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    29 October 2015, Volume 29 Issue 5
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    Engineering Geology and Environmental Geology
    New Advance on Gas Hydrate Survey and Research in Sanlutian of Muli, Qinghai
    WEN Huai-jun, LU Zhen-quan, LI Yong-hong, WANG Wei-chao, LIU Wen-jin, LI Xi
    2015, 29(5):  983-994. 
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    Since the first discovery of gas hydrate in 2008, gas hydrate survey and research has become a hot spot in Sanlutian of Muli, Qinghai. Within two years, Shenhua Qinghai Energy Development Co. Ltd. invested a project to carry out work on gas hydrate survey and evaluation in Sanlutian, which is the latest deployment and arrangement of gas hydrate survey and research in this area. At present a series of important new advances have been obtained. In this paper, these new advances are summarized: (1) new understanding on study of gas hydrate occurrence features; (2) new understanding on main geological controlling factors on gas hydrate formation and distribution; (3) gas hydrate geological formation pattern is summed up; (4) permafrost associated gas hydrate exploration methods such as geological, geophysical, geochemical and drilling methods are explored, and permafrost associated gas hydrate defining technical means such as pre-defining means by gas-logging while drilling, geological mark-recognition means, wire logging distinction means, indoor analytical identification means are established; (5) gas hydrate resource is estimated and its economical exploitation is preliminarily evaluated, which comprehensively suggests that gas hydrate resource is not economically exploitable in Sanlutian in the present.

    Study on the Nature of Gas Source for Permafrost-associated Gas Hydrate in Sanlutian of Muli, Qinghai
    2015, 29(5):  995-1001. 
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    Since the first drilling discovery of gas hydrate samples in Muli of Qilian Mountain permafrost in 2008, the gas source problem has become the focus. On the basis of previous work, the analysis of gas composition and methane carbon and hydrogen isotopes in adsorbed gases from cores, and of gas composition in headspace gas from cores was conducted in the paper. Those samples were collected from the gas hydrate drillings which were invested by Shenhua project in Sanlutian of Muli. Combined with gas composition and methane carbon and hydrogen isotope feature of gas hydrate in the relevant drillings, the analytical results are to further explore the genesis and properties of gas source for gas hydrate in this area. The results show that the gas source in the study area is of mixed genesis. The gas source is mainly composed of microbial gas partially mixed with a small amount of thermo-genic gas in the eastern part of the study area. The gas source is mainly composed of thermo-genic gas and secondarily of some microbial gas with a small amount of coal-derived gas in the middle to east of the study area. The thermo-genic gas is characteristic of crude oil cracking gas, and partly of microbial transformation gas.

    Geological Constraints on Gas Hydrate Formation and Distribution in Sanlutian Permafrost of Muli, Qinghai
    2015, 29(5):  1002-1013. 
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    Up to date, onshore permafrost-associated gas hydrate is just restricted to the area of Sanlutian of Muli in Qinghai in China; drilling holes revealed that gas hydrate distribution was very scattered in this area; then what geological constraints affect such heterogeneity of gas hydrate distribution in this area? Targeting this question, based on past work, the geological data and analytical results of various samples collected from the series of gas hydrate drilling holes invested by Shenhua Corporation are emphatically and comprehensively studied in Sanlutian of Muli in Qinghai, to discuss constraints of the genetic type of gas source, the available supply amount of gas source, the different occurrence and nature of faults on gas hydrate formation and distribution. Study results show that the genetic type of gas source is mainly thermal in the middle to west of the study area of Sanlutian where gas hydrate is found and it is mainly microbial in the east of the study area where gas hydrate is not found, indicating that the genetic type of gas source plays an important role in controlling gas hydrate formation and distribution in the study area. The available supply amount of gas source is the best in the west of the study area, and it is the second in the middle, and it is the worst in the east; this available supply circumstance of gas source directly affects the difference of gas hydrate formation and distribution between the middle to west and the east of the study area. Thrust faults F1 and F2 control gas hydrate formation and distribution in the middle to west of the study area. The occurrence and nature of the thrust fault F1 is various in the east to middle of the study area, which go against gas hydrate formation and distribution. The nature of normal fault F30 locally affects gas hydrate formation and distribution in the west of study area.

