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Gases in Southern Junggar Basin mud volcanoes: Chemical composition, stable carbon isotopes, and gas origin
Journal of Natural Gas Science and Engineering 14 (2013) 108e115
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Gases in Southern Junggar Basin mud volcanoes: Chemical composition, stable carbon isotopes, and gas origin Zhifeng Wan a, b, c, *, Qiuhua Shi a, c, Feng Guo d, Yun Zhong a, c, Bin Xia a, c a
School of Marine Sciences, Sun Yat-sen University, Guangzhou 510275, China Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, Ministry of Land and Resources, Qingdao 266071, China c Key Laboratory of Offshore Oil Exploration and Development of Guangdong Higher Education Institutes, Guangzhou 510275, China d Shengli Oilfield Co. Ltd, SINOPEC, Dongying 257000, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2013 Received in revised form 13 June 2013 Accepted 17 June 2013 Available online 9 July 2013
Mud volcanoes are a common geological phenomenon in tectonically compressed areas on land and offshore. Mud volcano eruptions hold great significance for research on tectonic activity, the sedimentary environment and oil and gas accumulation. Methane emitted from mud volcanoes is also a source of greenhouse gas. Many mud volcanoes have developed in the southern Junggar Basin, Northwest China, but they have been studied very little. In this study, the chemical composition, stable carbon isotopes and gas origin of these mud volcanoes are analysed. The major gas component from the mud volcanoes in the southern Junggar Basin is methane, with an average value of 92.81%. The other gas components are ethane (4.8e2.93%), propane (0.01e0.05%), CO2 (0.11e5.36%) and N2 (0e3.63%). The methane carbon isotope ratios (d13C1) are between 38.92& and 42.82&, and ethane carbon isotope ratios (d13C2) are 20.50& to 22.95&. All these data have similar characteristics to other mud volcanoes around the world. Based on the C1 (methane)/(C2 (ethane) þ C3 (propane)) and d13C1, d13C2 results, the released gas is a coal-type thermogenic gas. The gas is from a middle-low Jurassic coal-measure source. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Mud volcano Chemical composition Carbon isotope Gas origin Southern Junggar Basin
1. Introduction A mud volcano (MV) is a geological structure formed as a result of the emission of argillaceous material on the Earth’s surface or the sea floor (Kopf, 2002; Milkov, 2000; Etiope et al., 2009a). Mud volcanoes are a significant marker of modern crustal movement and new tectonic activity. The muddy sediments, groundwater, oil and gas erupting from a mud volcano can be used to research and evaluate deep strata, groundwater dynamics, and oil and gas accumulation. Methane emitted from mud volcanoes is also a greenhouse gas source (Dimitrov, 2002; Ershov et al., 2011). The formation of a mud volcano requires three elements: a mud source, a fault channel and gas. These three elements are also behind the most important conditions of petroleum geology theory, which are hydrocarbon generation, migration and accumulation. Oil and gas reservoirs will form if these three elements are suitable. When a fault opens and breaks to the surface, the oil and
* Corresponding author. School of Marine Sciences, Sun Yat-sen University, Guangzhou 510275, China. E-mail addresses: [email protected], [email protected] (Z. Wan). 1875-5100/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jngse.2013.06.006
gas reservoirs will be destroyed, and a mud volcano will form. Therefore, the presence of a mud volcano indicates that the area around it has been subject to oil and gas generation, migration, accumulation and destruction. A mud volcano can be used for early oil and gas evaluation of a basin. Nearly 40% of the world’s oil and gas fields were found through hydrocarbon seepage on the earth’s surface (Link, 1952; Jones and Drozd, 1983; Matthews, 1996; Abrams, 2005; Dai et al., 2012). A submarine mud volcano is a channel connected to deep oil and gas and surface gas hydrates. Here, natural gas from the deep strata migrates to the surface through the mud volcano, forming a “leakage type” gas hydrate. Thus, a submarine mud volcano is living evidence and an important symbol of gas hydrates (Milkov, 2000; Ben-Avraham et al., 2002; Van Rensbergen et al., 2002; Bohrmann et al., 2003; Chen et al., 2005; Depreiter et al., 2005; Hein et al., 2006; Sauter et al., 2006; Feseker et al., 2009; Egorov and Rozhkov, 2010). Mud volcanoes may provide evidence of high petroleum potential in the deep subsurface. Gas hydrates are often associated with deep-water mud volcanoes, which are a potential energy resource. Thus, studying oil and gas leakage characteristics and the formation mechanisms of mud volcanoes is of major significance for hydrocarbon accumulation and the formation of gas hydrate reservoirs in submarine mud volcanoes.
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The current data show that the total number of prominent mud volcanoes on land may be approximately 1800, and there are more than 20,000 submarine mud volcanoes. These mud volcanoes are mainly located in Indonesia, Russia, Trinidad, Barbados, Taiwan, the Caspian Sea, the Black Sea, the Mediterranean Sea, the Barents Sea, the Gulf of Mexico and other places (Milkov, 2000; Dimitrov, 2002; Etiope et al., 2009a; Skinner and Mazzini, 2009). Mud volcanoes have attracted the attention of geologists for more than 200 years. Their tectonic settings, activity and products, their mechanisms of formation and their importance for petroleum prospecting have all been delineated (Milkov, 2000; Dimitrov, 2002; Kopf, 2002; Etiope et al., 2009a; Skinner and Mazzini, 2009; Zoporowski and Miller, 2009). However, the mud volcanoes in Mainland China have been studied very little. Numerous mud volcanoes have developed in the southern Junggar Basin, Northwest China, especially in the town of Baiyanggou, the Sailiketi farm of Wusu city, and the Dushanzi oilfield (Wang, 2000; Gao et al., 2008; Nakada et al., 2011; Dai et al., 2012). These areas have developed large-scale mud volcano groups, providing a convenient access for mud volcano research. The main aim of this paper is to analyse the chemical composition and stable carbon isotopes of gas from mud volcanoes in the Southern Junggar Basin of China, to discuss the gas origin and formation mechanisms of mud volcanoes, and to provide bases for oil and gas accumulation and exploration in the southern margin of the Junggar Basin. 2. Geological background The Junggar Basin is a large superimposed NeopaleozoiceCaenozoic basin located among the Siberian plate, the Kazakhstan plate and the Tarim plate and is bounded by the Altay and Tianshan Mountains (Li et al., 2008; Wang et al., 2011). The southern margin of the Junggar Basin referred to here is the transition region between the Tianshan Fold Belt and the Junggar Block, which was affected by the two episodic Yanshanian and Himalayan movements, especially the structural movement at the end of Himalayan, compressing the Tianshan Mountains and extruding them northward, which is why the southern margin of the Junggar Basin has the characteristics of SeN zonation and EeW segmentation (Chen et al., 2001; Zhao et al., 2003; Zheng et al., 2007) (Fig. 1). Since the Neopaleozoic, the southern margin of the Junggar Basin has experienced a series of complex tectogenetic activities, such as fault depression, the development of an oceanic basin, plate butting, separation, subduction, collision and wedging of the basement. These activities formed a multi-environmental and multi-type structural system. According to the construction characteristics and deformation behaviour, the geological evolution
Fig. 1. The tectonic position of the mud volcano developed area, which is the transition region between the Tianshan Mountains and Junggar Basin.
