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Marine and Petroleum Geology 35 (2012) 166e175

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Marine and Petroleum Geology

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Gas hydrate formation and its accumulation potential in Mohe permafrost, China

Xingmin Zhao*, Jian Deng, Jinping Li, Cheng Lu, Jian SongInstitute of Mineral Resources, CAGS, 26 Baiwanzhuang, Beijing 100037, China

a r t i c l e i n f o

Article history:Received 9 August 2011Received in revised form24 March 2012Accepted 12 April 2012Available online 27 April 2012

Keywords:Gas hydratePermafrostPetroleum geologyMohe regionChina

* Corresponding author. Tel.: þ86 10 68999069.E-mail address: xxmmzh@163.com (X. Zhao).

0264-8172/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.marpetgeo.2012.04.003

a b s t r a c t

The Mohe region is an area of continuous permafrost in northernmost China with strong similarities toother known gas-hydrate-bearing regions. Permafrost thickness is typically 20e80 m; average surfacetemperature ranges from �0.5 �C to �3.0 �C, and the geothermal gradient is roughly 1.6 �C/100 m. Weestimate that 204.66 � 1012 m3 of hydrocarbon gases have been generated in the Mohe basin fromnearly 1000 m middle Jurassic dark mudstones, providing ample gas source for gas hydrate formation.Numerous folds in the shallow section provide opportunities to trap gas within sandstones andsiltstones reservoirs bounded by competent mudstone seals. Gas migration to the shallow section isenabled via fault fracture zones and fracture systems. Based on core description and observations ofgas releases from drilled wells, we infer that the Mohe region could hold large quantities of naturalgas in the form of gas hydrate.

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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. Actually, gas hydrate is a kind ofspecial-form (solid) or special-type (unconventional) natural gas(Pan, 1986; Dai et al., 1989; Zhang and Zhang, 1989). In nature, gashydrate frequently occurs either in submarine sediments oncontinental shelves or in association with permafrost (Sloan, 1998;Majorowicz and Hannigan, 2000; Makogon et al., 2007; Makogon,2010). Gas hydrate has significant implications for energyresources, geohazards, and climate change (Kvenvolden, 1988a, b;Macdonald,1990; Nisbet, 1990; Paull et al., 1991; Maslin et al., 1998;Bouriak et al., 2000; Milkov et al., 2000; Kennedy et al., 2001;Collett, 2002, Collett et al., 2009) and has attracted great interestfrom the scientists from many disciplines.

Although marine settings house the majority of global gashydrate resources, permafrost is an important area for the forma-tion and occurrence of gas hydrate, particularly those occurrenceswithin sand reservoirs that have the greatest current potential forenergy production. An example from the Yamal Peninsula in westSiberia indicates that the relic of gas hydrate probably occur60e80m below permafrost for its self-preservation effect (Chuvilin

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et al., 1998; Yakushev and Chuvilin, 2000), suggesting that gashydrate may have an even more widespread distribution inpermafrost than commonly thought. Recently, gas hydrate wasfound in Qilian mountains permafrost in Qinghai Province, China(Zhu et al., 2010), in a similar to that of the Mohe area.

The Mohe area, situated in the northernmost portion of China(Fig. 1), has the lowest average surface temperature in China andwidespread development of permafrost. In the past few years,a range of geochemical, geological, and geophysical investigationshave provided confirmation of the existence of the necessaryelements for a well-developed gas hydrate petroleum system (afterCollett et al., 2009) in the basin.

2. Regional geological setting and its control of gas hydrateformation

The Mohe basin occupies roughly 21,300 km2 of China north ofN 52� 200. The region is among the least explored for hydrocarbonsin China. Paleogeographically, the Mohe basin is attributed toMongolia-Okhotsk ocean (Zhang et al., 2003;Wu et al., 2006) and iscurrently tectonically situated on the Eerguna micro-plate withinthe Okhotsk folded belt (Zhang et al., 2003) on the suturedboundary between the Siberia plate and the Northeast China put-together plate. The southern margin of the basin is marked byProterozoic granite (Fig. 1). Tectonic processes, including Mongolia-Okhotsk ocean crust subduction, interaction between the Siberiaplate and China plate, and subsequent underthrusting of Pacificplate etc., have significantly influenced both the depositional

Figure 1. Geological map of Mohe basin. 1dQuaternary system; 2dGanhe Formation; 3dJiufengshan Formation; 4dGuanghua Formation; 5dLongjiang Formation;6dTamulangou Formation; 7dKaikukang Formation; 8dMohe Formation; 9dErshierzhan Formation; 10dXiufeng Formation; 11dDiorite-porphyrite of early Cretaceous;12dmonzonitic granite of late Jurassic; 13dJurassic granite; 14dmonzonitic granite of new Archean.

X. Zhao et al. / Marine and Petroleum Geology 35 (2012) 166e175 167

(including source rocks, reservoirs, and seals) and structural(including the development of trapping geometrys and fault/frac-ture related migration pathways).

