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Gas source for gas hydrate and its significance in the Qilian Mountain permafrost, Qinghai
Marine and Petroleum Geology 43 (2013) 341e348
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Gas source for gas hydrate and its significance in the Qilian Mountain permafrost, Qinghai Zhengquan Lu a, b, *, Youhai Zhu a, b, Hui Liu a, b, Yongqin Zhang c, Chunshuang Jin b, Xia Huang a, Pingkang Wang a a b c
Institute of Mineral Resources, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing 100037, China Oil & Gas Survey, China Geological Survey, Beijing 100029, China Institute of Exploration Techniques, Chinese Academy of Geological Sciences, Langfang, Hebei 065000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 June 2012 Received in revised form 28 December 2012 Accepted 14 January 2013 Available online 1 February 2013
Gas source for gas hydrate is not clear yet in the Muli of Qilian Mountain permafrost. In this paper a case is illustrated in the hole of DK-2 during gas hydrate drilling; gas composition and isotopes of gas hydrate and its associated gases are analyzed; organic geochemistry on mudstone, oily shale, coal, oil & gas indications are correlated within the interval of gas hydrate occurrences; the aim is to discuss the source of gases from gas hydrate and its implication to gas hydrate exploration in the study area. Results from gas composition and isotopes of gas hydrate and its associated gases reveal that the origin of gases from gas hydrate is mainly concomitant with deep oil or crude oil in the study area. Parameters for the abundance, type and thermal evolution of organic matter in mudstone, oil shale, coal in the same interval of gas hydrate occurrence suggest that these strata, especially within gas hydrate stability zone, play little role in gas sources for gas hydrate. Reservoir pyrolysis results for oil & gas indication-bearing cores reveal that oil & gas indications are closely associated with gas hydrate within its interval, indicating that they may serve as a sign of gas hydrate in the study area. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Gas hydrate Gas source Implication Qilian Mountain permafrost Qinghai
1. Introduction Gas hydrate is a crystal material formed from water and light gases (such as CH4, C2H6, C3H8, i-C4H10, H2S, CO2, etc.) under low temperature and high pressure conditions (typically around 273.15 K and 3e5 MPa) when gas concentration is greater than its solubility (Sloan, 1998, 2003, 2004). In nature, gas hydrate occurs widely in marine sediments at water depth of more than 300 m (about 3 MPa) (Kvenvolden et al., 1993) and in permafrost in greater than 130 m below ground (about 3.5 MPa) (Shi and Zheng, 1999). Due to great significance in energy resources (Kvenvolden et al., 1993), latent feedback to environment (Dickens, 2004), and possible submarine hazards (Mienert et al., 2005) or impact on safety of drilling platform (McConnell et al., 2012), gas hydrate becomes one of hot fields for survey and research. After gas hydrate was first discovered in the Messoyakha gas field in the western Siberia permafrost, Russia, in 1960s (Makogon, 2010), some relevant gas hydrates were continuously found in the
* Corresponding author. Oil & Gas Survey, China Geological Survey, Beijing 100029, China. Tel.: þ86 10 64697550; fax: þ86 10 64697599. E-mail address: [email protected] (Z. Lu). 0264-8172/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marpetgeo.2013.01.003
Mackenzie delta permafrost, Canada (Bily and Dick, 1974; Dallimore and Collett, 2005) and in the Mount Elbert permafrost of Alaska, USA (Collett et al., 2011), and other marine gas hydrates were also found offshore East Korea (Kim et al., 2011), India (Shankar and Riedel, 2011) and New Zealand (Schwalenberg et al., 2010) in latest years. At present, about 230 locations of gas hydrate were mapped out around the world (Makogon, 2010). In China, gas hydrate was successfully sampled by drilling in the Qilian Mountain permafrost in 2008 (Lu et al., 2011; Zhu et al., 2009) after it was discovered in the northern continental slope of the South China Sea in 2007 (Wu et al., 2009). In total, eight scientific drilling wells, namely DK-1, DK-2, DK-3, DK-4, DK-5, DK-6, DK-7 and DK-8, were completed in the Qilian Mountain permafrost, among which gas hydrates were encountered in wells of DK-1, DK2, DK-3, DK-7 and DK-8, where their distance was no more than 30 m. On the contrary, only some anomalous phenomena associated with gas hydrate, indicated by (1) release of flammable gas from the well when gas hydrate bearing layers were penetrated, (2) a large amount of gas release when gas hydrate bearing cores were placed within airtight conditions, (3) anomalously low core temperatures measured by a Flir infrared camera, etc., were observed in wells of DK-4, DK-5 and DK-6, although there were several hundred meters to 1000 m away from any one of DK-1, DK-2, DK-3, DK-7 and
