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A natural gas hydrate-oil-gas system in the Qilian Mountain permafrost area, northeast of Qinghai-Tibet Plateau
Zhen-quan Lu, Shi-qi Tang, Xiao-ling Luo, Gang-yi Zhai, Dong-wen Fan, Hui Liu, Ting Wang, You-hai Zhu, Rui Xiao
Citation: Zhen-quan Lu, Shi-qi Tang, Xiao-ling Luo, Gang-yi Zhai, Dong-wen Fan, Hui Liu, Ting Wang, You-hai Zhu, RuiXiao, 2020. A natural gas hydrate-oil-gas system in the Qilian Mountain permafrost area, northeast of Qinghai-Tibet Plateau, ChinaGeology, 3, 511-523. doi: 10.31035/cg2020075.
View online: https://doi.org/10.31035/cg2020075
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A natural gas hydrate-oil-gas system in the Qilian Mountain permafrost area, northeastof Qinghai-Tibet PlateauZhen-quan Lua,*, Shi-qi Tangb, Xiao-ling Luoc, Gang-yi Zhaia, Dong-wen Fana, Hui Liua, Ting Wanga,You-hai Zhua, Rui Xiaoa a Oil and Gas Resources Survey, China Geological Survey, Ministry of Natural Resources, Beijing 100083, Chinab Institute of Geophysical and Geochemical Exploration, Chinese Academy of Geological Sciences, Langfang 065000, Chinac Development and Research Center, China Geological Survey, Ministry of Natural Resources, Beijing 100037, China
A R T I C L E I N F O A B S T R A C T
Article history:Received 2 December 2020Received in revised form 18 December 2020Accepted 19 December 2020Available online 22 December 2020
Keywords:Natural gas hydrate-oil and gas systemHydrocarbon gas-generation seriesGas-fluid migration seriesHydrocarbon gas-accumulation seriesQilian Mountain permafrostSouthwest China
Natural gas hydrate, oil and gas were all found together in the Qilian Mountain permafrost area, northeastof Qinghai-Tibet Plateau, China. They are closely associated with each other in space, but whether theyare in any genetic relations are unknown yet. In this paper, a hydrocarbon gas-generation series, gas-fluidmigration series and hydrocarbon gas-accumulation series are analyzed to probe the spatial, temporal andgenetic relationships among natural gas hydrate, oil and gas. The subsequent results show that natural gashydrate, oil and gas actually form a natural gas hydrate-oil-gas system. Based on the Middle Jurassic andthe Upper Triassic hydrocarbon gas-generation series, it is divided into four major sub-systems in thestudy area: (1) A conventional Upper Triassic gas-bearing sub-system with peak hydrocarbon gas-generation in the late Middle Jurassic; (2) a conventional Middle Jurassic oil-bearing sub-system with lowto mature hydrocarbon gas-generation in the late Middle Jurassic; (3) a natural gas hydrate sub-systemwith main gas source from the Upper Triassic gas-bearing sub-system and minor gas source from theMiddle Jurassic oil-bearing sub-system as well as little gas source from the Middle Jurassic coal-bed gasand the microbial gas; (4) a shallower gas sub-system with microbial alteration of the main gas sourcefrom the Upper Triassic gas-bearing sub-system. This natural gas hydrate-oil-gas system and its sub-systems are not only theoretical but also practical, and thus they will play an important role in the furtherexploration of natural gas hydrate, oil and gas, even other energy resources in the study area.
©2020 China Geology Editorial Office.
1. Introduction
Natural gas hydrate or hydrate, is commonly known as“flammable ice”. It is composed of water and light-weightedgas molecules (such as methane, ethane, propane, isobutane,hydrogen sulfide, carbon dioxide, etc.), a kind of crystallinesolid substance formed with sufficient gases at lowtemperature (generally around 273.15 K) and high pressure(generally greater than 3 MPa to 5 MPa) (Sloan ED and KohCA, 2008). In nature, hydrates are usually distributed insubsurface sediments with a water depth of greater than 300 m(Kvenvolden KA, 1993) or in permafrost with a stratum depthof greater than 130 m below the surface (Shi D and Zheng
JW, 1999). In addition to drilled hydrates (Wu NY et al.,2009) even with test-production on the northern slope of theSouth China Sea (Li JF et al., 2018; Ye JL et al., 2018, 2020),China also collected hydrate samples by drilling for the firsttime in the Qilian Mountain permafrost, achieving abreakthrough in the investigation of hydrates in permafrostregions (Zhu YH et al., 2009; Lu ZQ et al., 2011). This is thefirst time in the world that hydrate has been discovered in themid-latitude permafrost.