Mohe basin is a continental basin formed through the Mesozoic.The approximately 6000 m or more of strata fill consists of clasticand volcanic rocks of the middle and upper Jurassic (with minorCenozoic sediments) which lies upon Devonian basement. Themiddle Jurassic, which is the majority of the basin fill, is mostlymade up of fluvial and lacustrine terrigenous clastics of the Xiufeng,Ershierzhan, Mohe, and Kaikukang Formations (Table 1). TheErshierzhan and Mohe Formations occur widely throughout thebasin and include the majority of source rocks and reservoir rocksfor potential gas hydrate formation. The Xiufeng Fm, also widelydistributed in the region, primarily embraces reservoir rocks. TheKaikukang Fm is restricted to the northeast part of the basin. Theupper Jurassic occurs only locally and primarily consists of volcanicand pyroclastic rocks interbedded with fluvial and lacustrineterrigenous clastic formations. During the late stage of basin filling,the interaction among Siberia plate, Northeast China put-togetherplate and Pacific plate resulted in the development of manynortheast-to-east northeast striking overthrust faults and associ-ated faults, alternating depressions and uplifts, as well as large-scale volcanic eruption in the region (Zhang et al., 2003). Thesefaults make up the primary conduits for continuous hydrocarbon-supply to the shallow section. The uplifts or salients act as thetraps for gas hydrate accumulation.

3. Gas hydrate petroleum system

3.1. Thickness of permafrost and geothermal gradients

Permafrost is continuously spread throughout Mohe region(Zhou et al., 2000; Jin et al., 2009) and thickens to the northwest.Thickness of permafrost ranges has been alternatively reported asranging from 50 to 100m (Zhou et al., 2000) and from 0 to 60m (Jinet al., 2009). Our geophysical surveying from 2003 to 2004

indicates that the thickness of permafrost to the west of the JingouForest Farm is about 20e80 m with thickness up to 140 m or solocally. These thicknesses are comparable to those reported inother gas-hydrate-bearing regions including Qilian mountainsregion in China (Zhu et al., 2010) and Yamal Peninsula in Siberiawhere is speculated on gas hydrate occurring (Chuvilin et al., 1998;Yakushev and Chuvilin, 2000).

Geotemperature, which is usually characterized by groundsurface temperature, geothermal heat flow and gradient, hasa profound impact on occurrence of gas hydrate. Prior data (Wangand Huang, 1988a, b) indicate that the surface temperature of thepermafrost in northeastern China is between 0.5 and �2.5 �C;geothermal flux is from 30 to 71 mW/m2; and geothermal gradientis from 1.0 to 4.54 �C/100 m. As far as it is further concerned, Moheregion has the lowest surface temperature of �1.0 to �2.5 �C, thenortheast part of Inner Mongolia shows the lowest geothermal heatflow of 40 mW/m2, and the north area of Heilongjiang Province hasthe lowest geothermal gradient of 1.2 �C/100 m. Our data reveal,however, that the surface temperature and geothermal gradient inMohe region is from �0.5 to �3.0 �C and around 1.6 �C/100 mrespectively, which is similar to those of Messoyakha in Siberia (�8to �12 �C (Romanovsky et al., 2007) and 1.0e3.0 �C/100 m(Makogon, 2010)), Prudhoe Bay on Alaska North Slope (�4.6 �C to�12.2 �C (Kamath et al., 1987) and 1.5 �Ce5.2 �C/100 m (Collettet al., 2011), and Qilian Mountains area in China (�1.5 �C to�2.4 �C and 2.2 �C/100 m; Zhu, personal correspondence). Conse-quently, Mohe permafrost has a favorable geothermal condition ofgas hydrate formation.

3.2. Temperature-pressure condition of gas hydrate formation

Temperature and pressure requisite for gas hydrate formation(Sloan and Koh, 2008) have a decisive effect on the phase balance ofgas hydrate petroleum system. In the subsurface, temperature andpore pressure are dominant controls on the stability of gas hydrate.In general, the thickness of the gas hydrate stability zone can be

Table 1Strata fills in Mohe basin (Revised after Zhang et al., 2003).

Strata Thickness (m) Dominant lithology

Overlain strata Quaternary Jinshan Fm 0e45 Light yellow clay, loam, fined-sand, even with gravelCretaceous Lower Cretaceous Yiliekede Fm 0e596 Pale yellow, dark gray and black almond-shape basalt is dominated

0e839 Majority of tuff, with a small amount of marl and clastic rocks in the lower;the acid volcanic rocks at the top

Jurassic Upper Jurassic Shangkuli Fm 0e1656 Tuff, with a small amount of marl and clastic rocks in the upper; the acidvolcanic rocks at the top

0e729 Dark red, purple and brown conglomerate, gray fine-grained sandstone,with a little amount of crystal-tuffaceous sandstones

Tamulangou Fm 0e1161 Purple, gray-green and dark gray block almond-shape or vesicular basalt;granule and pebble conglomerates, brecciform basaltic tuff

Middle Jurassic Kaikukang Fm 0e2734 Gray, brown, dark gray sandstones, feldspathic quartz sandstones, lithicquartz sandstones, pebbly sandstones and silty mudstones interbeddedwith coal streaks