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since the Neopaleozoic of the southern margin of the Junggar Basin can be divided into four stages: a rift and limited ocean basineearly foreland basin in the CarboniferouseEarly Permian; an intracontinental depression in the middle PermianeJurassic; a piedmont depression in the CretaceouseOligocene; and a unified intracontinental basin in MioceneeQuaternary (Ji et al., 2008; Chen et al., 2010; Sha et al., 2011). The oil and gas resources are rich in the southern margin of the Junggar Basin, fulfilling the strategic goal of oil and gas exploration. Numerous oil-gas fields and oil-gas structures have already been discovered, such as the Dushanzi, Qigu, Ganhezi, Santai, Hutubi, Kayindike, and Mahe sites (Jiao et al., 2007; Qiu et al., 2008). A developed area of mud volcanoes is located in the western segment of the southern margin of the Junggar Basin, which developed three suites of hydrocarbon source rocks, those of the Middle Permian, the Lower-middle Jurassic and the Anjihaihe Formation of the Paleogene. The Paleogene and Neogene layers and the Tugulu Group in the Cretaceous are the predominant reservoirs. Unconformity, a sand body and faults characterise the passage system. The main oil- and gas-bearing formations are Neogene, Paleogene and Jurassic. The Dushanzi and Kayindike oil field are the two main oil fields thus far discovered (Pan and Yang, 2000; Ding et al., 2003; Bao et al., 2011). 3. Mud volcano descriptions There are outcrops of mud volcanoes in the southern margin of the Junggar Basin in the middle segment of the northern Tianshan Mountains. The most representative mud volcanoes are in Wushu Baiyanggou, Wushu Aiqigou, and the Dushanzi Oil Field (Fig. 2). Each has several vents and overflows (Table 1). 3.1. The mud volcanoes in Baiyanggou, Wushu, Xinjiang This mud volcanic cluster is located 2.5 km from the centre of Baiyanggou Town, which is 43 km from Wushu City. The area belongs to the hilly country in the northern piedmont of the Tianshan Mountains. More than 20 mud volcanoes are erupting in this mud volcano cluster, which is distributed in a narrow zone 200 m long and less than 40 m wide. The smallest volcano is as small as a horsebean, while the largest one’s diameter is nearly 4 m. Four or
Fig. 2. Map of the study area showing the Baiyanggou, Aiqigou and Dushanzi mud volcanoes, and the main rivers and roads.
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Table 1 The development characteristics of the main active mud volcanoes in the Southern Junggar basin. Latitude (N)
Longitude (E)
Shape
Vent size
Eruption characteristics
BYG-1
44 100 56.7600
84 230 12.4200
Basin-shaped
2.1 m 2.5 m
BYG-2
44 100 56.8200
84 230 12.2400
Basin-shaped
0.92 m 0.8 m
BYG-3
44 100 57.0600
84 230 12.3600
Basin-shaped
1.05 m 0.8 m
BYG-4 AQG
44 110 0.4800 44 110 19.8800
84 230 20.4600 84 290 35.2300
Spray mud pool Spray-mud-cone
3.52 m 3.28 m Diameter: 5 m
DSZ-1 DSZ-2
44 180 17.0400 44 180 18.8400
84 500 50.0800 84 500 46.6600
Basin-shaped Spray-mud-cone
1.05 m 1 m Diameter: 0.08 m
This mud volcano erupts very intensely, up to 70 times per minute. A large gas bubble is accompanied with 3e5 small bubbles, the maximum diameter is about 15 cm. The position of the bubble relatively fixed, about 50 times per minute. The bubbles are not uniform with different size and rate, 50 times per minute Three scattered bubble points, about 40 times per minute Bubbling points mainly at the south of the vent, the largest diameter of about 16 cm There are two bubbling points, intermittent eruption Only one bubbling point with the diameter of 5 cm, but erupts intensely, up to 70 times per minute
five mud volcano vents are grouped together in the intensive area, with distances between each other of 1e2 m. Because of the low consistency of the erupted mud, not every vent forms a spray-mudcone; some only form some spray mud pools or holes whose sizes are not uniform. Caesious and reddish brown are the two main colours of the erupted mud. Some dark brown oily matter is floating on the water. The eruptive frequency of the violent mud volcano is greater than 70 times/min. This study chose 4 mud volcanoes, which are large and convenient for collecting the gas, as the key objects of observation (Fig. 3). Details of the scale and eruption features of the 4 mud volcanoes are listed in Table 1. 3.2. Mud volcanoes in Aiqigou, Wushu, Xinjiang There are two mud volcano cones that form infrequent “twin spray-mud-cone mud volcanoes” on the piedmont terrace of Aiqigou at Sailiketi Farm, Wushu City (Fig. 4). These two mud volcanoes are standard cones. The distance between the two craters is approximately 20 m, and the height of the cones is approximately 9 m. The northeastern mud volcano is still active; the diameter of its crater is 5 m, and the diameter of the platform on top of the cone is
7 m. Some of the mud is drying up. The southwest volcano is extinct; the diameter of its crater is 1 m, and the diameter of the platform on top of the cone is 6 m. Compared with the mud of the volcanoes in Baiyanggou, the mud of this volcano is stickier. The erupted mud is cold and silver grey with an oily appearance. This mud volcano is composed of cone and skirt-like mud layers, and the gradient grade decreases as the elevation decreases. Thousands of tons of the mud, peppered with a colourful oil film, has erupted from the two mud volcanoes and has flowed down to the low concave area, forming a peculiar landform: the Red River Valley. Mud would become consolidated because of the loss of water during flow; with the fluid erosion of the rainwater, different shades and the random distribution of ditches, the mud surface became very tough, which gave the mud the appearance of a boiling mudflow. 3.3. Mud volcanoes in Dushanzi, Xinjiang The third group of mud volcanoes is located on a loess hill with an elevation of approximately 950 m that is approximately 1 km southwest of the Dushanzi District, Karamay City, Xinjiang. There
Fig. 3. The vent photos of four major mud volcanoes in Baiyanggou. The location of these mud volcanoes is shown in Fig. 2.
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Fig. 4. The twin mud volcanoes in Aiqigou. These two mud volcanoes are standard cones with the distance of about 20 m. The location is seen in Fig. 2.
Fig. 6. The drainage method is used to collect Mud volcano gas. This figure shows the vent of northeastern mud volcano in Aiqigou.
are two mud vents at present in this area. The diameter of the large one is approximately 1 m, and the small one is approximately 8 cm (Fig. 5). The shape of the large mud volcano in the south is like a mudjetting basin. The mud is diluted and has a slight oily appearance, and the colour is celadon. The mud overflowed through the spillway in the southwest. The craters of the small mud volcanoes in the north are filled with sticky mud, which is bubbling continuously. The mud gushing out has formed a spray-mud-cone on the surface with a diameter of approximately 8 m at the bottom that is 2 m in height and has a chap mud block on the surface.
collected by drainage in the bucket, or it was conducted to the airbag directly. For convenience and safe transportation, this study used vacuum aluminium foil airbags to collect the gas.
4. Sampling and analytical methods 4.1. Sampling A drainage method was used to collect the gas (for more details, see Fig. 6). Some bubbles were large and sometimes several bubbles were discharged at a time, making it difficult to collect them simultaneously using a funnel or a flask for drainage. Therefore, we made a large funnel with a large contact area to collect gas. This special funnel was made from a 40-cm-diameter plastic basin. A 5cm-diameter hole was drilled at the bottom of the plastic basin, and a normal funnel was conglutinated tightly to the hole. A pipe was connected to the normal funnel. When collecting gas, the plastic basin was filled with water to the brim. The other end of the pipe was introduced to the bucket full of water. As the gas was collected, bubble would appear in the bucket. It was necessary to wait a moment to ensure there was no air in the pipe. The gas was then
4.2. Gas species and methane stable carbon isotope measurement Gas analysis was carried out at the State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. An HP6890/wasson-ECE made by the American Agilent Company was used for the measurement of gas components. This instrument, when connected with to a vacuum system, can complete a test analysis at one time through an auto-injection system. The device measures organic gas (C1, C2, C3, C4) and small inorganic gas concentrations (CO2, N2, H2S, SO2) by using external standardisation. IsoChrom II, made in England by the VG Company, was used to measure stable carbon isotopes. Samples fractionalised by GC to a single compound were introduced by carrier gas to the 850 Coxidation furnace. It was burned into carbon dioxide and water. The water was then removed through a cold trap. The remaining carbon dioxide was introduced by carrier gas to the mass spectrometer. The carbon dioxide in the MS ion source bombarded by electrons emitted from the filament was ionised to charged ions with qualities of 44, 45 and 46. The charged ions formed three beams after being accelerated, dispersed and focalised and then reached the receiver. Faraday triple golden cups received the ion currents and transformed them to electrical signals, which were received by a
Fig. 5. The two major vents of mud volcanoes in Dushanzi. The location is shown in Fig. 2.