    Study on the Accumulation Pattern for Permafrost-associated Gas Hydrate in Sanlutian of Muli, Qinghai
    2015, 29(5):  1014-1023. 
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    The distribution of gas hydrate is very complex in Qilian area, therefore it is urgent to study the controlling factors and the formation pattern of gas hydrate in this area. The geological data and analytical results of various samples collected from the series of gas hydrate drilling holes are emphatically studied in Sanlutian. Results show that gas source for gas hydrate in the study area is mainly composed of oil-typed thermo-genic gases and this kind of gas source is partially mixed with some biogenic gas or coal gas in the shallow. And the oil-typed thermo-genic gases are mainly derived from lower or deeper Upper Triassic or Permian. When gas source migrated upward and arrived at the shallow formation, gases were directly or indirectly blocked by fracture and other seals such as mudstone, oil shale, thus causing shallow gas accumulation. Shallow gas was locally added by microbial gas or coal gas. They together with water were formed into gas hydrate in gas hydrate stability zone after these gases were encountered into frozen formation which happened no later than early Middle Pleistocene. Parts of them still exist as abnormally high-pressured gas reservoir or free (adsorbed) gases in shallower strata when they are outside of the gas hydrate stable zone. Because of variation of gas source types and supply conditions, migration and accumulation conditions, and range of gas hydrate stability zone, these factors’matching relation is various at different location, which affects occurrence and distribution of gas hydrate in a horizontal plane and vertical profile in this area.

    Biogenetic and Sub-biogenetic Gas Resource and Genetic Types of Natural Gas Hydrates in Pearl River Mouth Basin, Northern Area of South China Sea
    2015, 29(5):  1024-1034. 
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    Marginal basins in the northern area of South China Sea have abundant natural gas resources. Not only a large amount of oilfield have been found in the shallow water of the northern area, but also great exploration breakthroughs of deepwater oil and gas have been made in the deep water of the southern area so far. A large number of biogenetic gas, sub-biogenetic gas and gas hydrate have been discovered in submarine and shallow layers of the deep water area. This paper, based on a large number of geochemical data of natural gas which received from gas exploration in recent years, as well as combining hydrocarbon accumulation geological conditions, makes a depth analysis and elucidation of the geological and geochemical characteristics of biogenetic gas and sub-biogenetic gas and the distribution feature of hydrocarbon source rocks in Neogene and Quaternary of Pearl River Mouth Basin in the northeast of South China Sea. On this basis, we preliminarily predict and estimate the generation amount and resource extent of biogenetic gas in Pearl River Mouth Basin. At the same time, by geochemical and geological analysis on the genetic type of gas hydrate at the deep water area in Baiyun sag in the southern area of Pearl River Mouth Basin, we further confirm that the gas hydrate discovered so far belongs to self-source diffusion model and biogenic mineralization hydrates type ,whose gas source is a kind of mixture dominated by biogenetic gas coming from original place and nearby, while the mineralization and accumulation model of gas hydrate can be classified into near-source migrated and accumulated model of self-generation and self-reservoir, which has tremendous resource potential.

    Distribution and Reservoir Characteristics of Gas Hydrates in Sanlutian of Muli, Qinghai
    2015, 29(5):  1035-1046. 
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    Based on the geological information derived from 14 gas hydrate drilling holes in Sanlutian of Muli，Qinghai, the basic features of gas hydrates’output, occurrence and spatial distribution are analyzed in this paper. According to the inner connection between inter-restriction and inter-stimulation of multivariate control factors, which is taken as a bridge, the gas hydrate reservoir is introduced as a study subject. From the aspects of reservoir analysis, it shows that the gas hydrate is controlled by lithofacies, which makes its spatial distribution appear to be selective, extensile and comparable to some extent, namely the relative stability of its reservoir appearance in Sanlutian. The gas hydrate distribution in the interior reservoir is also controlled by various occurrence of fracture besides the various occurrence of the reservoir in Sanlutian, leading to irregular distribution in space and frequently sudden disappearance or sudden appearance of gas hydrate.

    Winter Seasonal Characteristics of the Archaeal Communities in Different Alpine Ecosystem Topsoils Collected from the Gas Hydrate Drilling Area，Qilian Mountains
    2015, 29(5):  1047-1060. 
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    Qilian Mountain Permafrost，located in the north of Tibet Plateau, is one of the most important permafrost regions, and it is also known as an important area for gas hydrate distribution on land in China. Previous studies showed that two main types of alpine ecosystems, including alpine meadow and alpine swamp meadow, were observed around the gas hydrate drilling area. In order to insight into the archaeal community and its distribution in the both topsoils of alpine ecosystem, the geochemical parameters and archaeal community compositions were analyzed for the soil samples collected during the early winter of 2014. The results showed that pH of the soils from alpine meadow were neutral, whereas pH of the soils from alpine swamp meadow were slightly acidic. The TOC content and methane concentration in the soils of the drilling area were significantly higher than those in the background area (non-hydrate drilling area), however, they were shown no difference when compared with those in different topsoils of alpine ecosystems in background area. The microbial cell abundance in the drilling area (except for DZ2-14Q-5T sample) was 2 to 5 folds higher than that in the background area. The archaeal biodiversity was poor in the winter seasonal topsoils, and was comprised of three archaeal groups affiliated to Crenarchaeota and three groups affiliated to Eurarchaeota. The archaeal communities from alpine meadow topsoils were dominated by Group Ⅰ.1b affiliated to Crenarchaeota, while alpine swamp meadow topsoils were dominated by Methanosarcinales affiliated to Eurarchaeota.It was inferred that the moisture in topsoils may be one of the most important reasons causing the variation of microbial cell abundances and the different compositions of archaeal communities between the both topsoils of alpine ecosystem, and the methanogenic archaea dominated in the alpine swap meadow soils was more related to the higher TOC content. The results also showed that more abundance of methanogenic archaea inhabited in the alpine swamp meadow topsoils than that in the alpine meadow topsoils. It was well known that the methanogens oxidized the organic carbon to produce methane under anaerobic conditions, contributing of one source of soil methane from biogenesis.