Mohe Fm 1945e3979 Dark gray, black medium to coarse and fine-grained lithic arkoses, siltstones,and mudstones interbeded with varicolored conglomerates and coal streaks

Ershierzhan Fm 799e4400 Light gray, gray yellow, green gray, dark gray and black mudstones, siltymudstones, feldspathically lithic sandstones and pebbly sandstones interbededwith fine sandstones, siltstones and coal streaks

Xiufeng Fm 836e2806 In the lower and upper, majorly sandy conglomerates and cobble conglomeratesinterbeded with fine sandstones; lithic arkoses interbeded with sandy tuffs andcoal streaks in the middle

Basement (Devonian) Lower Devonian Huolongmen Fm 84e115 Metamorphic arkose, sericite slate, biomarl and phyllitzed silty- and fine-grainedsandstones

Figure 2. Temperature-pressure conditions in the Mohe basin. 1dBottom boundary of20 m thickness of permafrost; 2dbottom boundary of 80 m thickness of permafrost;3dgeothermal gradient of 20 m thickness of permafrost; 4dgeothermal gradient of80 m thickness of permafrost; 5dgeothermal gradient below permafrost; 6d phasebalance boundary of pure methane hydrate; 7dphase balance boundary of hydrate ofabsorbed hydrocarbon gases from drilled cores.

X. Zhao et al. / Marine and Petroleum Geology 35 (2012) 166e175168

calculated using surface temperature, geothermal gradient andstrata pressure.

In accordance with the data published before, Mohe area has anannual mean surface temperature of about �2.4 �C; its geothermalgradient calculated on the basis of only a temperature well log fromwell MD1 is about 1.6 �C/100 m. The strata pressure in this area,which is hydrostatic pressure plus lithostatic pressure, is calculatedaccording to the thickness and density of strata in the region, thedata we use here as follows: 20 m and 80 m of permafrost thick-ness, about 2600 kg/m3 of rock density and 1000 kg/m3 of porewater density.

In addition to temperature and pressure, the salinity of porewaters and gas composition also impact gas hydrate stability. Thelowered salinity of the underground water in Mohe area can beoverlooked, gas composition come from the absorbed gas collectedfrom drilled cores.

According to all mentioned above, temperature-pressurecondition of gas hydrate stability is determined by utilizingSloan’s (1998) software. The results indicate that, besides gascomposition, the thickness of permafrost in Mohe area has themost decisive impact on the thickness of gas hydrate stability zone.The gas hydrate phase diagram (Fig. 2) shows that, given 20 m ofpermafrost thickness and about 1.6 �C/100 m of geothermalgradient, no gas hydrate can form assuming pure methane gas.However utilizing gas chemistry data obtained from absorbed gasfrom drilled core, gas hydrate stability can occur. Given 80 m ofpermafrost thickness and the geothermal gradient above, thegeothermal gradient curve intersects with the hydrate phasebalance curve of both pure methane and absorbed gas from thedrilled cores, which indicates that hydrates of both pure methaneand absorbed gas collected from the drilled cores may form. Owingto the presence of more heavy hydrocarbon gases in the Mohe area,we determine that all areas with permafrost greater than 20 m inthickness offers the potential for gas hydrate formation.

3.3. Source and quantity of hydrocarbon gases

3.3.1. Source material for hydrocarbon gasesAccording to our present knowledge, the hydrocarbon gases

from gas hydrate can be either biogenic and thermogenic in origin.No matter how the gases were generated, the source material is

organic matter within sedimentary rocks or sediments. Therefore,the type, origin and quantity of organic matter from gas sourcerocks are very important for gas hydrate formation.

As discussed above, 6000 m thickness of the basin fill is mostlyterrigenous clastic sedimentary formations, which consist domi-nantly of sandstones and mudstones originated from shallow lake,braided river delta and braided river environments. In the light ofthe data acquired from regional surveying and drilled cores,

X. Zhao et al. / Marine and Petroleum Geology 35 (2012) 166e175 169

mudstones, mainly including dark gray or black mudstone andsilty mudstone, are the dominant gas source rocks (greater than85%) in the Mohe basin. Based on the measured results of 144samples from Mohe basin, the argillaceous rock (includingcarbonaceous shale) greater than 0.75% of total organic carboncontent is defined as gas source rock in this paper (Chen et al.,1997). According to this definition, total estimated thickness ofgas source rocks in the Mohe basin is 1000 m or more (Table 2), ofwhich there is 142e477 m in Xiufeng Fm, 120e660 m occurs inErshierzhan Fm, 797e1631 m in Mohe Fm, and none inKaikukang Fm.