Besides hydrates, various oil and gas indications (oilspots, oil immersions, etc.) associated phenomena arecommon in the Qilian Mountain permafrost, e.g. in drillingholes of DK-1, DK-2, DK-3, DK-7, DK-8, DK-9, DK-12, etc.(Lu ZQ et al., 2018, 2013a, 2013b, 2015a; Cheng B, 2018;Tang SQ, 2015a, 2020) and DK8-19, DK10-16, DK10-17,DK10-18, DK11-14, DK12-13, DK13-11, etc. (Wen HJ et al.,2015; Lu ZQ et al., 2015b; Wang WC et al., 2015; Li YH et
* Corresponding author: E-mail address: [email protected] (Zhen-quan Lu).
doi:10.31035/cg20200752096-5192/© 2020 China Geology Editorial Office.
China Geology 4 (2020) 511−523
China GeologyJournal homepage: http://chinageology.cgs.cn
https://www.sciencedirect.com/journal/china-geology
al., 2015; Tang SQ et al., 2015b). In particular, in DK-9borehole, apart from the thick layered hydrates, thick oil-bearing layers were encountered, and in DK-10 borehole anabnormally shallow natural gas layer was also encountered.This is the first time in the South Qilian Basin to obtain the oiland gas bearing layer by drilling (Lu ZQ et al., 2018). On theone hand, this new discovery indicates that the QilianMountain permafrost contains a good potential of hydrateresources and has a good prospect for oil and gas exploration,which can provide a new national backup option for hydrateand oil and gas resource security (Wu CG et al., 2011); on theother hand, it also poses a new important scientific questionwhether there is an inherent genetic link between hydrate andoil-bearing layer, abnormal gas layer or various oil and gasindications in the Qilian Mountain permafrost.
In the past, a detailed analysis of key elements wasconducted in the world ’s typical marine hydrateaccumulations, such as in the Blake Ridge offshore the UnitedStates, the Hydrate Ridge offshore Canada, the northern slopeof the Gulf of Mexico, and the Storegga landslide offshoreNorway and it revealed that they all developed a goodcombination of hydrocarbon gas-generation series, gas-fluidmigration series, and hydrocarbon gas-accumulation series,which together reflected the geological process and geologicalelements from the formation to preservation of hydrates. Thecombination constitutes a natural gas hydrate geologicalsystem (Lu ZQ et al., 2008). Collett TS et al. (2009, 2011),Max MD and Johnson AH (2014) published articles thatclearly put forward the concept of “natural gas hydrate andpetroleum system ”, and made a keynote report at the 8th
International Gas Hydrate Conference to further clarify thatthe geological control over hydrate occurrences was referredto as “natural gas hydrate and petroleum system” (Collett TS,2014). Since then, the natural gas hydrate geological systemor natural gas hydrate and petroleum system has graduallygained attention and application in practice, such as the“natural gas hydrate system” in the Krishna-Godavari basinoffshore India (Riedel M et al., 2010; Shankar U and RiedelM, 2010; Badesab FP et al., 2017) or “natural gas hydratepetroleum system” (Vedachalam N et al., 2016), “natural gashydrate system” in the southwest of the Barents Sea (Rajan Aet al., 2013; Vadakkepuliyambatta S et al., 2015), “natural gashydrate system ” in the Niger Delta Basin off Nigeria(Akinsanpe OT et al., 2017) or “natural gas hydrate petroleumsystem” offshore Angola (Nyamapfumba M and McMechanGA, 2012), “natural gas hydrate system ” in Black Sea(Hillman JIT et al., 2018), “natural gas hydrate system ”offshore Northwest Taiwan in the northern South China Sea(Han WC et al., 2019) or “natural gas hydrate system” (ShaZB et al., 2015; Liang YX et al., 2013) and its related“seepage system ” in Dongsha waters (Wu SG et al., 2010;Zhang W et al., 2018).
In this study, the internal connections between hydrate andoil-bearing layer, shallow gas layer are revealed by theanalysis of the sources of hydrate, oil-bearing layer, andshallow gas layer in the Qilian Mountain permafrost under the
natural gas hydrate geological system theory. For the firsttime, different natural gas hydrate-oil and gas subsystemshave been established in this area, and the understanding ofnatural gas hydrate accumulation rules are gradually improvedin the Qilian Mountain permafrost, which provides a newtheoretical guidance for hydrate and oil and gas investigationsin permafrost regions in China.
2. Geological background
It is dominated by mountainous permafrost in the QilianMountain permafrost, covering an area of about 100×103 km2.The permafrost thickness is generally 60–95 m. It is mainlydistributed in the middle and western sections of the QilianMountain. The lower boundary of the permafrost is roughlythe same as the isotherm equivalent of annual averagetemperature –2 − –2.5 °C (Zhou YW et al., 2000).