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preamplifier and sent to the computer. By using a carbon isotope ratio of carbon dioxide that is already known as the standard, the computer received the signals and calculated the carbon isotope ratio according to the predetermined program.
31.83& in the Aiqigou volcanoes and 23.95& and 24.96& in 13 Dushanzi. The d CCO2 from the Baiyanggou mud volcanoes could not be acquired due to the too low CO2 concentration. 6. Discussion
5. Results 6.1. Variation characteristics of mud volcano eruptions A total of 16 gas samples from the 7 mud volcanoes mentioned above were collected for the analysis of chemical composition and carbon isotopes, including 4 gas samples from the Aiqigou mud volcanoes and 12 from the other 6 volcanoes, with 2 samples each. The results are shown in Table 2. The dominant component of the natural gas that seeps from the mud volcanoes in the southern Junggar Basin is methane, with a concentration between 90.78% v/v and 95.82% v/v and an average value of 92.81% v/v (the volume percentage hereafter referred to as %). The mud volcano in the northern Dushanzi area emits the highest methane concentration, while that in the northeastern Aiqigou emits the lowest, though there is no large difference between them. The other major component is ethane, with a concentration between 4.8% and 2.93% and an average value of 4.08%. All the Baiyanggou volcanoes emit a much higher ethane content (>4.6%), while the Dushanzi volcanoes show the lowest. Propane only shows an average content of 0.03%. In addition to these alkanes (C1eC3), some air gases were detected in the mud volcano seeps, such as CO2 and N2, some with concentrations that cannot be neglected. The Dushanzi mud volcanoes emit CO2 with a concentration of approximately 1.3%, while the Baiyanggou mud volcanoes show the lowest CO2 concentration of 1.3%. N2 was detected in several mud volcano seeps. In the Baiyanggou mud volcanoes, for example, the mean N2 concentration was 2.63%, with the highest value of 3.63%. The hydrogen sulfide (H2S) and sulphur dioxide (SO2) weren’t detected in these samples. But it doesn’t represent that the mud volcano gas was not containing H2S and SO2. May be these gases were dissolved in the water. Table 2 shows the results of carbon isotope values of gaseous alkanes (mainly methane and ethane) and CO2. The average methane carbon isotope ratio (d13C1) was 40.53&, with the highest value of 38.92& from the Aiqigou mud volcano and the lowest value of 42.82& from the Baiyanggou mud volcano. The ethane carbon isotope ratio (d13C2) was between 20.50& and 22.95&, with an average value 21.45&, while the highest value was from Dushanzi and the lowest from Aiqigou. In addition, the 13 CO2 carbon isotope ratios ðd CCO2 Þ were between 31.14& and
In less than a year, from August 2011 to June 2012, we twice investigated the mud volcanoes in the southern margin of the Junggar Basin. Our findings, along with local geological survey reports, suggest the mud volcano activity is weakening. The number of active mud volcanoes in Baiyanggou is decreasing. According to the records from Baiyang town and Wusu city, there were approximately 200 active mud volcanoes in 2002. When we began our first field survey there in 2011, it was found that most of the mud volcanoes had dried up, with approximately 40 still erupting. When we conducted the second field trip in this area in 2012, only approximately 20 active mud volcanoes were found, suggesting the rapid disappearance of active mud volcanoes. Form the two photos (Fig. 7) taken at the Baiyanggou mud volcano group in August 2011 and June 2012, it can be found that there are 10 active mud volcanoes ranging in size from a diameter of 20 cm to the size of a broad bean in 2011 and that these mud volcanoes were all inactive, with only dried pits, in 2012. A similar case was also found with the twin mud volcanoes in Aiqigou. In 2011, both of the mud volcanoes were erupting, with different eruptive intensities between them (the eruption of the northeastern volcano was stronger than that of the other). In less than a year, however, the southwestern volcano had stopped erupting and had dried up. When the author stood on the dried surface of the crater, the sounds of bubbles breaking came from the interior of the volcanic cone, which indicated the inside mud was not dried up completely. Half of the surface area of the northeastern crater had dried up and it was expected that it would go completely dry soon after. The Dushanzi mud volcanoes were given much attention because of their location close to town. According to the geological record, these volcanoes have been erupting for several million years, and during the period of strong geological activity, the eructation from the mud volcanoes could reach a height of several metres because of the higher formation pressures. Today, these volcanoes are in the late stage of geological activity, with intermittent eruptions.
Table 2 The chemical component and stable carbon isotope of gases emitted from mud volcanoes in Southern Junggar Basin. Site
BYG 1-1 BYG 1-2 BYG 2-1 BYG 2-2 BYG 3-1 BYG 3-2 BYG 4-1 BYG 4-2 AQG 1-1 AQG 1-2 AQG 2-1 AQG 2-2 DSZ 1-1 DSZ 1-2 DSZ 2-1 DSZ 2-2
C1/(C2 þ C3)
Gas composition (vol.%) CH4
C2H6
C3H8
CO2
N2
92.35 91.94 92.73 91.74 93.84 93.01 91.54 92.91 91.20 91.15 90.87 90.78 95.17 94.12 95.77 95.82
4.67 4.66 4.80 4.75 4.69 4.64 4.67 4.74 3.71 3.71 3.86 3.85 3.33 3.29 2.93 2.93
0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
0.11 0.11 0.12 0.11 0.11 0.11 0.11 0.11 5.07 5.13 5.27 5.36 1.49 1.48 1.29 1.24
2.82 3.25 2.31 3.35 1.32 2.19 3.63 2.19 0.00 0.00 0.00 0.00 0.00 1.10 0.00 0.00
19.58 19.55 19.12 19.12 19.83 19.83 19.41 19.41 24.48 24.48 23.49 23.51 28.48 28.47 32.59 32.60
Stable carbon isotope, d13C (&) CH4
C2H6
CO2
42.82 42.56 41.11 41.11 41.40 41.29 41.42 41.34 39.41 39.36 38.97 38.92 40.15 39.85 39.34 39.41
20.63 20.60 20.59 20.51 21.47 21.14 21.52 21.38 22.45 22.43 22.82 22.95 22.06 21.43 20.50 20.64
31.50 31.83 31.14 31.37 23.95 24.08 24.75 24.96
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Fig. 7. Two photos of the same site in Baiyanggou, which was taken in 2011 and 2012, respectively, reflect that the number of active mud volcanoes reduced significantly.