3.3.2. Generation and quantity of hydrocarbon gasesGas source rocks are certainly of great importance to the

formation of gas hydrate. More important, however, is the abun-dance of organic matter in gas source rocks. The analysis of 125mudstone samples show that samples of higher than 0.75% of totalorganic carbon (TOC) exceed 80% with more than 65% in XiufengFm, around 73% and 89% in Ershierzhan Fm and Mohe Fmrespectively. The measured data of 22 mudstone samples alsoreveals that the samples higher than 0.0015% of chloroform bitu-minous A is about 50%, of which there is little in Xiufeng Fm, thereis about 55% in Ershierzhan Fm, and there is more than 65% inMohe Fm. Rock pyrolysis data from 62 samples shows that over70% of samples have more than 0.5 mg/g of hydrocarbon potential(S1 þ S2). Taking into account the fact that due to these samplesbeing taken from the ground surface in the field, weathering maymake such a significant loss of organic matter in rock that theabundance of organic matter in gas source rocks decrease greatly(Chen, personal correspondence), the abundance of organic matterfrom mudstone in Mohe region may be much higher than thatdescribed here. Therefore, in the light of the abundance of organicmatter, the study area has great richness of hydrocarbon-generating materials.

The microscopic analysis of kerogen from 65 mudstone samplesand vitrinite reflectance of 55 mudstone samples indicate that theorganic matter from the Middle Jurassic in Mohe basin is domi-nated by type III kerogen with vitrinite reflectance (Ro) usuallybetween 0.90% and 1.35%. According to the modern theory ofpetroleum geology, these organic matters are the parent materialsof thermogenic hydrocarbon gases. On the other hand, the modernoil and gas geochemical studies have shown that some of organicmatter (especially type II kerogen of lacustrine origin) from themudstones may be the major sources of biogenic hydrocarbongases (Liu et al., 2009). Furthermore, some recent research alsoindicates that biogenic hydrocarbon gases from organic matter inthe mudstones of permafrost (Ward et al., 2004) and non-permafrost (Rice, 1993; Scott et al., 1994. DATE; Smith andPallasser, 1996; Luo et al., 2003; Zhu et al., 2006; Manzur andSimith, 2001) are also common.

The hydrocarbon gases generated in Mohe basin include boththermogenic and biogenic gases, the amount of which werecalculated according to thermogenic (Schmoker,1994) and biogenicmodels (Wang et al., 1996) respectively. It was calculated that the

Table 2The occurrences of source rocks in Mohe basin.

Lithostrati-graphy Thickness inmeasured strata (m)

Thickness in measuredgas source rocks (m)

Ratio of srocks to s

Kaikukang Fm. 254.6 0.6 0.002Mohe Fm. 2284.27 946.89 0.41Ershierzhan Fm. 3079.16 466.06 0.15Xiufeng Fm. 977.3 163.8 0.17Total 6595.33 1577.35 0.24 (me

hydrocarbon gases generated in Mohe basin amounts to204.66 � 1012 m3 or so, of which thermogenic gases is about52.9� 1012 m3, and biogenic gases is up to around 151.76� 1012 m3.Obviously, such a great volume of hydrocarbon gases may be gassource for potential gas hydrate formation in Mohe basin. What ismore, the biogenic gases almost three times as much as the ther-mogenic is roughly consistent with the permafrost of Mohe regionin the formation time (late Pleistocene, Zhou et al., 2000). Conse-quently, biogenic gases are more favorable to gas hydrate formationand accumulation in Mohe region.

3.3.3. Composition and origin of hydrocarbon gasesTaking the absorbed gases from 59 drilled cores collected from

well Mk-1 in the Mohe basin as examples, the origin of hydro-carbon gases in this area is discussed. Gas chromatograph analysesof the absorbed gas from the drilled core show that it is mainlycomprised of methane(C1) with some wider range of heavierhydrocarbon(C2þ), that is about 51.53%e93.65% of methane(C1),1.51%e29.6% of ethane(C2), 0.05%e4.73% of ethylene(C2H4),0.81%e16.5% of propane(C3), 0.06%e3.82% of propylene(C2H6),0.45%e30.6% of normal butane(nC4), 0.08%e8.75% ofisobutane(iC4), 0.01%e3.8% of normal pentane(nC5), 0.01%e3.07%of isopentane(iC5) (Table 3).

Compared with some permafrost areas where gas hydrate werefound, the methane out of the absorbed gas in Mohe basin has alsoa wider range of abundance, which is less than both the AlaskaNorth Slope and Messsoyakha in West Siberia and roughly equiv-alent to or a bit higher than Qilian Mountains region in China(Table 4). In addition, Gas chromatograph analysis of dissolved gasfrom 42 spring water samples in Mohe area also shows that dis-solved gas is dominated by methane (about 78%e99%) with a littleof ethane and none of other heavier hydrocarbon gases. Themethane of water-dissolved gas in Mohe basin is a little higherthan the Canadian Shield permafrost (64%e87% of methane) wheregas hydrate may occur (Scotler et al., 2010). Therefore, as far as gascomposition is concerned, the hydrocarbon gases in theMohe basinis fully able to meet the basic requirements for gas hydrateformation.