    Lithofacies Palaeogeography of the Middle Jurassic in the Juhugeng Gas Hydrate Potential Area in Muli，Qinghai Province
    2015, 29(5):  1061-1072. 
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    The Juhugeng mining area in Qinghai Province is not only a major site for abundant coal resources, but also the primary discovery site of gas hydrate in high plateau of the mid-latitude worldwide. The gas hydrate exploration horizon is mainly in the Middle Jurassic coal-bearing series. In this paper, we studied the sedimentary environments and palaeogeography of the Middle Jurassic coal-bearing series in the Juhugeng mining area by analysis of data from borehole cores and logging curves. The lithofacies paleogeography of each lithological member was reconstructed by using combination of the quantitative and qualitative analysis methods, and the single borehole and cross section depositional section analyses. The Lower Member of the Muli Formation is mainly composed of fine conglomerates, pebbly sandstones, and coarse-grained sandstones, reflecting a braided river depositional environment. The Upper member of the Muli Formation consists of sandstones, siltstones, and two thick coal seams, and this member was formed mainly in the deltaic sedimentary environment, with the shore-shallow lake environment being locally developed in the southwestern part of the mining area. The Lower Member of the Jiangcang Formation is dominated by the fine-grained sandstones and mudstones, intercalated with several coarse-grained sandstone beds and thin coal seams, and this member was mainly deposited in the lower delta plain and delta front environments, with the shore-shallow lake environment developed in the southwestern part of the area. The Upper Member of the Jiangcang Formation, where the gas hydrates were frequently discovered, is dominated by a succession of oil shale and dark mudstone containing bivalve fossils, reflecting a deep to semi-deep lacustrine environment. During the Middle Jurassic the Juhgeng area has experienced an overall deepening process from the braided fluvial, deltaic plain, delta front and shore-shallow lake, to shallow to deep lake environments.

    Depositional Environments of the Middle Jurassic in the Sanlutian Mining Field of the Juhugeng Mining Area in Qinghai Province
    2015, 29(5):  1073-1086. 
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    The Juhugeng mining area in Qinghai Province is the primary discovery site of the gas hydrates in the mid-latitude of the global land, with the gas hydrates being preserved in the Middle Jurassic coal-bearing series of the Sanlutian mining field. This paper analyzed the depositional environments of the coal-bearing series in the Sanlutian mining field by using the borehole cores and logging curves. The palaeogeographic outlines of each lithological member of the Middle Jurassic have been reconstructed based on the single borehole facies analysis and the cross-section facies correlations. The lithology of the Lower Member of the Muli Formation is mainly fine conglomerates, pebbly sandstone and coarse-grained sandstone, reflecting a braided river-dominated environment. The Upper Member of the Muli Formation is characterized by the thick sandstone interbedded with thick coal seam, indicating a gradual deepening process and development of the deltaic environment. The Lower Member of the Jiangcang Formation is characterized by the fine -grained sandstones interbedded with mudstones and thin coal seams, signifying a further deepening process and development of the delta front and shore-shallow lake environments. The Upper Member of the Jiangcang Formation consists mainly of oil shale and dark mudstone containing bivalve fossils, reflecting a shallow to deep lake environment.