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Figure 1. A sketch map of tectonics in the Qilian Mountain permafrost (modified from Zhang and Yang, 2007) I1-the Alashan continental block; I2-the North Qilian suture in Neoproterozoic to Early Paleozoic era; I2-1-the Qilian-Menyuan magmatic arc belt in middle to late Early Paleozoic era (OeS); I3-the Middle Qilian continental block; I4-the South Shule Mt-Laji Mt suture in Early Paleozoic era; I5-the South Qilian continental block; I6-the Zongwulong Mt-South Qinghai Mt fault-depression trough in Late Paleozoic to Early Mesozoic era ( D-T2); I6-1-the Zongwulong Mt-Xinghai aulacogen (DeP); I6-2-the Zeku back-arc foreland basin (T1-2); I7-the Oulongbuluke continental block; I7-1-the Ebo Mt marginal craton basin; I7-2-the Dingzikou-Amunike Mt-Maoniu Mt magmatic arc belt in Neoproterozoic to late Early Paleozoic era (Pt3-S); I8-the marginal North Qaidam suture; I9-the Qaidam block; I10-the Qimantage-Doulan suture; I11-the middle East Kunlun continental block; I11-1-the middle East Kunlun magmatic arc belt (Pt3-J).
DK-8 (Lu et al., 2010a). On the one hand, it shows that gas hydrate distribution is complex (Lu et al., 2010b); on the other hand, it reveals that controlling factors for gas hydrate accumulation are unknown. In theory, gas hydrate is formed under suitable temperature and pressure conditions when gas source is sufficient (Sloan, 1998). In the Qilian Mountain permafrost, gas source may be a decisive factor for gas hydrate formation. Analyses on composition and isotope ratio of gases from gas hydrate indicate that hydrocarbon gases are thermogenic in the Qilian Mountain permafrost (Lu et al., 2010c), whereas it is thought of being from coalbed methane (Zhu et al., 2010). There is a remarkable fact that intervals of gas hydrate occurrences are well developed with coalbearing strata, oily shale, and oil & gas indications (such as oil stain, oil patch, oil immersion) which are often accompanied by gas hydrate in the Qilian Mountain permafrost (Lu et al., 2010b). In this paper the connection of gas source for gas hydrate with coal, mudstone, oily shale, and oil & gas indications is discussed with the help of conventional natural gas diagram, organic petrology and geochemistry.
2. Geological setting in the Muli of Qilian Mountain permafrost Wells of DK-1, DK-2, DK-3, DK-4, DK-5, DK-6, DK-7 and DK-8 were located in the Muli of Qilian Mountain permafrost, which belongs to the Sanlutian mining area of the Muli coal field. It is tectonically situated in the western Middle Qilian block formed in Caledonian Movement (513e386 Ma), adjacent to the South Qilian structural zone (Fig. 1; Feng, 1997), and it is also situated in the Muli Depression of the South Qilian Bain (Fu and Zhou, 1998, 2000). The Muli coal field is the biggest coal field in Qinghai province. Its destination layer is Jurassic lacustrine coal-bearing stratum, including Jiangcang Formation (J2j) and Muli Formation (J2m) of middle Jurassic (Wen et al., 2011). It is nowadays alpine-typed permafrost in the Qilian Mountain area, and is about 1 104 km2 in area in total, among which continuous permafrost has about 2.4 to 1.5 C and island permafrost has about 1.5 to 0 C on the yearly average in the atmosphere. The permafrost is about 50e139 m in thickness (Zhou et al., 2000).
Figure 2. Structural pattern in the study area (modified from Wen et al., 2011).
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Table 1 Composition of gases from gas hydrate in DK-1 (v/v, %). Sample
Depth/m
C1
C2
C3
i-C4
n-C4
i-C5
n-C5
Cþ 6
N2
CO2
Remark
G-5-1-2 G-6-1-2 S-1-2
134 143
42.90 10.47 59.01
5.40 1.62 6.23
5.68 3.38 9.43
0.70 0.35 0.93
3.48 0.53 1.01
0.45 0.05 0.13
0.70 0.05 0.12
2.55 0.50 1.71
35.98 76.76 19.27
2.16 6.28 2.16
* * **
Note: * e samples of gas dissociated from gas hydrate; ** e samples of gas collected from the drilling mud.