The study area is located in Muli Town, Tianjun County,Qinghai Province, in the third mining pit of Juhugeng CoalMine area in Muli Coal Field, with an altitude of 4026–4128 m.In tectonics, it is in the western section of the Central QilianBlock formed during the Caledonian tectonic movementperiod (513–386 Ma) (Wen HJ et al., 2006), adjacent to theSouth Qilian structural belt, belonging to the Muli Depressionof the South Qilian Basin. During the Yanshan Movement,the stratum in the study area was strongly uplifted, and theolder fault blocks in the flanking strata gradually rose. Itsmain fault properties changed from tension to compression,resulting in a series of secondary fault structures.Experiencing a series of the tectonic evolution it eventuallyformed the present-day thrust-nappe structure framework withNWW and NW (Guo JN et al., 2011). As a result of thetectonic movement and evolution, the central part of the studyarea is composed of an anticline consisting of Triassic, andthe north and south sides are individually syncline consistingof Jurassic coal-bearing strata. Along the north and southsides of the anticline-syncline complex, large scaled thrust-nappe faults develop, controlling the boundaries of theexisting depression. In the north and south synclines a set oflarge-scale northeast-trending shear faults develop, which cutsthe depression into intermittent blocks with different size.These faults become the natural boundary that divides themining pits in the Juhugeng coal mine area, making the studyarea present a structural feature of various north-south zonesand east-west blocks (Fig. 1).
In addition to the Quaternary, drilling results reveal thatthe exposed strata in the study area also include the MiddleJurassic and part of the Upper Triassic. The Upper Triassic iswidely exposed in the north and south of the study area. Thelithology is dominated by black mudstone and siltstone withthin coal seams, and is in angular unconformity contact withthe overlying Jurassic. The Middle Jurassic can be dividedinto Muli Formation (J2m) and Jiangcang Formation (J2j)from bottom to top. The Muli Formation can be subdividedinto upper and lower lithological sections: The lower sectionis mainly braided-river sediments, dominated by gray-white
512 Lu et al. / China Geology 4 (2020) 511−523
medium-coarse-grained sandstone, developed with bottom-conglomerate; the upper section is the main coal-bearingsection with two sets of exploitable coal layers and locallywith thin coal seams, and is mainly gray fine-medium-grainedsandstone and dark gray fine-grained to siltstone in lake-marsh environment. The Jiangcang Formation (Fm.) can alsobe subdivided into two lithological sections: The lowersection is composed of dark gray mudstone, siltstone and grayfine sandstone in a delta-lake environment, and containsmultiple thin coal seams; the upper section is composed ofdeposits in shallow lakes and semi-deep lakes, and darkmudstone and gray-black oil shale are developed.
3. Natural gas hydrate and oil and gas display characteristics
3.1. Natural gas hydrate characteristics
Since 2008, the China Geological Survey has deployed 12hydrate drilling holes in the study area, including DK-1, DK-2, DK-3, DK-7, DK-8, DK-9, DK-12, etc. Hydrate wasencountered in these boreholes. In addition, Shenhua Groupalso implemented 14 hydrate boreholes in the study area andhydrate was also obtained by drilling in DK8-19, DK11-14,DK12-13, DK13-11 and other boreholes. Among them, inDK-9 borehole, hydrate has the most obvious characteristicsand the largest thickness.
The main characteristics of hydrate observed in the field
include (Fig. 2): (1) White and milky white aggregates arefound on the fracture surface of the hydrate-bearing core; (2) thehydrate-bearing core can be directly burned upon ignition; (3) thetemperature of the hydrate-bearing core is measured byinfrared thermal imaging camera and then obvious lowtemperature anomalies are shown; (4) bubbles and waterdroplets can continuously emerge on the surface of thehydrate-bearing core; (5) the hydrate-bearing corecontinuously bubbles in the water; (6) abnormal gas is oftenencountered in the hydrate-bearing section during drilling andwhen the sample is put into the gas tank, a large amount ofgas can be resolved; (7) the honeycomb structure remains onthe surface of the hydrate-bearing core after being placed for acertain time; (8) the authigenic carbonate crystals and pyriteparticles are associated on the fracture surface of the hydrate-bearing core (Lu ZQ et al., 2010).
In many boreholes in the study area the drilling resultsrevealed that hydrate mainly occurs at a shallow depth of 400m or less. It is visible with the naked eyes as a thin white ice-like layer (smoky gray when mixed with mud) in fissures ofstrata or occurs as a fine disseminated state in pores of rockformations. The reservoir lithology is mainly mudstone, oilshale, siltstone, fine sandstone, etc. On the contrary, hydrate israrely seen in medium sandstone and coarse sandstone (LuZQ et al., 2010; Zhu YH et al., 2010; Wang WC et al., 2015).
3.2. Oil and gas display characteristics
Preliminary statistics showed that oil and gas displays
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Fig. 1. Geological characteristics and borehole locations in the study area.