    Structural Fracture Characteristics of Cores from Gas-hydrate Drillholes in Sanlutian of Muli Coalfield, Qinghai
    2015, 29(5):  1087-1095. 
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    Through observation and statistical analysis of gas-hydrate drilling cores in Sanlutian mining area, the characteristics of fractures were discussed from the point of linear density, dip angle and filling degree. The occurrence type of gas hydrates and the basic feature of structural fractures in the study area were confirmed. The influence of fractures on the formation of gas hydrate was also discussed. The results showed that the gas hydrate mainly occurred in siltstone, dark mudstone and oil shale, and about 62.5% of gas hydrate occurred in structural fractures,which are mainly low-dip angle fractures and unfilled fractures with proportion of 50.37% and 73.68%, respectively;in addition, the density of fractures in the overlying strata reduced to some extent. It is concluded that the development of fractures has important effect on the formation of gas hydrate in the study area: firstly, the unfilled fractures have provided migration passages and accumulation spaces for hydrocarbon gases;secondly, the fractures with low-dip angles make the strata have low permeability vertically and formations have poor connectivity, greatly limiting the upward diffusion of hydrocarbon gases; finally, the reduction of fractures in the overlying strata also inhibits the upward diffusion of hydrocarbon gases, playing the role of cap rocks.

    Tectonic Subsidence History of Sanlutian Mining Field in Muli,Qinghai
    2015, 29(5):  1096-1102. 
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    According to drillhole and geological survey report of Sanlutian mining field in Juhugeng mining area, we inversed and analyzed the subsidence history of Sanlutian mining field by back-stripping method and discussed the relationship between hydrate formation and tectonic evolution. The simulated results showed that the study area experienced four stages of subsidence and three stages of uplift since Carboniferous period as follows: subsiding slowly then rapidly with larger subsidence scope from Carboniferous to Late Triassic, uplifting and denudating in the end of Late Triassic by the late Indosinian movement, subsiding rapidly from Early Jurassic to Early Cretaceous, uplifting and denudating in Late Cretaceous by Yanshan movements,subsiding rapidly in Miocene epoch, uplifting rapidly with the uplift of the Tibetan Plateau in Pliocene epoch, and subsiding rapidly in Quaternary period. The simulation result of subsidence history provides a quantitative or semi-quantitative value of parameters for the tectonic evolution in the study area.The control of the tectonic subsidence on the formation of natural gas hydrate shows the two aspects, the maturity of source rocks and the formation of hydrate stable zone.

    Deformation Features of Triassic and Jurassic Strata in Mid-Qilian Orogenic Belt
    LIU Shi-bao1, FAN Wen-ke1, LI Yong-hong2, DOU Guang-yuan1, QI Chang-wei1, DANG H
    2015, 29(5):  1103-1109. 
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    Through sandbox modeling experiment the strain data were measured to analyze the deformation features of Triassic and Jurassic strata in mid-Qilian orogenic belt. The results show that the deformation feature of Jurassic is obviously different from that of Triassic.Triassic is a set of offshore sediments with sandstone and limestone and mudstone, and the mudstone was green with carbonaceous and calcareous cementation;Jurassic is a set of lacustrine sediments mainly composed by mudstone and sandstone and bearing coal.Triassic sandstones are mostly feldspar-quartz fine-sandstones, and Jurassic sandstones are argillaceous sandstones or gray-wackes with a lot of cuttings.During the late period of Indosinian movement,Triassic shows brittle fault-block motion features and develop fractures and folds, of which the axial planes are vertical,and the hinges strike east to west resulting from north-south direction extrusion;Jurassic develop tight folds because of the intensive squeezing action in Yanshanian.

    Classification and Identification of Gas Hydrate Reservoirs from Well Log Data in Sanlutian, Muli Coalfield, Qinghai
    HOU Jie1,2, ZOU Changchun1, QU Lu1, ZHU Jichang1, LI Kang1, YUE Xuyuan1, PEN
    2015, 29(5):  1110-1121. 
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    In Sanlutian well field, the drilling results reveal that gas hydrate mainly occurs in the fractured and porous reservoirs. Based on the results of drilling and log lithology identification, by using conventional and ultrasonic imaging log data, this article summarizes the log response characteristics of the both kinds of reservoirs respectively, and establishes identification methods which use log data to divide gas hydrate reservoirs in this area. The results show that:(1)compared to shale section，the fracture is developed, and sonic velocity is relatively high(in the range of 1.5 to 4.5 km/s) and the resistivity is up to 90 Ω·m in fractured gas hydrate reservoirs; compared to the aqueous layer of sandstone, the acoustic velocity is of relatively high value in the range of 2.0-4.0 km/s, the resistivity is also of relatively high value in the range of 90-180 Ω·m in porous hydrate reservoirs. (2)12 layers of fractured reservoirs and 15 layers of porous reservoirs which have a total thickness of 217.2 m are identified in the 14 wells of the study area. All the thick drilling reservoirs can be identified except a small amount of thin ones，and the most possible reason is that log response is unobvious to less content of gas hydrate. Overall, the application effect of log methods is good at the identification of gas hydrate reservoirs in the Sanlutian well field.