The fact of enhanced ethane and other heavier hydrocarbon ofhydrocarbon gases in Mohe basin disclose non-biogenic origin ofthem (Liu et al., 2009). However, to further explore the origin ofthese hydrocarbon gases, we analyzed the d13CCH4 of the absorbedgases from the drilled cores and dissolved gases in the spring water.The results show that the d13CCH4 of the absorbed gases typicallyrange from - 47.7& to �22.7& with sometimes less than �55&,which indicate that thermogenic and biogenic gases coexist. Thed13CCH4 of the dissolved gas are generally between �78.9& and�64&, which apparently suggest biogenic gases. Moreover, C1 toC2 þ C3 ratios of absorbed gases is typically less than 100 and thatd13C�CH4 of those gases are larger than �50&, further indicatingthermogenic origin (Fig. 3). The C1 to C2 þ C3 ratios of dissolvedgases with d13C�CH4 usually less than �65& are frequently lowerthan 300, which indicate mixed origin of the gas (Fig. 3). All thementioned above reveal that both thermogenic and biogenic gases

ourcetratigraphy

Source rocks (%)with >0.75% TOC

Thickness instrata fill (m)

Speculated thicknessin gas source rocks (m)

0.1 0e2734 1e590.4 1945e3979 797e163177.1 799e4400 120e66064.3 836e2806 142e477

an) 84.6 (mean) 3580e13,919 1060e2773

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

Samplenumber

Methane(C1)

Ethane(C2)

Ethylene(C2H4)

Propane(C3)

Propylene(C3H6)

Normal butane(nC4)

Isobutene(iC4)

Normal pentane(nC5)

Isopentane(iC5)

#�01 77.16 8.50 2.35 4.41 1.75 1.97 1.30 1.66 0.89#�02 68.38 14.78 2.45 7.22 2.95 2.62 0.66 0.55 0.39#�03 79.31 8.43 0.84 4.61 0.78 3.04 1.84 0.58 0.57#�04 71.82 10.50 2.45 5.03 2.59 6.10 0.80 0.42 0.28#�05 64.90 15.52 1.46 10.29 1.70 3.86 0.92 0.70 0.64#�06 55.98 7.39 2.87 6.71 2.99 18.42 2.51 1.93 1.20#�07 53.34 16.56 1.17 16.50 1.53 6.19 2.11 1.19 1.42#�08 55.72 21.81 4.73 8.97 3.82 3.42 0.56 0.50 0.46#�09 64.12 10.27 e 7.97 0.11 11.82 3.62 1.13 0.96#�10 57.63 19.00 0.91 12.68 1.42 5.09 1.07 1.25 0.96#�11 62.51 19.78 1.17 8.71 1.89 4.37 0.57 0.44 0.55#�12 53.83 17.61 1.20 12.61 1.37 10.98 1.46 0.35 0.60#�13 57.76 23.40 3.37 7.31 3.80 3.16 0.43 0.45 0.32#�14 54.81 19.20 0.56 15.28 0.77 6.96 1.47 0.42 0.52#�15 78.59 10.35 1.76 4.69 2.16 1.57 0.27 0.40 0.21#�16 51.53 14.22 1.03 10.52 1.62 17.19 2.40 0.75 0.75#�17 81.97 10.19 0.80 3.32 1.09 1.48 0.25 0.59 0.33#�18 84.53 8.16 0.48 3.65 0.67 1.81 0.32 0.19 0.17#�19 61.64 21.30 1.63 7.08 2.48 4.77 0.61 0.31 0.18#�20 85.91 5.03 1.22 2.47 0.94 3.75 0.28 0.27 0.13#�21 85.79 6.26 0.78 2.85 0.68 2.87 0.42 0.20 0.16#�22 63.50 15.62 1.47 10.34 1.81 4.71 0.96 0.99 0.60#�23 64.75 16.47 3.38 7.30 2.84 3.52 0.65 0.59 0.50#�24 56.52 1.79 e 5.14 e 30.60 4.12 1.05 0.77#�25 61.07 9.53 0.92 9.53 1.24 14.34 2.53 0.38 0.46#�26 84.16 8.25 1.34 2.51 1.35 1.78 0.24 0.14 0.24#�27 87.53 4.51 0.17 4.22 0.25 2.13 0.56 0.31 0.32#�28 69.55 11.83 1.15 6.85 0.98 7.31 1.23 0.56 0.56#�29 55.01 22.46 2.15 10.74 1.92 4.74 1.15 1.04 0.78#�30 93.65 1.51 0.57 0.81 0.32 2.54 0.27 0.22 0.11#�31 76.46 4.51 1.17 3.26 0.91 11.54 1.23 0.41 0.50#�32 84.92 6.56 1.33 2.52 1.52 2.20 0.19 0.59 0.17#�33 82.15 10.91 1.19 2.80 1.40 1.00 0.14 0.31 0.11#�34 76.24 14.73 0.57 5.13 0.59 1.90 0.30 0.35 0.19#�35 90.13 5.55 0.90 1.54 1.01 0.53 0.08 0.10 0.16#�36 72.34 12.75 1.92 6.34 2.25 3.09 0.55 0.48 0.28#�37 62.53 12.87 2.09 7.60 2.06 10.13 1.51 0.72 0.48#�38 93.38 3.28 0.90 1.17 0.61 0.48 0.08 0.08 0.03#�39 79.20 8.35 0.27 4.82 0.14 4.92 1.62 0.33 0.35#�40 79.08 4.11 e 3.05 0.06 8.93 3.77 0.35 0.65#�41 83.62 7.89 0.97 2.63 0.37 3.05 0.96 0.24 0.27#�42 79.52 11.28 0.61 3.71 0.30 3.18 0.98 0.22 0.20#�43 66.63 7.33 e 8.40 e 9.36 4.41 1.97 1.89#�44 63.62 20.53 2.70 6.46 1.37 3.19 0.80 0.80 0.53#�45 58.68 2.17 e 3.52 0.84 19.18 8.75 3.80 3.07#�46 78.36 12.37 0.95 4.49 0.99 1.80 0.40 0.44 0.20#�47 77.17 11.06 0.55 4.62 0.44 3.28 1.44 0.74 0.71#�48 67.73 23.36 2.22 3.85 2.08 0.46 0.13 0.04 0.12#�49 77.60 18.34 e 3.04 e e e e 1.02#�50 70.80 19.85 e 6.45 0.98 1.26 0.21 0.18 0.27#�51 62.30 29.60 e e 1.16 1.62 1.89 1.36 1.99#�52 70.89 16.40 2.47 4.75 2.90 0.94 0.57 e 1.07#�53 66.00 20.00 0.05 9.00 0.40 3.00 0.60 0.70 0.90#�54 84.00 12.80 e e 0.90 e 1.13 e 1.14#�55 67.00 17.20 e 11.90 e 3.62 e e e

