaternary glacial, the depth of permafrost in this area reached approximately 90 m, which

452

provided the temperature and pressure condition for gas hydrate formation. Finally, gas hydrate

453

reservoirs formed in the overlap of the gas hydrate stability and gas accumulation zones in the

454

Sanlutian area. 2. The permafrost-associated gas hydrate samples in China were only obtained beneath

456

permafrost in the range of 124.4-396.0 m in the Sanlutian region, and the gas composition was

457

complex. The original gas composition before gas hydrate formation in DK 12-13 was predicted to

458

be C1 94.091%, C2 4.13%, C3 0.845%, i-C4 0.0535%, n-C4 0.275%, C5+ 0.0655% and CO2 0.54% by

459

using CSMGem software, which was used to analyze the range of gas hydrate stability zone in

460

DK12-13 at 108-485 m. Combined with the range of gas hydrate samples, it can be inferred that the

461

gas hydrate stability zone in the Qilian Mountain permafrost was 100-400 m depth.

M AN U

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455

3. The results of numerical simulation of gas distribution in the interval of 128.70-138.28 m

463

indicated that the sealing property of the overburden was mainly controlled by porosity, rock

464

permeability and initial water saturation. When initial water saturation was 0.9 and permeability was

465

0.05 × 10-15 m2, gas saturation rapidly decreases from 128.5 to 129 m, which was close to the depth

466

of 128.7 m that was observed in drilling. This indicated that the overburden was an effective sealant,

467

which could achieve gas accumulation in the fracture zone, and provided adequate gas for gas

468

hydrate formation. This supported the hypothesis that gas hydrate formation occurred after the

469

formation of gas accumulation zone.

AC C

EP

TE D

462

470

4. Gas hydrate mainly occurred in the fracture of low-porosity and low-permeability rocks such

471

as Jurassic mudstone and oil shale. Through laboratory experiments and numerical simulations, it

472

was found that methane hydrate was more likely to form in mudstone fractures. This was mainly

473

controlled by effective water absorption and retention of the mudstone and water migration during

474

hydrate formation. The water film on the sandstone fracture was relatively thin; therefore, there was

475

little methane hydrate in the fracture. The formation of methane hydrate in the sandstone was mainly 28

ACCEPTED MANUSCRIPT controlled by the pore size and clay content, which further controlled the water distribution and gas

477

permeability. The occurrence of methane hydrate in the lab experiment was consistent with field

478

observations, which verified the hypothesis that the occurrence of gas hydrate was controlled by

479

lithology and the fractured system.

480

Acknowledgments

RI PT

476

This study has been supported by National Natural Science Foundation of China (Grant

482

No.51474112, Grant No.51304079), and China Geological Survey Project (GZHL201400307,

483

GZHL20110326).

484

Nomenclature

486

Fault 2

487

Fault 25

488

Fault 27

489

Fault 30

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Fault 1

TE D

485

SC

481

F-F Fracture-Filling gas hydrate reservoir

491

P-F Pore-Filling gas hydrate reservoir

492

NA No data

AC C

493

EP

490

Formation temperature (℃)

494

Annual average surface temperature (℃)

495

Temperature of the bottom of permafrost (℃)

496

Geothermal gradient of permafrost (℃/m)

497

Geothermal gradient below permafrost (℃/m)

498

Depth of formation (m) 29

ACCEPTED MANUSCRIPT !

Depth of the bottom of permafrost (m)

500

"#

Initial water saturation

501

References

502

Bily C, Dick J W L, 1974. Naturally occurring gas hydrates in the Mackenzie Delta, NWT. Bulletin

503

of Canadian Petroleum Geology. 22(3), 340-352.

RI PT

499

Cao DY, Li J, Wang D, et al, 2013. Study of the gas hydrate stability zone in Muri Coalfield,

505

Qinghai Province, China. Journal of China University of Mining & Technology. 42(1), 76-82[in

506

Chinese with English abstract].

SC

504

Chen LM, Cao DY, Jiang AL, et al, 2015a. Structural control of reservoir forming for natural gas

508

hydrate in Sanlutian well field, Qinghai. Science & Technology Review. 33(6), 91-96 [in

509

Chinese with English abstract].

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Chen LM, Qin RF, Jiang AL, et al, 2015b. Structural fracture characteristics of cores from gas

511

hydrate drillholes in Sanlutian of Muli Coalfield, Qinghai. Geoscience. 29(5), 1087-1095 [in

512

Chinese with English abstract].

TE D

510

Cheng B, Xu JB, Lu ZQ, et al, 2017. Hydrocarbon source for oil and gas indication associated with

514

gas hydrate and its significance in the Qilian Mountain permafrost, Qinghai, Northwest China.

515

Marine and Petroleum Geology (2017), https://doi.org/10.1016/j.marpetgeo.2017.02.019.

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Collett TS, 2002. Energy resource potential of natural gas hydrates. AAPG bulletin. 86(11),

AC C

516

EP

513

1971-1992.

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ACCEPTED MANUSCRIPT Highlights The mechanism of gas hydrate formation in the Qilian Mountain permafrost was inferred in detail.

numerical analysis and laboratory experiments.

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Some control factors of gas hydrate formation were analysed and validated through

The gas hydrate stability zone in the Qilian Mountain permafrost was at the depth of

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100-400 m.

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The low-porosity and low-permeability formation with high water saturation can effectively seal gas and form gas accumulation zone.

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The occurrence of gas hydrate was controlled by lithology and the fractured system.