The formation mechanisms of mud volcanoes have been the focus of debate by geologists (Mazzini et al., 2007, 2009; Mellors et al., 2007; Davies et al., 2008; Manga et al., 2009; Zoporowski and Miller, 2009; Roberts et al., 2011). Milkov (2000) summarised the geneses of volcanoes from the literature and classified them into four groups: geological effects, tectonic effects, geochemical effects and hydrogeologic effects. There are two essential factors contributing to the formation of mud volcanoes. First, an argilliterich formation with a high pore fluid pressure must exist in the deep part as the material basis and source of driving force for the formation of the mud volcano. Second, a dynamic mechanism that triggers the mud volcano eruption is essential, such as tectonic activity, fault activity and seismic activity. The obvious formation overpressure under the southern margin of the Junngar Basin was mainly caused by the sedimentary and tectonic effects (Luo et al., 2006, 2007; Cai, 2009). The rapid deposition of sediments led to the poor discharge of liquid within the formation, which was the dominant factor in the overpressure. The lateral stress from the piedmont tectonic belt also accelerated the accumulation of stress to some extent. Whether the trigger mechanism was the piedmont fault activity or seismic activity remains a question for lack of a theoretical foundation. It is a preliminary understanding that the mud volcano activity is getting weaker due to the release of the deep formation pressure. Whether the mud volcanoes will erupt again at a large scale after energy accumulation in the future is a question that requires further analysis of deep formation and tectonic setting. 6.2. Gas origin Small molecules of natural gas are easy to transport and mix, and natural gas usually experiences complex migration, aggregation, accumulation, transformation, and re-aggregation processes, causing the sources and geneses of natural gas to be confused (Yang et al., 2004; You et al., 2004; Cao et al., 2010; Ershov et al., 2011). The identification of genetic types of natural gas is generally given on the basis of the major molecular composition and geochemical parameters and indexes combined with the geological setting. Organic gas differs from inorganic gas in carbon isotope composition. The distinct differences between these gases are as follows. First, inorganic methane is rich in 13C. Generally, methane with d13C1 > 10& is considered inorganic, while d13C1 < 30& is considered organic. Second, the alkane carbon isotope composition is different between the two types of gas. The carbon isotope compositions of different alkanes from an organic source comprise a positive series, namely d13C1 < d13C2 < d13C3 < d13C4; in contrast, those from an inorganic source comprise a negative series, namely d13C1 > d13C2 > d13C3 > d13C4 (Dai et al., 1992; Xu et al., 1998). The natural gas from the mud volcanoes of the southern Junngar Basin
margin all have a low d13C1 value (<30&), with the lowest d13C1 value of 38.92&, and they also accord with the characteristics of the positive carbon isotope series, that is, d13C1 < d13C2. Consequently, the natural gas in the study area is organic. A d13C1 value of 55& is generally bounded in organic gas; lighter than 55& is biogenic gas and heavier than 55& is thermogenic gas (Dai and Chen, 1994; Dai et al., 2004). From Table 2, we can find that the d13C1 values of the mud volcano gases are all greater than 55& at the southern edge of the Junggar Basin, so these gases are thermogenic. The methane carbon isotope ratios in natural gas exhibit considerable variations, unlike ethane with its minor variations. The carbon isotope ratio of ethane, as a good parameter, can be used to identify genetic types of natural gas. Gas with d13C2 > 25.1& and d13C2 < 28.8& represented coal-type gas and oil-type gas, respectively (Zhang et al., 1988; Dai et al., 2009). The released gas in the study area has a d13C2 between 20.50& and 22.95&, belonging to the coal type. The CO2 emitted from the Aiqigou and Dushanzi mud volcanoes 13 has a higher d CCO2 value (>20&), indicating that after microbial degradation, the natural gas experienced a reduction of CO2, then generated methane (Nakada et al., 2011). The exploration of oil and gas in the southern Junngar Basin has confirmed that there are three sets of source rocks in this area: Middle Permian rocks over mature source rocks that mainly generate oil-type cracking gas; Middle-Low Jurassic mature-over mature coal-measure source rocks that mainly generated coaltype gas; and Paleogene immature-low mature gas source rocks that mainly generated biogenic gas and low mature gas (Pan and Yang, 2000; Ding et al., 2003; Jiao et al., 2007; Qiu et al., 2008; Bao et al., 2011). According to the geochemical analysis of the mud volcano gas above, it can be inferred that the mud volcano gas in study area was derived from a Middle-Low Jurassic coal-measure source. 6.3. Comparison of mud volcanoes worldwide Etiope et al. (2009a) compiled a global database of 143 terrestrial mud volcanoes from 12 countries and regions, including Azerbaijan, Italy, New Zealand, Guinea, Russia, Teli Dominican, Turkmenistan, the Ukraine, and Taiwan, to conduct a statistical analysis. The results indicated that 76% of mud volcanoes release thermogenic gas, with only 4% being biogenic and 20% with a mixed character. The average methane concentration and methane carbon isotope ratios d13C1 in natural gas are 90% and 46.4%, respectively. The natural gases emitted from mud volcanoes are richer in light hydrocarbons than gases originating from deep reservoirs (dominated by methane, together with minor ethane and propane).
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The dominant gas compositions of the three groups of mud volcanoes in the southern Junngar Basin margin are all methane, with a concentration between 90.78% and 95.82% and a d13C1 between 38.92& and 42.82&, as organic gas. All these data fall within the ranges of other mud volcano characteristics around the world, reflecting the common characteristics of natural gas from mud volcanoes: a mud volcanoes behaves just like a “natural refinery”, in which light hydrocarbons are separated from heavy hydrocarbons through a molecular fractionation process and seeps (Etiope et al., 2009b). 7. Conclusion The development characteristics, gas composition and carbon isotopes of mud volcanoes are analysed at the southern margin of the Junggar Basin, northwestern China. The major gas components are methane (90.78e95.82%), ethane (4.8e2.93%), propane (0.01e 0.05%), CO2 (0.11e5.36%) and N2 (0e3.63%). The methane carbon isotope ratios (d13C1) are between 38.92& and 42.82&, and ethane carbon isotope ratios (d13C2) are 20.50& and w22.95&. Based on the gas composition, geochemical parameters and indexes and geological setting, the released gas is a coal-type thermogenic gas from a Middle-Low Jurassic coal-measure source. Acknowledgements This work was supported by National Nature Science Foundation of China (No. 41102077), Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 201101711 20016), China Postdoctoral Science Foundation (No. 2011M501362) and Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, Ministry of Land and Resources (No. MRE 201103). References Abrams, M.A., 2005. Significance of hydrocarbon seepage relative to petroleum generation and entrapment. Marine and Petroleum Geology 22, 457e477. Bao, Z.D., Zhao, Y.J., Qi, L.Q., Si, M.N., Wu, B.R., Luo, X.Y., Zhao, H., Wu, X., 2011. Controlling factors of reservoir development in structural transfer zones: a case study of the Inner Junggar basin in Jurassic. Acta Petrologica Sinica 27, 867e877. Ben-Avraham, Z., Smith, G., Reshef, M., Jungslager, E., 2002. Gas hydrate and mud volcanoes on the southwest African continental margin off South Africa. Geology 30, 927e930. Bohrmann, G., Ivanov, M., Foucher, J.P., Spiess, V., Bialas, J., Greinert, J., Weinrebe, W., Abegg, F., Aloisi, G., Artemov, Y., Blinova, V., Drews, M., Heidersdorf, F., Krabbenhöft, A., Klaucke, I., Krastel, S., Leder, T., Polikarpov, I., Saburova, M., Schmale, O., Seifert, R., Volkonskaya, A., Zillmer, M., 2003. Mud volcanoes and gas hydrates in the Black Sea: new data from Dvurechenskii and Odessa mud volcanoes. Geo-Marine Letters 23, 239e249. Cai, X.Y., 2009. Overpressure development and oil charging in the central Junggar Basin, Northwest China: implication for petroleum exploration. Science in China Series D: Earth Sciences 52, 1791e1802. Cao, H.C., You, C.F., Sun, C.H., 2010. Gases in Taiwan mud volcanoes: chemical composition, methane carbon isotopes, and gas fluxes. Applied Geochemistry 25, 428e436. Chen, D.F., Huang, Y.Y., Yuan, X.L., Cathles, L.M., 2005. Seep carbonates and preserved methane oxidizing archaea and sulfate reducing bacteria fossils suggest recent gas venting on the seafloor in the Northeastern South China Sea. Marine and Petroleum Geology 22, 613e621. Chen, S.P., Zhang, Y.W., Tang, L.J., Bai, G.P., 2001. Tectonic evolution of the Junggar foreland basin in the late carboniferousepermian. Acta Geologica Sinica e English Edition 75, 398e408. Chen, Z.L., Lu, K.G., Wang, G., Chen, B.L., Wang, G.R., Zheng, E.J., Cui, L.L., Ding, W.J., 2010. Characteristics of Cenozoic structural movements in southern margin of Junggar basin and its relationship to the mineralization of sandstone-type uranium deposits. Acta Petrologica Sinica 26, 457e470. Dai, J.X., Chen, Y., 1994. Characteristics of carbon isotopes of alkane components and identification marks of biogenic gas in China. Science in China. Series B 37, 231e241. Dai, J.X., Pei, X.G., Qi, H.F., 1992. Natural Gas Geology in China, vol. 1. Petroleum Industry Press, Beijing, pp. 35e87.