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were encountered in 15 hydrate drilling holes such as in DK-1, DK-2, DK-3, DK-4, DK-5, DK-8, DK-9, DK-10, DK8-19,DK10-16, DK10-17, DK10-18, DK11-14, DK12-13, DK13-11 etc. in the study area. In particular, a nearly 8 m thick oil-bearing layer was encountered in DK-9 borehole. The oil-bearing layer is within 362.79 –370.58 m with the middle-sized sandstone of the Middle Jurassic. On the surface of theoil-bearing core it can be seen to be grayish brown to lightbrown, showing a large area of oil immersion or oil staining(Fig. 3). On-site gas logging shows that the content of totalhydrocarbons and that of methane have increasedsignificantly, with 2.19% –8.35% and 1.01% –5.62%respectively, while the content of total hydrocarbons and thatof methane in non-oil-bearing layers are only 0.10%–0.40%and 0.035%–0.225%.
In the meantime, in the hydrate scientific drilling test holeDK-10 in the study area, when drilled to a siltstone interval ata 52.9 m depth, a strong gas eruption occurred. The height ofthe burning flame exceeded 10 m. When the erupted gas wasintroduced to the area about 200 m away from the well
through a simple pipeline and was ignited, and gases burnedviolently. The flame height arrived at 3 –4 m (Fig. 4).According to on-site estimates, the flow rate of high-pressureabnormal gas encountered during drilling is greater than4800 m3/d. Due to the abnormally high-pressure gas layer,DK-10 borehole was finished and sealed. The final hole depthis 52.9 m. The main formation lithology is: Quaternary gravelsand at 0 –6.1 m, gray siltstone partially mixed with coal at6.1 –10.2 m, coal inter-bedded with silt-bearing mudstone,carbonaceous mudstone at 10.2 –37.21 m, and inter-beds ofargillaceous siltstone, fine sandstone, silty mudstone at37.21 –52.9 m. According to the characteristics of lithologycombination and the stratigraphic comparison amongboreholes, apart from Quaternary gravel-bearing sand layer,the strata encountered by the borehole drilling are coal-bearing strata of the upper part of Muli Formation (to thedepth of 37.21 m) in the shallower part, and it is the Triassicsiltstone and argillaceous siltstone of Galedesi Formation inthe lower part.
Fig. 2. Characteristics of natural gas hydrate collected from drilling holes in the study area.
Fig. 3. Characteristics of oil-bearing cores in DK-9 in the study area.
514 Lu et al. / China Geology 4 (2020) 511−523
4. Sources of natural gas hydrate and oil and gas
4.1. Source of natural gas hydrate
Studies have shown that the gas from hydrate itself ismainly light hydrocarbons in the study area, with thecharacteristics of moisture gas, and its isotopes arecharacterized by a series of positive carbon isotopes,indicating that the gas of hydrate is of organic origin, and ismainly sourced by pyrolysis, with a small amount ofmicroorganisms. Among them, the pyrolysis sourced gas ismainly related to crude-oil cracked gas and crude-oilassociated gas, and a small part is related to condensate-oilassociated gas, coal-derived gas, and kerogen cracked gas.The organic geochemical analysis of mudstone, oil shale, andcoal in the hydrate layer in the study area shows that themudstone, oil shale, and coal in the hydrate layer cannot bethe main gas source rock for hydrate by the abundance, type,thermal evolution degree and other parameters of organicmatter, indicating the source for gas of hydrate may be mainlyfrom deep oil or crude oil-associated gas or mature/over-mature gas from deep gas source rock formations.
With the help of thermal simulation experiment methods,cores such as mudstone, oil shale and coal are select toconduct thermal simulation experiments. Under thermalsimulation conditions, the composition and carbon isotopecomposition of newly produced hydrocarbon gases will befurther analyzed. Their gas composition and isotopiccharacteristics are compared with those of hydrate to explorethe source for gas of hydrate. The results show that under thecondition of low temperature below 300°C, the gas productsare mainly non-hydrocarbon CO2, and the content ofhydrocarbon gases is small. The amount of hydrocarbon gas-produced in mudstone is less than the amount of thatproduced in oil shale. And the latter is less than that producedin coal. As the thermal simulation temperature increases, theamount of produced hydrocarbon gases increasessignificantly, reaching the highest at 500°C; on the contrary,
the amount of CO2 gas production does not change much. Asthe temperature of the thermal simulation increases, thecarbon isotopes of the hydrocarbon gas-produced inmudstone, oil shale, and coal show the characteristics offirstly become lighter and then become heavier, indicating apositive carbon isotope sequence like δ13C1<δ13C2<δ13C3.