Table 2 Isotopic compositions of gases from gas hydrate in DK-1 (&). Sample
Type
Depth/mbs
d13C1
d13C2
d13C3
d13i-C4
d13n-C4
d13 CCO2
dD C1
dD C2
G-5-1-1 G-6-1-1 S-1-2
* * **
134 143
50.5 39.5 47.4
35.8 32.7 35.0
31.9 30.8 31.8
31.9 31.1 31.8
31.0 30.4 30.9
18.0 18.0 17.0
262 266 268
240 254
Note: * e samples of gas dissociated from gas hydrate; ** e samples of gas collected from the drilling mud.
In the study area, the middle part is an anticline composed of Triassic stratum, and the northern and southern flanks are two synclines composed of Jurassic lacustrine coal-bearing stratum. Large thrust faults were developed on the northern and southern flanks of the anticline and synclines. In two synclines, thrust faults caused a series of further NE direction shear fractures to develop, which cut depressions into interrupted blocks. Therefore, the study area is presented with north to south belts and west to east zones (Fig. 2). Gas hydrate and its associated anomalies such as voluminous gas release from wells or from tightened core samples, relatively low temperature record within cores, etc., occur within intervals of about 133e396 mbs (meter below surface) below permafrost in the study area. The base of permafrost is around 120 m revealed by drilling, during which at that depth ice flakes were observed upon cutting fresh cores. Some samples appear visible in fissures and are white ice-like lamina in smoky gray when mixed with drilling mud. Others are rarely visible and microscopically fillings in pores. Intervals with gas hydrate flakes were encountered in cores mainly composed of mudstone, oily shale, siltstone, and fine sandstone etc. However, gas hydrate was little obtained in medium sandstone and coarse sandstone. It is deemed that gas hydrate occurrences are in close relation with fractures rather than sandstone; in the meanwhile it is constrained by gas hydrate stability zone, especially by permafrost basic features (Lu et al., 2010b).
3. Samples and analyses Gas hydrate samples were individually collected at 134 m and 143 m beneath the ground in DK-1. Upon recovery, samples were wrapped up by tin paper and placed in cloth bag, then stored in liquid nitrogen. Gas samples released from drilling mud due to gas hydrate dissociation were also collected in DK-1, which were obtained by water-gas displacement and linked to a 20 ml vacuum vial. These gas samples were then stored in a refrigerator under low temperature conditions (<5 C). After gas hydrate samples were taken into laboratory, they were put in a vacuum container with a rubber stopper and gas hydrate was then naturally dissociated under laboratory conditions. Gas samples from gas hydrate were subsequently transported into a vacuum vial by water-gas displacement. In addition, 99 mudstone and oily shale samples together with 8 coal samples were also obtained at different intervals in DK-2. Gas samples were measured with gas composition and isotope ratios, which were analyzed in the state key laboratory of gas geochemistry, Lanzhou Center for Oil and Gas Resources, Institute of Geology and Geophysics, CAS. Rock samples were measured with microscopic components, vitrinite reflectance (RO), total organic carbon content (TOC), Rock-Eval of source rock and carbon isotope of organic matter. Microscopic components observations and vitrinite reflectance measurements were based on GB/T 8899-1998 and GB/T6948-2008 of China National Standard with a microscope of LEICA DM 2500P and a photometer of MSP200 under conditions
13
Figure 3. Plot of CH4/(C2H6 þ C3H8) to d CCH4 of gases from gas hydrate in DK-1. I1: bacterial gas, I2: mixed gas of bacterial and sub-bacterial gas, I3: sub-bacterial gas, II1: oil-concomitant gas, II2: oil-cracking gas, III1. mixed gas of oil-cracking gas and coal-genetic gas, III2: mixed gas of condensate oil-concomitant gas and coal-genetic gas, IV: coal-genetic gas, V1: abiogenic gas, V2: mixed gas of coal-genetic gas and abiogenic gas.
2
13
Figure 4. Plot of d HCH4 to d CCH4 of gases from gas hydrate in DK-1. A: bacterial gas, B: transition gas from bacterial gas to thermal gas, C: oil-concomitant gas, D: condensate oil-concomitant gas, E: coal-typed gas, F: marine over-matured gas.