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Gases in Southern Junggar Basin mud volcanoes: Chemical composition, stable carbon isotopes, and gas origin Zhifeng Wan a, b, c, *, Qiuhua Shi a, c, Feng Guo d, Yun Zhong a, c, Bin Xia a, c a
School of Marine Sciences, Sun Yat-sen University, Guangzhou 510275, China Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, Ministry of Land and Resources, Qingdao 266071, China c Key Laboratory of Offshore Oil Exploration and Development of Guangdong Higher Education Institutes, Guangzhou 510275, China d Shengli Oilfield Co. Ltd, SINOPEC, Dongying 257000, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2013 Received in revised form 13 June 2013 Accepted 17 June 2013 Available online 9 July 2013
Mud volcanoes are a common geological phenomenon in tectonically compressed areas on land and offshore. Mud volcano eruptions hold great significance for research on tectonic activity, the sedimentary environment and oil and gas accumulation. Methane emitted from mud volcanoes is also a source of greenhouse gas. Many mud volcanoes have developed in the southern Junggar Basin, Northwest China, but they have been studied very little. In this study, the chemical composition, stable carbon isotopes and gas origin of these mud volcanoes are analysed. The major gas component from the mud volcanoes in the southern Junggar Basin is methane, with an average value of 92.81%. The other gas components are ethane (4.8e2.93%), propane (0.01e0.05%), CO2 (0.11e5.36%) and N2 (0e3.63%). The methane carbon isotope ratios (d13C1) are between 38.92& and 42.82&, and ethane carbon isotope ratios (d13C2) are 20.50& to 22.95&. All these data have similar characteristics to other mud volcanoes around the world. Based on the C1 (methane)/(C2 (ethane) þ C3 (propane)) and d13C1, d13C2 results, the released gas is a coal-type thermogenic gas. The gas is from a middle-low Jurassic coal-measure source. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Mud volcano Chemical composition Carbon isotope Gas origin Southern Junggar Basin
1. Introduction A mud volcano (MV) is a geological structure formed as a result of the emission of argillaceous material on the Earth’s surface or the sea floor (Kopf, 2002; Milkov, 2000; Etiope et al., 2009a). Mud volcanoes are a significant marker of modern crustal movement and new tectonic activity. The muddy sediments, groundwater, oil and gas erupting from a mud volcano can be used to research and evaluate deep strata, groundwater dynamics, and oil and gas accumulation. Methane emitted from mud volcanoes is also a greenhouse gas source (Dimitrov, 2002; Ershov et al., 2011). The formation of a mud volcano requires three elements: a mud source, a fault channel and gas. These three elements are also behind the most important conditions of petroleum geology theory, which are hydrocarbon generation, migration and accumulation. Oil and gas reservoirs will form if these three elements are suitable. When a fault opens and breaks to the surface, the oil and
* Corresponding author. School of Marine Sciences, Sun Yat-sen University, Guangzhou 510275, China. E-mail addresses: [email protected], [email protected] (Z. Wan). 1875-5100/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jngse.2013.06.006
gas reservoirs will be destroyed, and a mud volcano will form. Therefore, the presence of a mud volcano indicates that the area around it has been subject to oil and gas generation, migration, accumulation and destruction. A mud volcano can be used for early oil and gas evaluation of a basin. Nearly 40% of the world’s oil and gas fields were found through hydrocarbon seepage on the earth’s surface (Link, 1952; Jones and Drozd, 1983; Matthews, 1996; Abrams, 2005; Dai et al., 2012). A submarine mud volcano is a channel connected to deep oil and gas and surface gas hydrates. Here, natural gas from the deep strata migrates to the surface through the mud volcano, forming a “leakage type” gas hydrate. Thus, a submarine mud volcano is living evidence and an important symbol of gas hydrates (Milkov, 2000; Ben-Avraham et al., 2002; Van Rensbergen et al., 2002; Bohrmann et al., 2003; Chen et al., 2005; Depreiter et al., 2005; Hein et al., 2006; Sauter et al., 2006; Feseker et al., 2009; Egorov and Rozhkov, 2010). Mud volcanoes may provide evidence of high petroleum potential in the deep subsurface. Gas hydrates are often associated with deep-water mud volcanoes, which are a potential energy resource. Thus, studying oil and gas leakage characteristics and the formation mechanisms of mud volcanoes is of major significance for hydrocarbon accumulation and the formation of gas hydrate reservoirs in submarine mud volcanoes.
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The current data show that the total number of prominent mud volcanoes on land may be approximately 1800, and there are more than 20,000 submarine mud volcanoes. These mud volcanoes are mainly located in Indonesia, Russia, Trinidad, Barbados, Taiwan, the Caspian Sea, the Black Sea, the Mediterranean Sea, the Barents Sea, the Gulf of Mexico and other places (Milkov, 2000; Dimitrov, 2002; Etiope et al., 2009a; Skinner and Mazzini, 2009). Mud volcanoes have attracted the attention of geologists for more than 200 years. Their tectonic settings, activity and products, their mechanisms of formation and their importance for petroleum prospecting have all been delineated (Milkov, 2000; Dimitrov, 2002; Kopf, 2002; Etiope et al., 2009a; Skinner and Mazzini, 2009; Zoporowski and Miller, 2009). However, the mud volcanoes in Mainland China have been studied very little. Numerous mud volcanoes have developed in the southern Junggar Basin, Northwest China, especially in the town of Baiyanggou, the Sailiketi farm of Wusu city, and the Dushanzi oilfield (Wang, 2000; Gao et al., 2008; Nakada et al., 2011; Dai et al., 2012). These areas have developed large-scale mud volcano groups, providing a convenient access for mud volcano research. The main aim of this paper is to analyse the chemical composition and stable carbon isotopes of gas from mud volcanoes in the Southern Junggar Basin of China, to discuss the gas origin and formation mechanisms of mud volcanoes, and to provide bases for oil and gas accumulation and exploration in the southern margin of the Junggar Basin. 2. Geological background The Junggar Basin is a large superimposed NeopaleozoiceCaenozoic basin located among the Siberian plate, the Kazakhstan plate and the Tarim plate and is bounded by the Altay and Tianshan Mountains (Li et al., 2008; Wang et al., 2011). The southern margin of the Junggar Basin referred to here is the transition region between the Tianshan Fold Belt and the Junggar Block, which was affected by the two episodic Yanshanian and Himalayan movements, especially the structural movement at the end of Himalayan, compressing the Tianshan Mountains and extruding them northward, which is why the southern margin of the Junggar Basin has the characteristics of SeN zonation and EeW segmentation (Chen et al., 2001; Zhao et al., 2003; Zheng et al., 2007) (Fig. 1). Since the Neopaleozoic, the southern margin of the Junggar Basin has experienced a series of complex tectogenetic activities, such as fault depression, the development of an oceanic basin, plate butting, separation, subduction, collision and wedging of the basement. These activities formed a multi-environmental and multi-type structural system. According to the construction characteristics and deformation behaviour, the geological evolution
Fig. 1. The tectonic position of the mud volcano developed area, which is the transition region between the Tianshan Mountains and Junggar Basin.