The hydrocarbon gas-composition, carbon isotopecomposition produced by thermal simulation are comparedwith gas composition and isotopic characteristics of gas fromhydrate. The results show that the gas composition and carbonisotope composition of the hydrocarbon gas-produced bymudstone at 350–400°C or oil shale at 380–400°C are similarto those of gas from hydrate (Figs. 5, 6). It is speculated thatthe gas source for hydrate corresponds to the deep mudstoneor oil shale according to the equivalent thermal simulationtemperature. It has a geochemical relationship. On thecontrary, although the hydrocarbon gas-produced in coal issimilar in composition to that of gas in hydrate, the isotopiccomposition of them is quite different. It is deemed that thesource for gas of hydrate is not much related to coal (Xue XHet al., 2013; Lu ZQ et al., 2013c). This conclusion is basicallyconsistent with the view of other scholars (Zhai GY et al.,2014).
4.2. Source of oil-bearing layer
DK-9 hole is taken as an example to compareconventional source rock with oil source and to comparethermal simulation product with oil source. The MiddleJurassic and Upper Triassic low-maturity source rock samplesare selected to conduct thermal simulation experiments at fivetemperature points of 300°C, 350°C, 390°C, 410°C and 460°Cto simulate the process of hydrocarbon gas-generation andexpulsion from deep source rocks. The newly-producedhydrocarbon gases are then compared with the oil and gasdisplay by their components to further explore the oil and gasdisplay sources.
Conventional oil source analysis shows that the oil andgas in this area can be divided into two categories. Type I oiland gas may suffer from bio-degradation and have a slightlyhigher maturity, while Type II oil and gas have a slightlylower maturity (Fig. 7); source rocks are mainly divided intothree classes, corresponding to depths of 163.30 –207.42 m,207.42 –348.50 m, 357.90 –586.50 m. Comparison ofconventional oil sources shows that Type I oil and gas arehomologous to Type I source rocks, and Type II oil and gasmay be mainly homologous to Type II source rocks, and mayalso be similar to or related to Type III source rocks or deepersources.
Thermal simulation experiments show that the bio-markerparameters from the Middle Jurassic thermal simulationsamples begin to coincide with those of Type II oil and gasfrom 390°C. It is inferred that the liquid hydrocarbonproduced by the Middle Jurassic source rocks above 390°C
Fig. 4. Blowout of gases from the abnormal gas layer in DK-10 inthe study area.
Lu et al. / China Geology 4 (2020) 511−523 515
are equivalent to Type II oil and gas (Fig. 8). At the sametime, starting from 410°C, the thermal simulation biomarker
parameters of the Upper Triassic samples began to coincidewith the parameters of those of Type II oil and gas. It is
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Fig. 6. Comparison of carbon isotopes between natural gas hydrate and thermal simulation products.
516 Lu et al. / China Geology 4 (2020) 511−523
inferred that the liquid hydrocarbon produced by the UpperTriassic samples above 410°C are equivalent to Type II oiland gas (Fig. 9).
Combining conventional oil source comparison, thermalsimulation experiments and geological conditions analysis, itis speculated that Type I oil and gas are mainly homologousto Type I source rocks; Type II oil and gas are mainlyhomologous to Type II source rocks, and there may also be
some contribution from Type III source rocks or deepersource rocks. Namely the parent material sources of Type IIoil and gas are related to both the Middle Jurassic sourcerocks and the Upper Triassic source rocks (Lu ZQ et al.,2015a; Tang SQ et al., 2015a).
4.3. Source of shallow gas layer
It was encountered with abnormally high pressure shallow
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Fig. 7. Characteristics of normal alkanes (a), terpanes (b), and steranes (c) from oil samples in DK-9 in the study area. Pr –Pristine;Ph–Phytane; Ts–22, 29, 30-Trisnorneohopane-II; Tm–22, 29, 30-Trisnorneohopane.
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Fig. 8. Contrast of oil and gas displays with thermal simulation products from the Middle Jurassic samples in DK-9 in the study area.1–Ts/(Ts+Tm); 2–C30RH/C29(H+Ts); 3–C29(H+Ts)/C30H; 4–C27D/C27-29-St; 5–C27αααR/C27-St; 6–C28αααR/C28-St; 7–C29αααR/C29-St; 8–C29-S/(S+R); 9–C29-ββ/(αα+ββ).
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gas at 52.9 m in the DK-10 borehole in the study area.Although in the upper coal-bearing formation, gas logging hasa total hydrocarbon and methane value of about 3%, the high-pressure abnormal shallow gas encountered in drilling can beexcluded from the influence of coal-bed methane because thesurface casing with a diameter of 146 mm is put down tillabout 41 m. And the possibility of direct decomposition ofhydrate can also be excluded within this depth range.
According to on-site mud gas logging, the totalhydrocarbon content of the drilling mud gas is about 60% inDK-10 in the study area. The hydrocarbon gas-component ismainly methane, and its content is about 60%. The ethanecontent is about 0.25%, and the other components are less
than 0.01% (Table 1), showing the composition characteristicsof pyrolysis gas.