#�56 67.00 21.00 0.60 7.10 1.80 1.60 0.26 0.40 0.28#�57 81.50 10.77 e 2.42 2.35 0.93 1.07 e 0.97#�58 74.00 17.30 0.47 4.90 1.45 1.05 0.22 0.01 0.01#�59 72.32 15.96 1.21 3.16 2.86 1.29 0.91 1.23 1.07

Note: - a little.

X. Zhao et al. / Marine and Petroleum Geology 35 (2012) 166e175170

contribute to gas hydrate formation and are practically gas sourcefor potential gas hydrate formation.

3.4. Trap condition

Trap is a concept of petroleum geology and the requisite for oiland gas accumulation. In the past ten years, great importance hasbeen attached to it in the activity of gas hydrate exploration andresearch (Bunz et al., 2003; Collett et al., 2008, 2011; Boswell et al.,2011; Makogon, 2010; Scotler et al., 2010; Winters et al., 2011). This

is because, whether on the Alaska North Slope in Prudhoe Baywhere gas hydrate was detected by drilling, or in the Beaufort-Mackenzie area of north Canada where gas hydrate productiontest was conducted, gas hydrate deposits are gathered in a largeanticline crest, or uplift site (Majorowicz and Hannigan, 2000;Safronov et al., 2010).

Trap condition of Mohe basin has been unclear due to its lowerexplored, but the preliminary investigation reveals that “adepression between two uplifts” structural framework (i.e. theHeilongjiang Coast Uplift to the north, the Gulian River Uplift to the

Table 4Comparison dominated hydrocarbon gas content from the drilled cores in Mohe area to some other permafrost.

Hydrocarbon gas Mohe area Alaska North Slope(Majorowicz and Hannigan, 2000)

Messsoyakha, Siberia(Makogon, 2010)

Qilian Mountains, China(Zhu et al., 2010)

Methane (%) 51.53e93.65 91.19e99.53 98.6 54e76Ethane (%) 1.51e29.6 0.1 8e15Propane (%) 0.81e16.5 0.1 4e21

X. Zhao et al. / Marine and Petroleum Geology 35 (2012) 166e175 171

south and the Central Depression) and patterns of alternating sagsand salients on this framework likely occur. Such structures arefairly favorable to migration of hydrocarbons generated in low-lying area to the structural highs, that is, migration of hydro-carbon generated in the depressions or sags to their adjacent upliftsor salients of the basin and promotes formation of gas hydrate bytransferring of the hydrocarbon gases in the region to the near-surface permafrost.

The reservoir, an important component of traps, plays a signifi-cant role in gas hydrate accumulation, which was recognized bymost of the explorer and researchers in gas hydrate area. This isbecause both gas hydrate accumulation theory and its successfulpractice of exploration have demonstrate that gas hydrate canaccumulate to high concentrations in the sandy sediments, sand-stones (Makogon, 2010; Safronov et al., 2010; Collett et al., 2008,2011) and all the fractured rocks (Collett et al., 2011). For example,gas hydrates in Alaska North Slope are frequently limited to thesandstones and siltstones (Collett et al., 2011; Winters et al., 2011).As described above, the sandstone, conglomerate of the MiddleJurassic, particularly fractured siltstone and fine sandstone of MoheFm (Fig. 4), are abundant in Mohe basin. There should be widelydistributed reservoirs for gas hydrate accumulation throughout thisarea.