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since the Neopaleozoic of the southern margin of the Junggar Basin can be divided into four stages: a rift and limited ocean basineearly foreland basin in the CarboniferouseEarly Permian; an intracontinental depression in the middle PermianeJurassic; a piedmont depression in the CretaceouseOligocene; and a unified intracontinental basin in MioceneeQuaternary (Ji et al., 2008; Chen et al., 2010; Sha et al., 2011). The oil and gas resources are rich in the southern margin of the Junggar Basin, fulfilling the strategic goal of oil and gas exploration. Numerous oil-gas fields and oil-gas structures have already been discovered, such as the Dushanzi, Qigu, Ganhezi, Santai, Hutubi, Kayindike, and Mahe sites (Jiao et al., 2007; Qiu et al., 2008). A developed area of mud volcanoes is located in the western segment of the southern margin of the Junggar Basin, which developed three suites of hydrocarbon source rocks, those of the Middle Permian, the Lower-middle Jurassic and the Anjihaihe Formation of the Paleogene. The Paleogene and Neogene layers and the Tugulu Group in the Cretaceous are the predominant reservoirs. Unconformity, a sand body and faults characterise the passage system. The main oil- and gas-bearing formations are Neogene, Paleogene and Jurassic. The Dushanzi and Kayindike oil field are the two main oil fields thus far discovered (Pan and Yang, 2000; Ding et al., 2003; Bao et al., 2011). 3. Mud volcano descriptions There are outcrops of mud volcanoes in the southern margin of the Junggar Basin in the middle segment of the northern Tianshan Mountains. The most representative mud volcanoes are in Wushu Baiyanggou, Wushu Aiqigou, and the Dushanzi Oil Field (Fig. 2). Each has several vents and overflows (Table 1). 3.1. The mud volcanoes in Baiyanggou, Wushu, Xinjiang This mud volcanic cluster is located 2.5 km from the centre of Baiyanggou Town, which is 43 km from Wushu City. The area belongs to the hilly country in the northern piedmont of the Tianshan Mountains. More than 20 mud volcanoes are erupting in this mud volcano cluster, which is distributed in a narrow zone 200 m long and less than 40 m wide. The smallest volcano is as small as a horsebean, while the largest one’s diameter is nearly 4 m. Four or
Fig. 2. Map of the study area showing the Baiyanggou, Aiqigou and Dushanzi mud volcanoes, and the main rivers and roads.
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Table 1 The development characteristics of the main active mud volcanoes in the Southern Junggar basin. Latitude (N)
Longitude (E)
Shape
Vent size
Eruption characteristics
BYG-1
44 100 56.7600
84 230 12.4200
Basin-shaped
2.1 m 2.5 m
BYG-2
44 100 56.8200
84 230 12.2400
Basin-shaped
0.92 m 0.8 m
BYG-3
44 100 57.0600
84 230 12.3600
Basin-shaped
1.05 m 0.8 m
BYG-4 AQG
44 110 0.4800 44 110 19.8800
84 230 20.4600 84 290 35.2300
Spray mud pool Spray-mud-cone
3.52 m 3.28 m Diameter: 5 m
DSZ-1 DSZ-2
44 180 17.0400 44 180 18.8400
84 500 50.0800 84 500 46.6600
Basin-shaped Spray-mud-cone
1.05 m 1 m Diameter: 0.08 m
This mud volcano erupts very intensely, up to 70 times per minute. A large gas bubble is accompanied with 3e5 small bubbles, the maximum diameter is about 15 cm. The position of the bubble relatively fixed, about 50 times per minute. The bubbles are not uniform with different size and rate, 50 times per minute Three scattered bubble points, about 40 times per minute Bubbling points mainly at the south of the vent, the largest diameter of about 16 cm There are two bubbling points, intermittent eruption Only one bubbling point with the diameter of 5 cm, but erupts intensely, up to 70 times per minute
five mud volcano vents are grouped together in the intensive area, with distances between each other of 1e2 m. Because of the low consistency of the erupted mud, not every vent forms a spray-mudcone; some only form some spray mud pools or holes whose sizes are not uniform. Caesious and reddish brown are the two main colours of the erupted mud. Some dark brown oily matter is floating on the water. The eruptive frequency of the violent mud volcano is greater than 70 times/min. This study chose 4 mud volcanoes, which are large and convenient for collecting the gas, as the key objects of observation (Fig. 3). Details of the scale and eruption features of the 4 mud volcanoes are listed in Table 1. 3.2. Mud volcanoes in Aiqigou, Wushu, Xinjiang There are two mud volcano cones that form infrequent “twin spray-mud-cone mud volcanoes” on the piedmont terrace of Aiqigou at Sailiketi Farm, Wushu City (Fig. 4). These two mud volcanoes are standard cones. The distance between the two craters is approximately 20 m, and the height of the cones is approximately 9 m. The northeastern mud volcano is still active; the diameter of its crater is 5 m, and the diameter of the platform on top of the cone is
7 m. Some of the mud is drying up. The southwest volcano is extinct; the diameter of its crater is 1 m, and the diameter of the platform on top of the cone is 6 m. Compared with the mud of the volcanoes in Baiyanggou, the mud of this volcano is stickier. The erupted mud is cold and silver grey with an oily appearance. This mud volcano is composed of cone and skirt-like mud layers, and the gradient grade decreases as the elevation decreases. Thousands of tons of the mud, peppered with a colourful oil film, has erupted from the two mud volcanoes and has flowed down to the low concave area, forming a peculiar landform: the Red River Valley. Mud would become consolidated because of the loss of water during flow; with the fluid erosion of the rainwater, different shades and the random distribution of ditches, the mud surface became very tough, which gave the mud the appearance of a boiling mudflow. 3.3. Mud volcanoes in Dushanzi, Xinjiang The third group of mud volcanoes is located on a loess hill with an elevation of approximately 950 m that is approximately 1 km southwest of the Dushanzi District, Karamay City, Xinjiang. There
Fig. 3. The vent photos of four major mud volcanoes in Baiyanggou. The location of these mud volcanoes is shown in Fig. 2.
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Fig. 4. The twin mud volcanoes in Aiqigou. These two mud volcanoes are standard cones with the distance of about 20 m. The location is seen in Fig. 2.
Fig. 6. The drainage method is used to collect Mud volcano gas. This figure shows the vent of northeastern mud volcano in Aiqigou.
are two mud vents at present in this area. The diameter of the large one is approximately 1 m, and the small one is approximately 8 cm (Fig. 5). The shape of the large mud volcano in the south is like a mudjetting basin. The mud is diluted and has a slight oily appearance, and the colour is celadon. The mud overflowed through the spillway in the southwest. The craters of the small mud volcanoes in the north are filled with sticky mud, which is bubbling continuously. The mud gushing out has formed a spray-mud-cone on the surface with a diameter of approximately 8 m at the bottom that is 2 m in height and has a chap mud block on the surface.
collected by drainage in the bucket, or it was conducted to the airbag directly. For convenience and safe transportation, this study used vacuum aluminium foil airbags to collect the gas.
4. Sampling and analytical methods 4.1. Sampling A drainage method was used to collect the gas (for more details, see Fig. 6). Some bubbles were large and sometimes several bubbles were discharged at a time, making it difficult to collect them simultaneously using a funnel or a flask for drainage. Therefore, we made a large funnel with a large contact area to collect gas. This special funnel was made from a 40-cm-diameter plastic basin. A 5cm-diameter hole was drilled at the bottom of the plastic basin, and a normal funnel was conglutinated tightly to the hole. A pipe was connected to the normal funnel. When collecting gas, the plastic basin was filled with water to the brim. The other end of the pipe was introduced to the bucket full of water. As the gas was collected, bubble would appear in the bucket. It was necessary to wait a moment to ensure there was no air in the pipe. The gas was then
4.2. Gas species and methane stable carbon isotope measurement Gas analysis was carried out at the State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. An HP6890/wasson-ECE made by the American Agilent Company was used for the measurement of gas components. This instrument, when connected with to a vacuum system, can complete a test analysis at one time through an auto-injection system. The device measures organic gas (C1, C2, C3, C4) and small inorganic gas concentrations (CO2, N2, H2S, SO2) by using external standardisation. IsoChrom II, made in England by the VG Company, was used to measure stable carbon isotopes. Samples fractionalised by GC to a single compound were introduced by carrier gas to the 850 Coxidation furnace. It was burned into carbon dioxide and water. The water was then removed through a cold trap. The remaining carbon dioxide was introduced by carrier gas to the mass spectrometer. The carbon dioxide in the MS ion source bombarded by electrons emitted from the filament was ionised to charged ions with qualities of 44, 45 and 46. The charged ions formed three beams after being accelerated, dispersed and focalised and then reached the receiver. Faraday triple golden cups received the ion currents and transformed them to electrical signals, which were received by a
Fig. 5. The two major vents of mud volcanoes in Dushanzi. The location is shown in Fig. 2.