The collected shallow gas samples were further sent to thelaboratory for testing, and the results showed that the gassamples contained lots of nitrogen and oxygen (Table 2),which was caused by the inevitable mixing of a certainamount of air during the collection process. According to thegas composition and isotopic characteristics, the gas fromDK-10 borehole seemingly shows the properties of biogenicgas (Fig. 10). Considering that the sample mixed with acertain amount of air may affect the results of hydrocarbongas-analysis and testing, it is speculated that the abnormallyhigh pressure shallow gas may be due to the microbial
Table 1. Gas composition recorded by gas logging in DK-10 in the study area (%).Depth/m Total HC CH4 C2H6 C3H8 iC4H10 nC4H10 iC5H12 nC5H12 CO2
50 57.27 55.99 0.25 0.004 0.0004 0 0 0 0.4451 63.15 62.02 0.25 0.005 0.0006 0.0002 0.0002 0.0001 0.2052 52.23 51.61 0.22 0.004 0.0007 0.0005 0.0004 0.0004 0.21
Table 2. Gas constituents (V-%) and isotopes (V-‰) of shallow gas in DK-10 in the study area.
Sample No. N2 O2 CO2 Ar He CH4 C2H6 C3H8 δ13C-C1 (V-PDB) δD-C1 (V-SMOW)DK10-K-01 26.66 2.54 1.43 0.32 0.12 68.81 0.12 0.00 –61.7 –248.6DK10-K-02 30.89 0.76 2.40 0.36 0.11 65.37 0.11 0.00 –60.1 –244.6DK10-K-03 18.35 0.51 1.03 0.21 0.14 79.62 0.14 0.00 –61.7 –247.3DK10-K-04 35.79 6.56 0.94 0.43 0.09 56.09 0.10 0.00 –61.6 –247.4DK10-K-05 33.17 5.41 0.71 0.39 0.10 60.12 0.10 0.00 –60.6 –248.1DK10-K-06 17.07 0.77 0.92 0.20 0.14 80.76 0.14 0.00 –61.6 –247.9DK10-K-07 36.12 1.18 2.74 0.46 0.03 59.47 0.00 0.00 –59.8 –240.0DK10-K-08 27.08 0.58 2.46 0.36 0.04 69.35 0.14 0.00 –60.5 –245.4DK10-K-09 30.36 1.08 2.48 0.39 0.04 65.53 0.12 0.00 –60.5 –242.5DK10-K-10 16.55 0.98 1.37 0.22 0.05 80.68 0.15 0.00 –60.9 –247.7Notes: Analyzed and tested by the laboratory of Lanzhou Oil and Gas Resources Research Center, Chinese Academy of Sciences.
DK9-M-53
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Fig. 9. Contrast of oil and gas displays with thermal simulation products from the Upper Triassic samples in DK-9 in the study.1–Ts/(Ts+Tm); 2–C30RH/C29(H+Ts); 3–C29(H+Ts)/C30H; 4–C27D/C27-29-St; 5–C27αααR/C27-St; 6–C28αααR/C28-St; 7–C29αααR/C29-St; 8–C29-S/(S+R); 9–C29-ββ/(αα+ββ).
518 Lu et al. / China Geology 4 (2020) 511−523
transformation from the lower or deep part during thepyrolytic gas migration to the shallow part.
5. Natural gas hydrate-oil and gas system
5.1. Natural gas hydrate and hydrocarbon gas-generationserials
According to the analyses of hydrate and oil and gassources, although they have very few microbial sources oforganic matter, they are mainly sources of pyrolysis of sourcerocks. Previous studies on the hydrocarbon gas-generationpotential of the South Qilian Basin have shown that there arefour main sets of potential source rocks in the study area,namely the Middle Jurassic coal-bearing strata, the UpperTriassic mudstone of Galedsi Formation, the Lower Permianlimestone of Caodigou Formation and the Carboniferousmudstone or limestone (Fu JH and Zhou LF, 1998, 2000). Thefour sets of source rocks have relatively high abundance oforganic matter. The organic matter types are mainly Type II2and Type III, and the degree of organic matter evolution isgenerally mature or over-mature with some in immature stage.They have good hydrocarbon generation potentials (Hao ASet al., 2016; Tang SQ et al., 2015c; Ren YJ and Ji YL, 2000;Xie QF et al., 2011, 2015; Cheng QS et al., 2016; Zhang JZ etal., 2017; Gong WQ et al., 2013).