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Table 3 Statistics of various indicators for organic matter abundance in oily shale in the DK-2 hole. Parameter H/C TOC (%) “A” (%) THC (106) THC/TOC (%)
Extremely good >3.5
Percent 87.50%
Good
Percent
Medium
Percent
Bad
Percent
No potentials
Percent
1.7e1.3 3.5e1.0 >0.12 >500 >6
87.50% 12.50% 100.00% 100.00% 25.00%
1.3e1.0 1.0e0.6 0.12e0.06 500e250 6e3
0.00% 0.00% 0.00% 0.00% 62.50%
1.0e0.5 0.6e0.4 0.06e0.01 250e100 3e1
12.50% 0.00% 0.00% 0.00% 12.50%
1.0e0.5 <0.4 <0.01 <100 <1
12.50% 0.00% 0.00% 0.00% 0.00%
Note: TOC e total organic carbon, THC e total hydrocarbon content.
Table 4 Statistics of various indicators for kerogen types in mudstone and oily shale in DK-2. Type (Hu et al., 1991)
S1 þ S2 (kg/t)
Percentage
IH (mg/g)
Percentage
S2/S3
Percentage
D (%)
Percentage
I II1 II2 III
>20 5e20 2e5 <2
20.20% 31.31% 24.24% 24.24%
>600 250e600 120e250 <120
15.15% 19.19% 33.33% 32.32%
>20 5e20 2.5e5 <2.5
5.05% 23.23% 26.26% 45.45%
>50 20e50 10e20 <10
18.18% 24.24% 28.28% 29.29%
Type (Tissot and Welter, 1989)
S1 þ S2 (kg/t)
Percentage
IH (mg/g)
Percentage
IO (mg/g)
Percentage
D (%)
Percentage
I II III IV
>6 6e4 <2 <2
49.49% 12.12% 24.24% 24.24%
>800 800e500 <150
5.05% 19.19% 40.40%
<40 60e40 150e50
22.22% 13.13% 53.54%
>50 10e50 <10 <10
18.18% 52.53% 29.29% 29.29%
Type (van Krevelen, 1982)
S1 þ S2 (kg/t)
Percentage
IH (mg/g)
Percentage
S2/S3
Percentage
D (%)
Percentage
I II III
>20 2e20 <2
20.20% 55.56% 24.24%
>600 120e600 <2.5
15.15% 52.53% 45.45%
>5 2.5e5 <10
28.28% 26.26% 29.29%
>50 10e50 <120
18.18% 52.53% 32.32%
of oil immersion and reflection, and completed in National Coal Quality Supervision and Inspection Center. Other measurements were completed in labs of Exploration and Development Research Institute of CNPC, among which total organic carbon analysis is based on GB/T19145-2003 with a carbon & sulfur analyzer of LECO CS-400 under normal temperature and pressure conditions; RockEval analysis is based on GB/ T18602-2001 with an instrument of Rock-Eval 2 Plus imported from France (Wu, 1986); carbon isotope of organic matter follows analysis based on SY/T5238-2008 with an instrument of Finngan MAT-252 and reference gas of GBW04405.
Figure 5. Plot of IH to IO in mudstone and oily shale in DK-2.
4. Results and discussion 4.1. Gas composition and isotope ratios Gas composition of gas hydrate and that of gas samples from drilling mud due to gas hydrate dissociation are presented in Table 1, and their isotope ratios are listed in Table 2. It is shown that 13 values of CCH4 =ðCC2 H6 þ CC3 H8 Þ are less than 10 and values of d CCH4 (PDB standard, &) are more positive (relative to 55& or 50&). 13 According to plot of CCH4 =ðCC2 H6 þ CC3 H8 Þ with respective to d CCH4 , gases from gas hydrate can be divided into bacterial gas, thermogenic gas and mixed gas. In the study area, it accordingly seems that gas from gas hydrate is mainly thermogenic gas (Fig. 3). For conventional nature gas, gas source is divided into bacterial gas, mixed gas of bacterial and sub-bacterial gas, sub-bacterial gas,
Figure 6. Plot of degradation rate (D) to Tmax in mudstone and oily shale in DK-2.