Analysis of 131 sandstone samples show that porosity andpermeability of the sandstones from Mohe region range from 0.5%to 9.1% and from 0.03md to 2.11 md respectively. Although they areinferior to reservoir of gas hydrate deposits in the other permafrost(Makogon, 2010; Safronov et al., 2010), such reservoirs won’tprevent gas and groundwater from migrating and accumulating.Furthermore, these units (such as sandstones, conglomerates andmudstones etc.) are extensively fractured, adding significant addi-tional permeability and bulk porosity to the units. Therefore, interms of gas hydrate formation, a great number of reservoirs occurthroughout the Mohe region.

Figure 3. Origin diagram on of hydrocarbon gases from Mohe basin. 1dDissolvedhydrocarbon gases in spring water; 2dabsorbed hydrocarbon gases from drilled cores.

The seal, another important part of traps, also plays an impor-tant role in gas hydrate formation. Whether it is the Beaufort-Mackenzie Basin in northern Canada (Majorowicz and Hannigan,2000), or Prudhoe Bay on the Alaska North Slope (Boswell et al.,2011), or the Qilian Mountain region in China, all of the large-scale, high saturation gas hydrate deposits are sealed by thickermudstones (>10 m). As mentioned above, Mohe Fm, usuallydistributed on the upper section in Mohe Basin (therefore close tothe permafrost), has a great amount of mudstone (>40%) that mayact as a cap rock for formation and accumulation of gas hydrates inMohe region.

In addition, the Mohe Fm includes thick section of interbededsandstone and mudstones. The upper part of the Mohe Fm, often inthe upper section of stratigraphy fill of Mohe basin, is usuallyspread in permafrost. Therefore, Mohe Fm is the best stratigraphicunit of source rocks, reservoirs and seals for gas hydrate formationand accumulation.

3.5. Migration conditions

Similar to conventional oil and gas accumulations, fluid migra-tion has an active influence on the gas hydrate formation andaccumulation. For a large-scale commercial gas hydrate deposits,fluid migration is certainly essential as the gas occurring in thickgas hydrate deposits typically exceeds what could be producedlocally (Collett et al., 2009). As result, it is essential for the forma-tion of large-scale gas hydrate deposits that external hydrocarbongases be supplied continuously by faults, fractures and permeablestrata etc. (Safronov et al., 2010; Boswell et al., 2011).

The role of non-local gas sourcing along pathways such as faultsin gas hydrate formation is demonstrated in the Gulf of Mexico, thesea to the north California, the Cascadia edge of the northernOregon (Brooks et al., 1986, 1991; Kastner et al., 1993), northernSiberia (Yakushev and Chuvilin, 2000; Safronov et al., 2010),Prudhoe Bay on the Alaska North Slope (Collett et al., 2011; Dai andLee, 2010; Lorenson et al., 2010) and the Muli region of the QilianMountains in China.

In the Mohe area, the fractures acting as potential migrationsystem is not known well, but preliminary investigation andexploration shows that both northeast and nearly east-westtrending faults and fractures are well-developed. These faults andfractures are interconnected in space and provide connectedpathways for fluid migration in this area, which is confirmed byfrequent and great loss of drilling mud fluid during drilling in thisarea.

3.6. Groundwater salinity

An effect of salinity on the formation and accumulation of gashydrate is well established, however, recent studies have shownthat, when the salinity is less than 4�10�2, it has little effect on theformation and stability of gas hydrate (Husebø et al., 2009). Ourdata show that the salinity of groundwater in northern Hei-longjiang Province is generally less than 1 � 10�3. In addition,analyses of three spring water samples from Mohe region showgroundwater salinity (Cl-ion concentration) of w2.94e17.6 � 10�6

Figure 4. Fractures in sandstones in Mohe basin.

X. Zhao et al. / Marine and Petroleum Geology 35 (2012) 166e175172

and higher than the North Slope’s Prudhoe Bay, Alaska(<1.0e19 � 10�12) (Collett et al., 2011), but lower than Messoya-kha, Siberia (�1.5 � 10�2) (Makogon, 2010). Salinity throughoutMohe area therefore has little effect on gas hydrate formation andcan be ignored.

4. Indications of gas hydrate occurrence

4.1. Authigenic calcite and pyrite

Examination of cores recovered from the Mohe region reveallarge numbers of calcite veins and epigenetic pyrite that occurswithin fissures (Fig. 5), in intergranular and intragranular pores insandstones, and within fractures in individual clastic grains (Fig. 6).These features are indicative of the strong underground fluxes ofhydrocarbon activity that, in the presence of anaerobic bacteria, canlead to the following chemical reactions:

CH4 þ SO42� / HS� þ HCO3

� þ H2O

HCO3� anion reacts with Ca2þ anion as follows:

HCO3� þ Ca2þ / Ca(HCO3)2

Ca(HCO3)2 / CaCO3Y þ CO2[ þ H2O

Here, CaCO3 precipitate is the calcite filled the fractures in therocks. Meantime, in the reduction circumstances caused by oxida-tion of hydrocarbons, sulfur and iron ions react to product pyrite asfollows:

HS� þ Fe2þ / FeS2 (pyrite)Y þ Hþ

We infer that epigenetic calcite and its associated pyrite, whichis characteristic of common secondary alteration above oil and gasaccumulations, indicate the intense activity of the underground

hydrocarbon fluid. Authigenic pyrite and calcite have been reportedin association with gas-hydrate-bearing sediments both in seaflooror permafrost settings (Mazzini et al., 2004; Sassen et al., 2004;Rose et al., 2011). Combined with gas hydrate formation conditiondiscussed above, these authigenic minerals can be indicative of theformation of potential gas hydrate.