Based on analyses of current geothermal fieldcharacteristics, and basin thermal history, sedimentary burialhistory, tectonic subsidence history, source rock maturityevolution history, hydrocarbon gas-generation and expulsionhistory, etc., the source rocks of the Middle Jurassic and theUpper Triassic Galedesi Formation were simulated in thestudy area. The results showed that the thermal evolution ofthe source rocks was controlled by the paleo-temperature fieldand reached the maximum in the late Middle Jurassic. TheMuli and Jiangcang Formations of the Middle Jurassicexperienced one stage of hydrocarbon gas-generation andexpulsion in the late Middle Jurassic, and the degree of
maturity was from immature to mid-mature stage, mainly withoil generation and very little gas production. The UpperTriassic Galedesi Formation experienced two stages ofhydrocarbon gas-generation and expulsion, respectively in theLate Triassic and the Late Middle Jurassic. However, thesource rock did not experience the peak of hydrocarbon gas-generation in the first stage, and the most of the source rockexperienced a peak of hydrocarbon gas-generation in thesecond stage. The middle and lower parts of source rockreached the gas generation stage, and the gas generation wasdominant on the whole (Zuo YH et al., 2016).
5.2. Natural gas hydrate and oil and gas migration serials
According to the regional geological structure evolutiondata, the NW-SE thrust fault was formed from the late MiddleJurassic to the early Early Cretaceous and is the mostimportant fault in the study area. The spatial mutual cuttingrelationship of these different faults indicates that the F1 andF2 faults were formed at the relatively late stage of faulting inthis period. Judging from the current structural framework ofthe study area, the Himalayan tectonic activities have aninherited influence on the F1 and F2 faults in this area.Therefore, the thrust nappe faults of the F1 and F2 faultsformed in the Yanshanian period in the study area have thecharacteristics of continuous compression, which can beserved as a good blocking and sealing effect on the liquid andgaseous hydrocarbons migrated from the deep part, togetherwith the Middle Jurassic mudstone or shale. These faults arebeneficial to the final formation of hydrate and theaccumulation of oil and gas. They are the main control faultsfor hydrate and oil and gas display in the study area.
The drilling data in the study area show that the hydratedistribution is closely related to the F1 and F2 faults not onlyon the plane but also on the borehole profile, and isparticularly obviously controlled by the F2 fault. For example,the hydrate intervals are mainly distributed in the foot wall ofthe F2 fault. Hydrate is more obviously controlled by the foot
−80 −70 −60 −50 −40 −30 −20 −10 0
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Fig. 10. Diagram of δ13C1 vs. C1/(C2+C3) (a) and δ13 vs.δ13 (b) in DK-10 in the study area. I1‒microbial; I2‒mixture of microbial andsub-mic rob ia l ; I 3‒sub-microbial; II1‒crude oil associated; II2‒oil-typed cracking; III1‒mixture of oil-typed cracking and coal-generagted;I I I 2‒mixture of condensate-oil associated and coal-generated; IV‒coal-generated; V1‒inorganic; V2‒mixture of inorganic and coal-generated.A‒biogenic; B‒transtion from microbial to thermal catalyzed; C‒oil associated; D‒condensate-oil associated; E‒coal-typed; F‒marine-phasetransitional.
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wall of the F1 and F2 thrust faults (Lu ZQ et al., 2015c, d).The analysis and test results of the headspace of borehole
cores in the study area also show that methane in theheadspace components is often accompanied by heavyhydrocarbon components such as ethane, propane, isobutane,n-butane, isopentane, and n-pentane. The appearance ofbutane often indicates the leakage and diffusion of deep gas,indicating that hydrocarbon gases are characteristic of deepmigration (Han WC et al., 2019; Sha ZB et al., 2015; LiangYX et al., 2013; Wu SG et al., 2010).
The drilling revealed that faults and fracture zones aregenerally developed in the boreholes in the study area. Manyboreholes show that the deep hydrocarbon gases have thecharacteristics of migrating upward along the faults orfracture zones. In the shallow fracture zones, hydrate and itsanomalies (Han WC et al., 2019; Sha ZB et al., 2015; LiangYX et al., 2013; Wu SG et al., 2010) indicate that differentfault systems in the study area can provide upward migrationchannels for hydrocarbon gases in the deep, and shallowfaults or fracture zones can also provide space for hydrate toaccumulate.
5.3. Natural gas hydrate and hydrocarbon gas-accumulationserials
According to the evolution history of hydrocarbon gases,
hydrate accumulation in the study area is mainly limited to thepermafrost formation stage. The formation of permafrost inthe Qilian Mountains is mainly related to the Mesozoic-Cenozoic tectonic activity and uplift process. Through thestudy of moraine sediments on the periphery of the study area,it was found that there existed the penultimate or maximumglacial period formed between 0.5 Ma and 0.7 Ma, the bottomthird glacial period of 0.13 Ma to 0.30 Ma, and the last glacialperiod of 10 Ka to 70 Ka. The average annual temperature ofthe bottom third glacial period was not higher than –7.5−–10°C, which was at least 9.2−11.7°C lower than the present.The main part of the study area entered the cryosphere. Theannual average temperature of the penultimate glacial periodwas not higher than –9.5− –10°C, which was at least 11°Clower than the present (Qi BS et al., 2014). It is inferred thatthe hydrate formation in the study area was not later than theearly Middle Pleistocene.