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preamplifier and sent to the computer. By using a carbon isotope ratio of carbon dioxide that is already known as the standard, the computer received the signals and calculated the carbon isotope ratio according to the predetermined program.
31.83& in the Aiqigou volcanoes and 23.95& and 24.96& in 13 Dushanzi. The d CCO2 from the Baiyanggou mud volcanoes could not be acquired due to the too low CO2 concentration. 6. Discussion
5. Results 6.1. Variation characteristics of mud volcano eruptions A total of 16 gas samples from the 7 mud volcanoes mentioned above were collected for the analysis of chemical composition and carbon isotopes, including 4 gas samples from the Aiqigou mud volcanoes and 12 from the other 6 volcanoes, with 2 samples each. The results are shown in Table 2. The dominant component of the natural gas that seeps from the mud volcanoes in the southern Junggar Basin is methane, with a concentration between 90.78% v/v and 95.82% v/v and an average value of 92.81% v/v (the volume percentage hereafter referred to as %). The mud volcano in the northern Dushanzi area emits the highest methane concentration, while that in the northeastern Aiqigou emits the lowest, though there is no large difference between them. The other major component is ethane, with a concentration between 4.8% and 2.93% and an average value of 4.08%. All the Baiyanggou volcanoes emit a much higher ethane content (>4.6%), while the Dushanzi volcanoes show the lowest. Propane only shows an average content of 0.03%. In addition to these alkanes (C1eC3), some air gases were detected in the mud volcano seeps, such as CO2 and N2, some with concentrations that cannot be neglected. The Dushanzi mud volcanoes emit CO2 with a concentration of approximately 1.3%, while the Baiyanggou mud volcanoes show the lowest CO2 concentration of 1.3%. N2 was detected in several mud volcano seeps. In the Baiyanggou mud volcanoes, for example, the mean N2 concentration was 2.63%, with the highest value of 3.63%. The hydrogen sulfide (H2S) and sulphur dioxide (SO2) weren’t detected in these samples. But it doesn’t represent that the mud volcano gas was not containing H2S and SO2. May be these gases were dissolved in the water. Table 2 shows the results of carbon isotope values of gaseous alkanes (mainly methane and ethane) and CO2. The average methane carbon isotope ratio (d13C1) was 40.53&, with the highest value of 38.92& from the Aiqigou mud volcano and the lowest value of 42.82& from the Baiyanggou mud volcano. The ethane carbon isotope ratio (d13C2) was between 20.50& and 22.95&, with an average value 21.45&, while the highest value was from Dushanzi and the lowest from Aiqigou. In addition, the 13 CO2 carbon isotope ratios ðd CCO2 Þ were between 31.14& and
In less than a year, from August 2011 to June 2012, we twice investigated the mud volcanoes in the southern margin of the Junggar Basin. Our findings, along with local geological survey reports, suggest the mud volcano activity is weakening. The number of active mud volcanoes in Baiyanggou is decreasing. According to the records from Baiyang town and Wusu city, there were approximately 200 active mud volcanoes in 2002. When we began our first field survey there in 2011, it was found that most of the mud volcanoes had dried up, with approximately 40 still erupting. When we conducted the second field trip in this area in 2012, only approximately 20 active mud volcanoes were found, suggesting the rapid disappearance of active mud volcanoes. Form the two photos (Fig. 7) taken at the Baiyanggou mud volcano group in August 2011 and June 2012, it can be found that there are 10 active mud volcanoes ranging in size from a diameter of 20 cm to the size of a broad bean in 2011 and that these mud volcanoes were all inactive, with only dried pits, in 2012. A similar case was also found with the twin mud volcanoes in Aiqigou. In 2011, both of the mud volcanoes were erupting, with different eruptive intensities between them (the eruption of the northeastern volcano was stronger than that of the other). In less than a year, however, the southwestern volcano had stopped erupting and had dried up. When the author stood on the dried surface of the crater, the sounds of bubbles breaking came from the interior of the volcanic cone, which indicated the inside mud was not dried up completely. Half of the surface area of the northeastern crater had dried up and it was expected that it would go completely dry soon after. The Dushanzi mud volcanoes were given much attention because of their location close to town. According to the geological record, these volcanoes have been erupting for several million years, and during the period of strong geological activity, the eructation from the mud volcanoes could reach a height of several metres because of the higher formation pressures. Today, these volcanoes are in the late stage of geological activity, with intermittent eruptions.
Table 2 The chemical component and stable carbon isotope of gases emitted from mud volcanoes in Southern Junggar Basin. Site
BYG 1-1 BYG 1-2 BYG 2-1 BYG 2-2 BYG 3-1 BYG 3-2 BYG 4-1 BYG 4-2 AQG 1-1 AQG 1-2 AQG 2-1 AQG 2-2 DSZ 1-1 DSZ 1-2 DSZ 2-1 DSZ 2-2
C1/(C2 þ C3)
Gas composition (vol.%) CH4
C2H6
C3H8
CO2
N2
92.35 91.94 92.73 91.74 93.84 93.01 91.54 92.91 91.20 91.15 90.87 90.78 95.17 94.12 95.77 95.82
4.67 4.66 4.80 4.75 4.69 4.64 4.67 4.74 3.71 3.71 3.86 3.85 3.33 3.29 2.93 2.93
0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
0.11 0.11 0.12 0.11 0.11 0.11 0.11 0.11 5.07 5.13 5.27 5.36 1.49 1.48 1.29 1.24
2.82 3.25 2.31 3.35 1.32 2.19 3.63 2.19 0.00 0.00 0.00 0.00 0.00 1.10 0.00 0.00
19.58 19.55 19.12 19.12 19.83 19.83 19.41 19.41 24.48 24.48 23.49 23.51 28.48 28.47 32.59 32.60
Stable carbon isotope, d13C (&) CH4
C2H6
CO2
42.82 42.56 41.11 41.11 41.40 41.29 41.42 41.34 39.41 39.36 38.97 38.92 40.15 39.85 39.34 39.41
20.63 20.60 20.59 20.51 21.47 21.14 21.52 21.38 22.45 22.43 22.82 22.95 22.06 21.43 20.50 20.64
31.50 31.83 31.14 31.37 23.95 24.08 24.75 24.96
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Fig. 7. Two photos of the same site in Baiyanggou, which was taken in 2011 and 2012, respectively, reflect that the number of active mud volcanoes reduced significantly.