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gases from gas hydrate are mainly composed of oil-concomitant gas in the study area (Fig. 4). 4.2. Gas source from mudstone, oily shale and coal
Figure 7. Plot of S2/S3 to Tmax in mudstone and oily shale in DK-2.
4.2.1. Abundance of organic matter Based on TOC data of 99 samples from mudstone and oily shale at different depths in DK-2, all samples have TOC values greater than 0.4%. According to classification by Hu et al. (1991) and Chen (1994), samples with TOC ranging from 0.4% to 0.6% account for 1.01% in total samples; those ranging from 0.6% to 1.0% account for 10.10%; those from 1.0% to 2.0% account for 23.23%; and those greater than 2.0% account for 64.65%. It shows that gas source conditions of mudstones and oily shale are mainly of high-quality in the Muli of Qilian Mountain permafrost. Certainly coal is undoubtedly a good kind of gas source rock if judged from TOC. Particularly, statistics of indicators for abundance of organic matter from 8 samples of oily shale at different depths in DK-2, is presented in Table 3. H/C ratio, TOC, chloroform “A” content, total hydrocarbon content (THC), and THC/TOC ratio suggest that most of these samples are in conditions of very good and good source rock (Hu et al., 1991; Chen, 1994).
oil-concomitant gas, oil-cracking gas, mixed gas of oil-cracking gas and coal-genetic gas, mixed gas of condensate oil-concomitant gas and coal-genetic gas, coal-genetic gas, abiogenic gas, mixed gas of coal-genetic gas and abiogenic gas (Schoell, 1980, 1983; Whiticar et al., 1986; Whiticar, 1999). Plot of CCH4 =ðCC2 H6 þ CC3 H8 Þ to d13 CCH4 indicates that gases from gas hydrate and those collected from drilling mud are nearly oil-concomitant gas, occasionally with a little mixed gas of condensate oil-concomitant gas and coalgenetic gas in the study area (Fig. 3). Based on features of hydrogen isotope of natural gas from 2 onshore sedimentary basins in China, it is believed that d HCH4 (SMOW standard, &) in natural gas generated from marine (saline lake) source rock are generally more positive (relative to 190&), whereas, those from freshwater environment are more negative 2 (relative to 190&) (Shen and Xu, 1993). On the basis of d HCH4 and 13 d CCH4 values, normal nature gases from onshore sedimentary basins in China can be divided into bacterial gas, transition gas from bacterial gas to thermal gas, oil-concomitant gas, condensate oilconcomitant gas, coal-typed gas, and marine over-matured gas 2 13 (Shen and Xu, 1991, 1993). Plot of d HCH4 to d CCH4 indicates that
4.2.2. Type of organic matter Rock-Eval data of source rocks from 99 samples of mudstone and oily shale at different depths in DK-2 are presented in Table 4. Parameters of hydrocarbon-generation potential (S1 þ S2), hydrogen index (IH), oxygen index (IO), type index (S2/S3), and degradation rate (D) of mudstone and oily shale indicate that types of organic matter range from type I, type II and type III at gas hydrate bearing intervals (van Krevelen, 1982; Tissot and Welter, 1989; Hunt, 1991), of which type II (II1 and II2) is slightly predominant; type III comes second, and type I has a small population. Plots of IH to IO, D to Tmax and S2/S3 ratio to Tmax in mudstone and oily shale clearly display this kind of features (Figs. 5e7). Carbon isotopes in kerogen of 38 samples from mudstone and oily shale in DK-2 (Table 5) indicate that organic matter (OM) is composed of standard sapropelic OM (type I1), humic sapropelic OM (type I2), intermediate or mixed OM (type II), sapropelic humic OM (type III1) and standard humic OM (type III2) (Huang et al., 1984). For organic microscopic component observations from samples of oily shale in DK-2 (Table 6), results indicate that all samples are mainly composed of exinite and secondly of sapropelinite, although
Table 5 Characteristics of carbon isotope in kerogen of mudstone and oily shale in DK-2. Type (Huang et al., 1984)
III2
III1
II
I2
I1
d13C in kerogen (&PDB)
22.5 to 24.5 28.95%
24.5 to 26.0& 10.53%
26.0 to 27.0& 10.53%
27.0 to 28.0& 18.42%
28.0 to 29.0 5.26%
Percentage
Table 6 Characteristics of microscopic components and RO in oily shale in DK-2. Sample
Sapropel
Exinite
Vitrinite
Inertinite
Type index
Type
RO (%)
Maturity
d13C in kerogen
DK-2-S-1 DK-2-S-2 DK-2-S-3 DK-2-S-4 DK-2-S-5 DK-2-S-6 DK-2-S-7 DK-2-S-8
10 5 10 5 20 5 30 5
83 87 83 85 70 83 60 80
4 5 5 6 8 9 7 10
3 3 2 4 2 3 3 5
46 42 46 39 47 37 57 33
II1 II1 II1 II2 II1 II2 II1 II2
0.35 0.62 0.78 0.58 0.70 0.55 0.66 0.60
Immure Lowly mature Mature Lowly mature Lowly mature to mature Lowly mature Lowly mature Lowly mature
32.7 32.0 30.2 31.1 27.7 30.3 32.8 30.4
(&PDB)
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Figure 8. Plot of H/C to O/C in kerogen of oily shale in DK-2.