    Lithological Identification from Well Logs in Gas-hydrate Drilling Holes, Sanlutian of Muli Coalfield, Qinghai
    2015, 29(5):  1122-1129. 
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    The developed faults,lost and repeated formations and diversified lithology are the features of Sanlutian bore field, therefore, it is difficult to identify the lithology. As a result,log data are used specifically to study the formation lithology of Sanlutian bore field. By analyzing the log response characteristics for different lithological in fourteen gas hydrate drillings, Gamma ray log, neutron log and density log are optimized as the sensitive parameters to lithology identification. The lithological identification plate is made by cross plot technique and the identification criterion is built, which are used to identify and subdivide the lithology in Sanlutian bore field. Seven kinds of lithology can be identified by the log data, including sandstone, siltstone, argillaceous siltstone, silty mudstone, mudstone, oil shale and coal. According to the lithology identification result, the horizontal distribution of lithology in Sanlutian bore field is that sandstone in the east has good physical property and the well -developed mudstone is in the west; vertical distribution is that the sand to shale ratio is 4.48 in Muli Formation which has coal beds of 204.5 meters, while 0.84 in Jiangcang Formation that has abundant mudstone and oil shale which means better for the hydrate occurrence.The logging identification results of lithology lay the foundation of locating the favorable reservoir of gas hydrate in Sanlutian bore field.

    3D Seismic Detection for Natural Gas Hydrates in Muli Area, Qinghai
    2015, 29(5):  1130-1137. 
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    The 3D seismic data were firstly acquired to explore gas hydrates in permafrost area in Muli of Qinghai. According to the features of permafrost thickness, formation lithology, the pore type and hydrate-bearing depth of reservoir, the scheme of 3D seismic data acquisition and processing was designed, and a satisfied result was gained. Based on the analysis of these seismic data it is found that the structure and seismic response characteristics of the formation in this area are obviously different from aboard hydrate-bearing permafrost zones. The gas hydrate in Muli area is mainly filled in fractures of reservoirs, occasionally in the pores; in addition, the hydrate formation is also controlled by the depth and lithology of permafrost. As a result, the characteristics of seismic responses, such as amplitude, frequency and speed have some unique features. Combined with the geological data of the study area, it is revealed that the seismic responses of hydrate-bearing intervals manifest low frequency and disorderly medium oscillation amplitude. These characteristics can provide valuable proof to recognize gas hydrates in the study area.

    Structural Features and the Relation with Gas Hydrate Distribution in Sanlutian of Muli, Qinghai
    2015, 29(5):  1138-1143. 
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    Through processing 3D seismic data acquired in Sanlutian mining field in Muli permafrost area, Qinghai, based on the petrophysical analysis and the seismic responsefeature analysis, the deep structure and special seismic processing results such as AVO characteristics of the study area were obtained. The results reveal that there exists extremely close relationship between the distribution of gas hydrates and the structure; the distribution of gas hydrates is mainly controlled by the structure. In the area where faults and fractures are developed, the distribution feature of gas hydrates are obvious; it is coincident with the drilling result.

    Field Study on Gas Hydrates in Sanlutian of Muli in Qinghai Based on Real-time Identification Method
    2015, 29(5):  1144-1150. 
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    The identification of gas hydrates is the foundation for its exploration and research. The exploration level of gas hydrate in permafrost areas in China is low, and it is different from marine gas hydrate in existing environment and occurrence shape and accumulation mechanism. Therefore, it is urgent to find a new identification method for gas hydrate in permafrost areas. A continuous research was undertaken to identify gas hydrates in 14 drill holes located in Sanlutian of Muli, Qinghai which were carried out by Qinghai No.105 Coal Geological Exploration Team. Based on this research this paper builds a real-time identification method for gas hydrates, namely recognizing the obvious sign of ice-shape gas hydrates and boiling water and water seep age detected by gas logging and core analysis. After further verification, this new real-time identification method which is supported by physical and chemical characteristics of gas hydrates is proved to be more reliable and applicative. Finally, it summarizes the abnormal symbol and obvious sign and recognizing method for gas hydrates identification.

    Application of Gas Logging to Natural Gas Hydrate Prognosis in Sanlutian of Muli, Qinghai
    2015, 29(5):  1151-1156. 
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    With the advantages of sensitive and quick monitoring hydrocarbon gases in different stratigraphies, gas logging has become a mature technology for exploring the conventional or unconventional oil and gas. Considering that while the drilling tool is grinding strata bearing gas hydrate, the gas hydrate could decompose and release numbers of hydrocarbon gases which can be sensitively monitored by gas logging, this paper tries to discuss and establish a real-time prognosis method to recognize gas hydrates in permafrost regions in Muli of Qinghai. Taking drillhole DK8-19 in Sanlutian coal field in Muli of Qinghai as an example, the application, problems and suggestions of this realtime prognosis through gas logging was discussed. The practical application shows that the goodness of fit between the result of singly using gas logging and that of combining geological analysis and testing is up to 85.71%, and the result of combining gas logging, geological analysis and testing will be better.