The formation mechanisms of mud volcanoes have been the focus of debate by geologists (Mazzini et al., 2007, 2009; Mellors et al., 2007; Davies et al., 2008; Manga et al., 2009; Zoporowski and Miller, 2009; Roberts et al., 2011). Milkov (2000) summarised the geneses of volcanoes from the literature and classified them into four groups: geological effects, tectonic effects, geochemical effects and hydrogeologic effects. There are two essential factors contributing to the formation of mud volcanoes. First, an argilliterich formation with a high pore fluid pressure must exist in the deep part as the material basis and source of driving force for the formation of the mud volcano. Second, a dynamic mechanism that triggers the mud volcano eruption is essential, such as tectonic activity, fault activity and seismic activity. The obvious formation overpressure under the southern margin of the Junngar Basin was mainly caused by the sedimentary and tectonic effects (Luo et al., 2006, 2007; Cai, 2009). The rapid deposition of sediments led to the poor discharge of liquid within the formation, which was the dominant factor in the overpressure. The lateral stress from the piedmont tectonic belt also accelerated the accumulation of stress to some extent. Whether the trigger mechanism was the piedmont fault activity or seismic activity remains a question for lack of a theoretical foundation. It is a preliminary understanding that the mud volcano activity is getting weaker due to the release of the deep formation pressure. Whether the mud volcanoes will erupt again at a large scale after energy accumulation in the future is a question that requires further analysis of deep formation and tectonic setting. 6.2. Gas origin Small molecules of natural gas are easy to transport and mix, and natural gas usually experiences complex migration, aggregation, accumulation, transformation, and re-aggregation processes, causing the sources and geneses of natural gas to be confused (Yang et al., 2004; You et al., 2004; Cao et al., 2010; Ershov et al., 2011). The identification of genetic types of natural gas is generally given on the basis of the major molecular composition and geochemical parameters and indexes combined with the geological setting. Organic gas differs from inorganic gas in carbon isotope composition. The distinct differences between these gases are as follows. First, inorganic methane is rich in 13C. Generally, methane with d13C1 > 10& is considered inorganic, while d13C1 < 30& is considered organic. Second, the alkane carbon isotope composition is different between the two types of gas. The carbon isotope compositions of different alkanes from an organic source comprise a positive series, namely d13C1 < d13C2 < d13C3 < d13C4; in contrast, those from an inorganic source comprise a negative series, namely d13C1 > d13C2 > d13C3 > d13C4 (Dai et al., 1992; Xu et al., 1998). The natural gas from the mud volcanoes of the southern Junngar Basin
margin all have a low d13C1 value (<30&), with the lowest d13C1 value of 38.92&, and they also accord with the characteristics of the positive carbon isotope series, that is, d13C1 < d13C2. Consequently, the natural gas in the study area is organic. A d13C1 value of 55& is generally bounded in organic gas; lighter than 55& is biogenic gas and heavier than 55& is thermogenic gas (Dai and Chen, 1994; Dai et al., 2004). From Table 2, we can find that the d13C1 values of the mud volcano gases are all greater than 55& at the southern edge of the Junggar Basin, so these gases are thermogenic. The methane carbon isotope ratios in natural gas exhibit considerable variations, unlike ethane with its minor variations. The carbon isotope ratio of ethane, as a good parameter, can be used to identify genetic types of natural gas. Gas with d13C2 > 25.1& and d13C2 < 28.8& represented coal-type gas and oil-type gas, respectively (Zhang et al., 1988; Dai et al., 2009). The released gas in the study area has a d13C2 between 20.50& and 22.95&, belonging to the coal type. The CO2 emitted from the Aiqigou and Dushanzi mud volcanoes 13 has a higher d CCO2 value (>20&), indicating that after microbial degradation, the natural gas experienced a reduction of CO2, then generated methane (Nakada et al., 2011). The exploration of oil and gas in the southern Junngar Basin has confirmed that there are three sets of source rocks in this area: Middle Permian rocks over mature source rocks that mainly generate oil-type cracking gas; Middle-Low Jurassic mature-over mature coal-measure source rocks that mainly generated coaltype gas; and Paleogene immature-low mature gas source rocks that mainly generated biogenic gas and low mature gas (Pan and Yang, 2000; Ding et al., 2003; Jiao et al., 2007; Qiu et al., 2008; Bao et al., 2011). According to the geochemical analysis of the mud volcano gas above, it can be inferred that the mud volcano gas in study area was derived from a Middle-Low Jurassic coal-measure source. 6.3. Comparison of mud volcanoes worldwide Etiope et al. (2009a) compiled a global database of 143 terrestrial mud volcanoes from 12 countries and regions, including Azerbaijan, Italy, New Zealand, Guinea, Russia, Teli Dominican, Turkmenistan, the Ukraine, and Taiwan, to conduct a statistical analysis. The results indicated that 76% of mud volcanoes release thermogenic gas, with only 4% being biogenic and 20% with a mixed character. The average methane concentration and methane carbon isotope ratios d13C1 in natural gas are 90% and 46.4%, respectively. The natural gases emitted from mud volcanoes are richer in light hydrocarbons than gases originating from deep reservoirs (dominated by methane, together with minor ethane and propane).
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The dominant gas compositions of the three groups of mud volcanoes in the southern Junngar Basin margin are all methane, with a concentration between 90.78% and 95.82% and a d13C1 between 38.92& and 42.82&, as organic gas. All these data fall within the ranges of other mud volcano characteristics around the world, reflecting the common characteristics of natural gas from mud volcanoes: a mud volcanoes behaves just like a “natural refinery”, in which light hydrocarbons are separated from heavy hydrocarbons through a molecular fractionation process and seeps (Etiope et al., 2009b). 7. Conclusion The development characteristics, gas composition and carbon isotopes of mud volcanoes are analysed at the southern margin of the Junggar Basin, northwestern China. The major gas components are methane (90.78e95.82%), ethane (4.8e2.93%), propane (0.01e 0.05%), CO2 (0.11e5.36%) and N2 (0e3.63%). The methane carbon isotope ratios (d13C1) are between 38.92& and 42.82&, and ethane carbon isotope ratios (d13C2) are 20.50& and w22.95&. Based on the gas composition, geochemical parameters and indexes and geological setting, the released gas is a coal-type thermogenic gas from a Middle-Low Jurassic coal-measure source. Acknowledgements This work was supported by National Nature Science Foundation of China (No. 41102077), Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 201101711 20016), China Postdoctoral Science Foundation (No. 2011M501362) and Key Laboratory of Marine Hydrocarbon Resources and Environmental Geology, Ministry of Land and Resources (No. MRE 201103). References Abrams, M.A., 2005. Significance of hydrocarbon seepage relative to petroleum generation and entrapment. Marine and Petroleum Geology 22, 457e477. Bao, Z.D., Zhao, Y.J., Qi, L.Q., Si, M.N., Wu, B.R., Luo, X.Y., Zhao, H., Wu, X., 2011. Controlling factors of reservoir development in structural transfer zones: a case study of the Inner Junggar basin in Jurassic. Acta Petrologica Sinica 27, 867e877. Ben-Avraham, Z., Smith, G., Reshef, M., Jungslager, E., 2002. Gas hydrate and mud volcanoes on the southwest African continental margin off South Africa. Geology 30, 927e930. Bohrmann, G., Ivanov, M., Foucher, J.P., Spiess, V., Bialas, J., Greinert, J., Weinrebe, W., Abegg, F., Aloisi, G., Artemov, Y., Blinova, V., Drews, M., Heidersdorf, F., Krabbenhöft, A., Klaucke, I., Krastel, S., Leder, T., Polikarpov, I., Saburova, M., Schmale, O., Seifert, R., Volkonskaya, A., Zillmer, M., 2003. Mud volcanoes and gas hydrates in the Black Sea: new data from Dvurechenskii and Odessa mud volcanoes. Geo-Marine Letters 23, 239e249. Cai, X.Y., 2009. Overpressure development and oil charging in the central Junggar Basin, Northwest China: implication for petroleum exploration. Science in China Series D: Earth Sciences 52, 1791e1802. Cao, H.C., You, C.F., Sun, C.H., 2010. Gases in Taiwan mud volcanoes: chemical composition, methane carbon isotopes, and gas fluxes. Applied Geochemistry 25, 428e436. Chen, D.F., Huang, Y.Y., Yuan, X.L., Cathles, L.M., 2005. Seep carbonates and preserved methane oxidizing archaea and sulfate reducing bacteria fossils suggest recent gas venting on the seafloor in the Northeastern South China Sea. Marine and Petroleum Geology 22, 613e621. Chen, S.P., Zhang, Y.W., Tang, L.J., Bai, G.P., 2001. Tectonic evolution of the Junggar foreland basin in the late carboniferousepermian. Acta Geologica Sinica e English Edition 75, 398e408. Chen, Z.L., Lu, K.G., Wang, G., Chen, B.L., Wang, G.R., Zheng, E.J., Cui, L.L., Ding, W.J., 2010. Characteristics of Cenozoic structural movements in southern margin of Junggar basin and its relationship to the mineralization of sandstone-type uranium deposits. Acta Petrologica Sinica 26, 457e470. Dai, J.X., Chen, Y., 1994. Characteristics of carbon isotopes of alkane components and identification marks of biogenic gas in China. Science in China. Series B 37, 231e241. Dai, J.X., Pei, X.G., Qi, H.F., 1992. Natural Gas Geology in China, vol. 1. Petroleum Industry Press, Beijing, pp. 35e87.
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