they all include vitrinite and inertinite, suggesting that their organic matter corresponds to type II1 and type II2, with relatively major type II1. Values of carbon isotopes in kerogen from these oily shale samples are less than 30.0&, except for one with 27.7& (Table 6), indicating that their kerogen is superior to type I. Ratios of H/C and O/C in kerogen from oily shale samples also illustrate that kerogen is preferable type I (Fig. 8). Of course, different parameters do not produce the same judgment for type of organic matter. It is believed that ratios of H/C and O/C in kerogen and S2/S3 in sedimentary rocks are effective parameters in determining kerogen types while IH has a high correlation with ratio of H/C (Hou and Tian, 1990). Thus, type of organic matter from oily shale samples is at least between type I and type II. Organic microscopic components observations and carbon isotopes in kerogen from coal samples in DK-2 are listed in Table 7. Organic microscopic components of coal samples are mainly composed of vitrinite, whose contents are mostly more than 60%, and secondly of inertinite and exinite with general contents of no more than several percent. In addition, organic microscopic components have a certain amount of mineral matter. These features indicate that their type of coal samples is mainly type III. Carbon isotopes in kerogen from six coal samples are between 23.7& and 24.8&. Based on criterion by Huang et al. (1984), type of organic matter in coal samples is type III1 and III2. 4.2.3. Thermal evolution of organic matter Plot of IH to Tmax in mudstone and oily shale in DK-2 demonstrates that thermal evolution of organic matter is mostly in mature
Figure 9. Plot of IH to Tmax in mudstone and oily shale in DK-2.
stage and only several samples are in highly mature or over mature stage (Fig. 9). RO values of oily shale in DK-2 are between 0.35% and 0.78% (Table 6), showing that most of samples are lowly mature, except for few samples being immature or intermediately mature. RO values of coal samples in DK-2 are between 0.86% and 1.13% (Table 7), indicative of being in mature stage. Based on thermal evolution stages of organic matter, namely five stages of being immature, oil-generation, condensate oil, wet gas, and dry gas (van Krevelen, 1982; Huang et al., 1984; Tissot and Welter, 1989; Hu et al., 1991; Hunt, 1991; Chen, 1994), mudstone, oily shale and coal samples are at the most in condensate oil stage, without reaching wet gas and dry gas stages in the study area, suggesting they have not yet produced abundant hydrocarbon gases to meet gas source conditions of gas hydrate formation. It should be noted that mudstone and oily shale samples were restricted within gas hydrate stability zone due to drilling limitation in this study. Therefore, it is not clear whether or not samples of mudstone and oily shale in deeper strata meet for gas source conditions of gas hydrate formation. Given that deeper strata would experience higher thermal evolution, mudstone and oily shale in deeper strata might be a gas source for gas hydrate. In the study area, coal beds occurred in shallow strata (Wen et al., 2011), where it is nearly overlapped with gas hydrate stability zone; no coal beds are expected in deep strata; therefore, coal beds is difficult to serve as a major gas source for gas hydrate formation.