    Microbial Geochemical Exploration and Research on Gas Hydrate in Sanlutian of Muli, Qinghai Province
    2015, 29(5):  1157-1163. 
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    The gas hydrate-discovering area in Muli of Tianjun, Qinghai Province was chosen as the research area, and the gas hydrate-discovering well was chosen as the analog model in this test to study the applicability of the microbial geochemical exploration (MGCE) method in gas hydrate exploration. Two different sampling grids of 250 m×500 m and 100 m×100 m were used in this study. The results showed that the research area had significant microbial anomaly, indicating the presence of hydrocarbon enrichment. The sparse grid study showed a favorable area of hydrocarbon enrichment, and the tight grid investigation showed the detail distribution of gas hydrate. This result verified the previous research result. The source of gas hydrates in the research area were also studied using geochemical method, and the results showed a very complex source, mainly thermogenic coalbed gas and oil-formed gas. This microbial geochemical exploration result can describe the detailed characteristics of the gas hydrate distribution and gas sources. Using geomicrobial, geochemical, geology and geophysical analysis (‘4G’), it can provide a new research method for the identification of gas hydrates in permafrost, and reduce the exploration risk and improve the success rate of exploration.

    Geochemical Characteristics of Trace Elements in Soil above Sanlutian Natural Gas Hydrates in the Qilian Mountains
    2015, 29(5):  1164-1172. 
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    The efficiency test of trace element geochemical exploration was carried out for natural gas hydrates in the Qilian Mountains permafrost region, the study area covered 150 km2 and sampling density of 2 sampling sites per km2 was used. Results showed that Ba, V, Fe and Ca in soils presented the apical pattern, and had identical anomalies. Natural gas hydrates could be higher successfully prospected by trace elements. The dry wells ( DK-4, DK-5, DK-6)drilled before this survey were located in background area, and the hydrate wells DK-1, DK-2, DK-3, DK-7 drilled before the survey and DK3-11, DK2-13, DK1-14 drilled after the survey were both located in anomaly area. Two prospective target regions were predicted according to the test results. Test has shown that trace elements were the effective index for prospecting gas hydrate in the Qilan Mountain permafrost region. Combined with permafrost conditions and geological characteristics, the comprehensive interpretation of trace elements and other geochemical index could improve the success rate of prediction for natural gas hydrate in permafrost area.

    Application of the Pyrolysis-Desorbed Hydrocarbon Technology to the Geochemical Exploration of Gas Hydrate in the Sanlutian Area in Muli Permafrost, Qinghai
    2015, 29(5):  1173-1179. 
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    Absorbed hydrocarbon (acidolysis hydrocarbon and pyrolysis-desorbed hydrocarbon) technique has displayed favorable affection in normal oil and gas geochemical exploration. In this paper, based on the soil absorbed hydrocarbon test conducted in Muli area in Qinghai, the analysis of the characteristic value and the distribution features of the geochemical anomaly reveals that hydrocarbon annular anomalies is consistent with the detected mineral area, the spatial distribution of which is controlled by underground resources. According to the analysis of the methane carbon isotope, the origin of soil absorbed hydrocarbon anomaly is proved to be pyrolysis, which has the similar thermogenic characteristics with gas hydrate. The experimental results indicate that the soil heat-release hydrocarbon technology applicable to the geochemical exploration of gas hydrate can delineate the hydrate deposits region, which could be the supplement of other geochemical exploration methods.

    Raman Spectroscopic Characteristics of Natural Gas Hydrates from Juhugeng Drilling Area, Qinghai
    2015, 29(5):  1180-1188. 
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    The structure characteristics of natural gas hydrates play a significant role in the exploration and evaluation of natural gas hydrate resource. In this paper, the hydrate samples, recovered from DK8 -19, DK11-14 and DK12-13 holes in the Juhugeng drilling area in Qinghai, were investigated by micro-Raman spectroscopy for the structure characteristics. The results show that gas hydrates are widely distributed in the Juhugeng drilling area. In the vertical direction, gas hydrate are distributed discontinuously between the depth of 126.1 and 322.2 m. The Raman characteristics of gas hydrates from the three sites with different depths are almost the same, and show structure Ⅱ hydrate with various gas molecules. Besides hydrocarbons such as CH4, C2H6, C3H8 and C4H10, N2 is generally observed in these gas hydrates samples. In addition, the Raman signals attributed to H2S in hydrate phase is found in the gas hydrate sample at depth of 126.1 m from DK8-19 hole, and it is the first time that H2S in hydrate phase is found in Qilian Mountain hydrates,suggesting that H2S gas molecules exist in some particular areas and gas hydrates form. The Raman spectroscopic characteristics of natural gas hydrates recovered from the Juhugeng drilling area may provide some new revelations for the study on the accumulation and distribution of gas hydrates located in the permafrost area in Qinghai.