Combined with geological analysis and previous research,the following hydrate-oil and gas accumulation relationshipmodel can be preliminarily established (Fig. 11): The gassource of hydrate in the study area is dominated by oil-typedpyrolysis gas, and only a small part is mixed with somemicrobial origin in the shallow part and a small amount ofcoal-type gas. The gas source of this oil-typed pyrolysis originis mainly provided by the lower Upper Triassic or deeper
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Fig. 11. Illustration for natural gas hydrate-oil and gas system in the study area.
520 Lu et al. / China Geology 4 (2020) 511−523
source rocks, secondarily followed by the Middle Jurassicmudstone or shale source rocks. When the source rockgenerates liquid and gaseous hydrocarbons and then migratesto the shallow part, they are directly or indirectly blocked toaccumulate by the F1 and F2 compressive faults formed in thelate Middle Jurassic to the early Early Cretaceous togetherwith mudstone or oil shale. Oil and gas accumulation in theshallow part was partially supplied with microbialhydrocarbon gases or coal-typed gas. Coupled with the island-like permafrost formed no later than the early MiddlePleistocene, it was combined with water in hydrate stabilityzone to form hydrate. When gases were outside the hydratestability zone, they would exist in shallower abnormally highpressure gas layer or as free gas or adsorbed gas in formationsat different depths. The abnormally high pressure gas layer inthe shallow part would be modified by microorganisms andhad the characteristics of microbial gas (Lu ZQ et al., 2019).
In particular, from the view point of natural gas hydrate-oil and gas system, it can be subdivided into several mainnatural gas hydrate-oil and gas subsystems, according to thehydrocarbon gas-generation serials in the Middle Jurassic andUpper Triassic source rocks, and hydrocarbon gas-accumulation serials in the study area. The first one is theconventional Upper Triassic gas-bearing sub-system withpeak hydrocarbon gas-generation in the late Middle Jurassic.The second one is the conventional Middle Jurassic oil-bearing sub-system with low to mature hydrocarbon gas-generation in the late Middle Jurassic. The third one is thenatural gas hydrate sub-system with main gas source from theUpper Triassic gas-bearing sub-system and minor gas sourcefrom the conventional Middle Jurassic oil-bearing sub-systemas well as little gas source from the Middle Jurassic coal-formed gas and the shallower microbial gas. The fourth one isthe shallower gas sub-system with microbial alteration of themain gas source from the Upper Triassic gas-bearing sub-system. Once the gas hydrate-oil and gas system is furtherapplied in the Qilian Mountain permafrost, it will play a moreimportant role in the exploration of natural gas hydrate, oiland gas, even other energy resources in the study area.Accordingly, it is of important theoretical and practicalsignificance.
6. Conclusions
(i) Natural gas hydrate, oil, and gas have been discoveredin the Qilian Mountain permafrost area. They are closelyrelated with each other in temporal, spatial, and genetic terms,and they mutually form a natural gas hydrate-oil-gas system.
(ii) This natural gas hydrate-oil-gas system in the QilianMountain permafrost area can be at least subdivided into fourmajor natural gas hydrate-oil and gas sub-systems: Aconventional Upper Triassic gas-bearing sub-system withpeak hydrocarbon gas-generation in the late Middle Jurassic;a conventional Middle Jurassic oil-bearing sub-system withlow to mature hydrocarbon gas-generation in the late MiddleJurassic; a natural gas hydrate sub-system with main gas
source from the Upper Triassic gas-bearing sub-system andminor gas source from the Middle Jurassic oil-bearing sub-system as well as little gas source from the Middle Jurassiccoal-bed gas and the microbial gas; a shallower gas sub-system with microbial alteration of the main gas source fromthe Upper Triassic gas-bearing sub-system.
CRediT authorship contribution statement
Zhen-quan Lu and Gang-yi Zhai conceived of thepresented idea. Shi-qi Tang and Dong-wen Fan carried out theexperiments. Zhen-quan Lu and Shi-qi Tang wrote themanuscript with support from Xiao-ling Luo. Hua Liu andYou-hai Zhu helped supervise the project. Ting Wangcontributed to the interpretation of the results. Shi-qi Tangand Rui Xiao designed the figures. All authors providedcritical feedback and helped shape the research, analysis andmanuscript.
Declaration of competing interest
The authors declare no conflicts of interest.
Acknowledgment
This work was supported by the projects of ChinaGeological Survey (DD20160223, DD20190102).
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