Table 7 Characteristics of microscopic components and carbon isotope in kerogen of coal in DK-2. Sample
DK-2-C-1 DK-2-C-2 DK-2-C-3 DK-2-C-4 DK-2-C-5 DK-2-C-6 DK-2-C-7 DK-2-C-8
d13C in kerogen
Microscopic components Mineral
Exinite
Vitrinite
Inertinite
31.3% 43.6% 4.0% 5.5% 15.5% 2.1% 1.4% 1.5%
0.3% 0.0% 5.2% 10.2% 3.6% 5.3% 0.3% 1.2%
65.6% 56.4% 89.5% 79.0% 62.8% 85.2% 66.7% 69.9%
2.8% 0.0% 1.2% 5.3% 18.1% 7.4% 31.6% 27.4%
Type
RO (%)
Maturity
24.8 24.8
III1 III1
24.2
III2
24 24.5 23.7
III2 III2 III2
0.96 0.94 0.86 0.89 1 0.96 1.13 1.06
Mature Mature Mature Mature Mature Mature Mature Mature
(&PDB)
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Figure 10. Oil and gas indications within gas hydrate intervals in DK-2. Note: left e oil stain seepage from fissures of carbonate lamina inter-bedded with mudstone at about 298.50 m; middle e oil stains from interfaces between carbonate lamina and mudstone at about 299.62 m; right e asphalt observed within fissure of silt mudstone at about 316.30 m.
4.3. Connection of gas hydrate with oil & gas indications Composition and isotope of gases from gas hydrate and its associated gas in the Muli of Qilian Mountain permafrost indicate that gas source for gas hydrate is mainly from deep oil or crude oil concomitant gases. Abundance, type and thermal evolution of organic matter in mudstone, oily shale and coal indicate that they did not supply sufficient gases for gas hydrate within gas hydrate stability zone. Gas hydrate was encountered in DK-1, DK-2, DK-3, DK-7 and DK-8, where various gas hydrate-associated anomalies together with oil & gas indications (oil stain, oil patch, oil immersion) had been observed during the drilling. For example, oil stains seep from fissures or interfaces between carbonate lamina and mudstone at about 298.50 m and 299.62 m, and asphalt occurs within fissure of silt mudstone at about 316.30 m in DK-2 (Fig. 10). Based on pyrolysis analysis of reservoir rock with oil & gas indications (Fig. 11), it proves that oil & gas indications which are often associated with gas hydrate are common on the one hand; it also indicates that crude oil represented by these oil & gas indications are mainly composed of intermediate-weighted oil with a small amount of heavy oil, over-heavy oil and even asphalt. On the other hand, close association of oil & gas indications with gas hydrate provides a sound proof for gas source of gas hydrate coming from oil or crude oil associated gases, which was deduced by compositions and isotopes of gases from gas hydrate and its
Figure 11. Classification diagram for gas hydrate-associated oil & gas indications in DK-2. Note: O1 ¼ (S0 þ S1)/(S0 þ S1 þ S21 þ S22 þ S23), O2 ¼ (S21)/(S0 þ S1 þ S21 þ S22 þ S23), O3 þ 4 ¼ (S22 þ S23)/(S0 þ S1 þ S21 þ S22 þ S23).
associated gas. On the contrary, in DK-4, and especially in DK-5 and DK-6, gas hydrate samples had not been obtained although various anomalies had been observed; in the meanwhile, oil & gas indications were not watched. Therefore, it is deduced that gas source of gas hydrate has a close relationship with oil & gas indications in the Muli of Qilian Mountain permafrost. 5. Conclusions (1) Gas source for gas hydrate is mainly from deep or crude oilconcomitant gases in the Muli of Qilian Mountain permafrost. (2) Gas hydrate is closely connected with oil & gas indications in the study area. Acknowledgments This work is jointly financed by National Basic Research Program of China (973 Program, grant No.: 2009CB219501), National Natural Science Foundation of China (NSFC, grant No.: 41073040), National Special Program for Gas Hydrate Exploration and Test-production (grant No.: GZHL20110310), and Ministry of Finance (MOF, grant No.: K1003). References Bily, C., Dick, J.W.L., 1974. Naturally occurring gas hydrates in the Mackenzie Delta, N.W.T. Bulletin of Canadian Petroleum Geology 22, 340e353. Chen, R., 1994. Geology of Oil and Natural Gas. China University of Geosciences Press, pp. 1e262 (in