    CT Image Characterization of Pores and Fissures in Rock Core from Juhugeng Gas Hydrate Area in Qinghai
    2015, 29(5):  1189-1193. 
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    Microstructure of pores and fissures is an important factor which influences the rock physical parameters such as acoustic velocity, permeability. These physical parameters are the important foundation for the gas hydrate accumulation and exploration in permafrost. The rock core samples recovered from Juhugeng district in Qinghai Province were studied in this paper. The pores and fissures of these samples were detected by the micro-focus computer tomography (micro CT). The results show that fissures widely exist in mudstone in good connectivity with the widths between 100 to 200 μm. However, pores are distributed dispersedly and there are poorly connected throats in sandstone with volumes of most pores lower than 0.1 mm3. The porosities of cores are estimated to be 2.36%-2.89% (sandstone samples) and 1.04%-3.64% (mudstone samples) respectively by the method of counting the voxel of 3D gray images.

    Gas Composition of Hydrate-bearing Cores in Juhugeng Drilling Area in Qinghai and Its Indicative Significance
    2015, 29(5):  1194-1200. 
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    Studying the composition characteristics of gases dissociated from hydrate-bearing cores in Juhugeng drilling area in Qinghai can be helpful for finding out the gases genesis and origin of natural gas hydrates, and can provide scientific guidance for gas hydrates exploration and exploitation. The gas molecular and stable isotopic compositions are analyzed for the totally 18 hydrate-bearing cores from 5 drilling holes (DK8-19, DK10-17, DK11-14, DK12-13 and DK13-11). The relevant diagrams of Cl/ (C2+C3)-δ13CC1, δDC1-δ13CC1 and δ13CC2-δ13CC1 are investigated in this paper. The results show that gases dissociated from hydrate-bearing cores mainly composed of light hydrocarbons with features of wet gas in Juhugeng drilling area in Qinghai and the isotopes are characterized as normal carbon isotopic series. In the drilling area, gases from most hydrate-bearing cores are mainly thermogenetic gas origin, and typical organic genetic gases formed in freshwater environment. However, it contains probably a small amount of biogenic gases in the shallow cores of DK8-19 hole.

    The Indicative Significance of Gas Composition of Headspace Gases from the Gas Hydrate Drilling Holes in the Sanlutian Mine of the Muli Mining Area,Qinghai
    2015, 29(5):  1201-1213. 
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    Through the analysis on hydrocarbon gas components at different depths in headspace gases from the cores in DK8-19, DK10-16, DK10-17, DK11-14, DK12-13, DK13-11 in Sanlutian area of Muli, the distribution relationship of the sections with anomalistic gas component content, gas hydrate output layers, oil and gas output layers, faults or fracture zones output layers was discussed. The results show that the anomalistic gas component content sections can be compared with those of gas hydrate and its major anomaly output layers, and oil and gas output layers; the abnormal section can be used as an indication of gas hydrate and its major anomalies and oil and gas. The relationship between variation of hydrocarbon gas content in headspace gases and the location of faults or fracture zones shows that different levels of fault system from the deep can provide a channel for deep hydrocarbon gas migration or existing space for the formation of gas hydrate.The results show that the gas component of headspace gas has important significance for gas hydrate and its major anomalies, the oil and gas and the migration of hydrocarbon.

    Organic Geochemical Characteristics of Gas Source Rocks in the Sanlutian Mine of the Muli Mining Area,Qinghai
    2015, 29(5):  1214-1222. 
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    The organic geochemical indicators were analyzed emphatically on the hydrocarbon (gas) source rocks of the Middle Jurassic and the Upper Triassic in gas hydrate occurrence interval in Sanlutian area of Muli,Qinghai. Results showed that the organic matter contents are at high levels in hydrocarbon (gas) source rocks of the Middle Jurassic and the Upper Triassic in the study area. Most TOC contents are more than 0.6%, that is medium, good and very good source rocks.The main organic matter is typed II2 and Ⅲ. The vitrinite reflectance of most samples are between 0 .7% to 1.3% and only a small part of the vitrinite reflectances are less than 0.7%. So the organic matter in most samples are at mature levels, or at condensate levels, not reach the level of wet to dry gas.This kind of gas source rocks may not produce large amounts of hydrocarbon gases to meet the prerequisite formation conditions for gas hydrate in the study area.The maturity of organic matter in the Upper Triassic is not high, even lower than that in the Middle Jurassic in the study area, which is probably caused by ground or shallow occurrence of the Upper Triassic reversed by faults.