4.2. Dissolved methane

The dissolved methane from some spring water in Mohe areais as high as 14,000 ppm or more. During hydrogeological drillingwithin the Mohe basin in the winter of 2010, a continuous flamewas unexpectedly produced when the wellhead was heated witha torch in order to thaw the frozen drill stem, then the flamecaused the surface of mud pools to catch fire though the mudflute and keep on burning while mud liquid from the well wassupplied continuously. We have determined that the fuel for thisfire was mainly composed of methane (>90%) and suspect thata source of this methane may have been dissociation of gashydrate.

5. Discussion

In Mohe area, there are the development of the petroleumsystems elements that are consistent with gas hydrate formation,including thick permafrost, necessary temperature-pressureconditions and geothermal gradient, abundant gas sources,migration conduits in the form of faults and fractures, and gashydrate-hosted traps composed of reservoirs and seals. Theconditions in the Mohe area are noted to be very similar to otherareas where permafrost-associated gas hydrate has been confirmedby drilling and observation.

Thousands meters of dark mudstone from the Middle Jurassicin Mohe basin provides the significant parent materials forhydrocarbon gases (including both thermogenic and biogenicgases) required for the formation of gas hydrate. A great amount

Figure 5. Calcite fills in the cores drilled in Mohe basin.

X. Zhao et al. / Marine and Petroleum Geology 35 (2012) 166e175 173

of dark mudstone in this region, of which total organic carbon ismore than 0.75%, kerogen is dominantly type III, and vitrinitereflectance is frequently more than 0.90%, can yield large volumesof thermogenic gases requisite for gas hydrate formation. Thetype II lacustrine organic matter in dark mudstone is the betterparent materials of biogenic gases and can produce quite a little ofbiogenic gases for hydrate formation. The huge amount ofhydrocarbon gases generated from the Middle Jurassic in Mohebasin can form a great deal of gas hydrate. According to

preliminary calculations, the middle Jurassic in Mohe basin cangenerate about 204.66 � 1012 m3 hydrocarbon gases for gashydrate formation, of which thermogenic gases amounts to about52.9 � 1012 m3, biogenic gases is 151.76 � 1012 m3 or so. Especially,biogenic gases of up to three quarters of hydrocarbon gases mayroughly be in accordance with the permafrost of Mohe region inthe formation time (late Pleistocene (Zhou et al., 2000)) so thatthey are quite conducive to the formation and accumulation of gashydrate.

Figure 6. Pyrite fills in the cores in Mohe basin. AdPyrite filled in intergranular and intragranular pores and fissures; Bdpyrite filled in intragranular pores and fissures; Cdpyritefilled in intergranular pores; Ddpyrite filled in intergranular and intragranular pores.

X. Zhao et al. / Marine and Petroleum Geology 35 (2012) 166e175174

The tectonic history of the region is helpful to make the trapsrequisite for gas hydrate formation. The uplifts to south and northrespectively and the secondary salients within them in Mohe basincreate the traps for gas hydrate formation. Within the traps, thewidely -spreading sandstones, sandy conglomerates and fracturedrocks from all the section of strata, especially fined-grained sand-stones, siltstones in Mohe Fm, are the common reservoirs for gashydrate formation, interbedded mudstones in Mohe Fm acts as theseals for gas hydrate accumulation.

The great amount of hydrocarbon gases, the richer heavyhydrocarbon gases and the lower groundwater salinity in Moheregion are favorable to the formation of gas hydrates. The fire at thewellhead of drilled wells, the adequacy of hydrocarbon gases in theregion, the richness of heavy hydrocarbon gases in hydrocarbon

gases, and groundwater salinity as low as about 10 ppm areconducive to the formation of gas hydrates.

Some signs show the occurrence of gas hydrate in Mohe region.The epigenetic calcite and pyrite in drilled cores which are similarto gas hydrate-discovered region such as the Gulf of Mexico, theLake Baikal and Alaskan North Slope, and burning on the surface ofmud pool suggest the occurrence of gas hydrate.

Overall, the temperature and pressure conditions, the richnessof hydrocarbon gases and its parent materials, the gas compositionand the groundwater salinity, the traps required for the formationand accumulation of gas hydrate, the presence of reservoirs andseal, the huge thickness of dark mudstone, the richness of organicmatter, suggest that the northwest Mohe basin is extremelyfavorable for the accumulation of gas hydrate.

X. Zhao et al. / Marine and Petroleum Geology 35 (2012) 166e175 175

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