    Mineralogical and Geochemical Characteristics of Carbonates Occurred at Gas Hydrate-bearing Intervals in Sanlutian of Muli of Qilian Mountains
    2015, 29(5):  1223-1233. 
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    It is a common phenomenon that gas hydrate occurs with carbonate in Sanlutian of Muli of Qilian Mountains. In order to discuss the formation environment of carbonate, the X ray diffraction analysis, the main-trace element and the carbon and oxygen isotope test of the carbonates were carried out. X ray diffraction results show that the carbonate minerals in the samples are mainly composed of calcite, dolomite and a small amount of siderite. Trace elements and rare earth elements show that the carbonate minerals are formed in the dry and oxidizing environment. δ13CVPDB is -2.3‰ to 3.77‰，and the average value is 2.43‰；δ18OVPDB is -17.90‰ to -10.69‰，and the variation is small. Carbon and oxygen isotopes show that the carbon in carbonate may be influenced by the combined effects of gas hydrate dissociation, dolomite precipitation and lake water pollution, and the possible sources of oxygen may be influenced by the atmospheric rainfall.

    Experimental and Numerical Study on the Hydrate Dissociation Front Induced by Depressurization in Unconsolidated Sediment
    2015, 29(5):  1234-1241. 
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    Hydrate dissociation front is one of objects of the field monitoring during gas hydrate production, and its velocity is closely related to the efficiency of gas hydrate production. However, there are few literatures about experimental data of the hydrate dissociation front. In view of this, an experiment of hydrate dissociation front induced by depressurization in unconsolidated sediment was developed. The hydrate saturation was measured by the time domain reflection measurement, and the movement of hydrate dissociation front was analyzed. An axisymmetric model for hydrate dissociation was proposed and its applicability was performed. A sensibility analysis for the velocity of hydrate dissociation front was carried out. Based on the experimental and numerical conditions, main conclusions were made as follows: first, propagation distance of hydrate dissociation front is approximatively linear with the square root of propagation time; second, velocity of the hydrate dissociation front decreases obviously when the propagation distance becomes longer; finally, velocity of the hydrate dissociation front increases when the reference value of the absolute permeability, initial gas saturation, and environmental temperature increase, however, it decreases when the initial hydrate saturation, reduction index, and outlet pressure increase.

    Geochemical Prospect Evaluation of Gas Hydrate in the Sanlutian Area, Muli, Qinghai Province
    2015, 29(5):  1242-1250. 
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    The intensive gas hydrate geochemical exploration has been taken in Sanlutian field, Muli, Qinghai. The work area is 10 square kilometers, and sampling density is 16 points per square kilometer, the soil acid-extracted and headspace gas of each point has been analyzed. The explaining of geochemical anomaly focused on the difference of gas hydrate mine and natural gas mine, and also focused on the gas hydrate target forecast. The result shows that comprehensive geochemical anomaly and the thickness of permafrost can effectively differentiate gas hydrate between shallow gases; and the ratio of acid-extracted hydrocarbon and headspace gas heavy hydrocarbon can effectively delineate the gas hydrate target area. Eight gas hydrate wells (DK-1, DK-2, DK-3, DK-7, DK-9, DK12-13, DK11-14, and DK10-17) are in the target area, while all dry wells (DK-4, DK-10, DK10-16, DK10-18, DK7 -20, DK-6, DK6-21, and DK4-24) are in background area.

    Preliminary Evaluation on Gas Hydrate Resources in Sanlutian of Muli, Qinghai
    2015, 29(5):  1251-1258. 
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    On the basis of predecessors’work and a series of drilling data obtained from the gas-hydrate drillholes carried out by Qinghai No-105 Coal Geological Exploration Team in Sanlutian of Muli, Qinghai together with the analysis of 3D-seismic, logging, geochemical and microbial data, the plane distribution of gas hydrates in this area was delineated with the use of hydrate-bearing intervals’ lithology, depth, thickness, gas-content anomaly and main faults. The gas-hydrate reserve in the study area was further estimated using the volume method, and the results show that the gas-hydrate reserve in prospects is about 2.138,5 million cubic meters and the inferred gas-hydrate reserve is about 4.526 million cubic meters. After the comprehensive analysis of the geological information, the gas-hydrate reserve in the study area and the production technics of gas hydrates, it comes to the conclusion that the gas-hydrate reservoir in Sanlutian is of no industrial exploitation value because of the complex occurrence situation, low reserve and immature mining technology at present.

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