Chinese). Collett, T.S., Lee, M.W., Agena, W.F., Miller, J.J., Lewis, K.A., Zyrianova, M.V., Boswell, R., Inks, T.L., 2011. Permafrost-associated natural gas hydrate occurrences on the Alaska North Slope. Marine and Petroleum Geology 28 (2), 279e 294. Dallimore, S.R., Collett, T.S., 2005. Scientific results from the Mallik 2002 gas hydrate production research well program, Mackenzie Delta, Northwest Territories, Canada. Geological Survey of Canada Bulletin 585, 1e140. Dickens, G.R., 2004. Global change: hydrocarbon-driven warming. Nature 429 (6991), 513e515. Feng, Y., 1997. Investigatory summary of the Qilian orogenic belt, China: history, presence and prospect. Advances in Earth Science 12 (4), 307e314 (in Chinese with English abstract). Fu, J., Zhou, L., 1998. Carboniferous-Jurassic stratigraphic provinces of the southern Qilian Basin and their petro-geological features. Northwest Geoscience 19 (2), 47e54 (in Chinese with English abstract). Fu, J., Zhou, L., 2000. Triassic stratigraphic provinces of the southern Qilian basin and their petro-geological features. Northwest Geoscience 21 (2), 64e72 (in Chinese with English abstract). Hou, D., Tian, S., 1990. A study of relationship between kerogen parameters and the validity in the classification of kerogen by multi-variate statistic analysis. Petroleum Exploration and Development 5, 33e37 (in Chinese with English abstract). Hu, J., Huang, D., Xu, S., Gan, K., Xue, S., Ying, F., 1991. Continental Petroleum Geology Theoretical Principle in China. Petroleum Industry Press, pp. 1e322 (in Chinese). Huang, D., Li, J., Zhang, D., 1984. Kerogen types and study on effectiveness, limitation and interrelation of their identification parameters. Acta Sedimentologica Sinica 2 (3), 18e32 (in Chinese with English abstract).
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Hunt, J.M., 1991. Generation of gas and oil from coal and other terrestrial organic matter. Organic Geochemistry 17 (6), 673e680. Kim, G.Y., Yi, B.Y., Yoo, D.G., Ryu, B.J., Riedel, M., 2011. Evidence of gas hydrate from downhole logging data in the Ulleung Basin, East Sea. Marine and Petroleum Geology 28 (10), 1979e1985. Kvenvolden, K.A., Ginsburg, G.D., Soloviev, V.A., 1993. Worldwide distribution of subaquatic gas hydrates. Geo-Marine Letters 13, 32e40. Lu, Z., Zhu, Y., Zhang, Y., Wen, H., Jia, Z., Li, Y., Liu, C., Wang, P., Guo, X., 2010a. Major evidence for gas hydrate existence in the Qilian Permafrost, Qinghai. Geoscience 24 (2), 329e336 (in Chinese with English abstract). Lu, Z., Zhu, Y., Zhang, Y., Wen, H., Li, Y., Jia, Z., Liu, C., Wang, P., Li, Q., 2010b. Basic geological characteristics of gas hydrates in Qilian Mountain permafrost area, Qinghai Province. Mineral Deposits 29 (1), 182e191 (in Chinese with English abstract). Lu, Z., Zhu, Y., Zhang, Y., Wen, H., Li, Y., Jia, Z., Wang, P., Li, Q., 2010c. Study on genesis of gases from gas hydrate in the Qilian Mountain permafrost, Qinghai. Geoscience 24 (3), 581e588 (in Chinese with English abstract). Lu, Z., Zhu, Y., Zhang, Y., Wen, H., Li, Y., Liu, C., 2011. Gas hydrate occurrences in the Qilian Mountain permafrost, Qinghai Province, China. Cold Regions Science and Technology 66 (2e3), 93e104. Makogon, Y.F., 2010. Natural gas hydrates e a promising source of energy. Journal of Natural Gas Science and Engineering 2 (1), 49e59. McConnell, D.R., Zhang, Z., Boswell, R., 2012. Review of progress in evaluating gas hydrate drilling hazards. Marine and Petroleum Geology 34 (1), 209e223. Mienert, J., Vanneste, M., Bünz, S., Andreassen, K., Haflidason, H., Sejrup, H.P., 2005. Ocean warming and gas hydrate stability on the mid-Norwegian margin at the Storegga Slide. Marine and Petroleum Geology 22 (1e2), 233e244. Schoell, M., 1980. The hydrogen and carbon isotopic composition of methane from natural gases of various origins. Geochimica et Cosmochimica Acta 44, 649e661. Schoell, M., 1983. Genetic characterization of natural gases. American Association Petroleum Geologists Bulletin 67, 2225e2238. Schwalenberg, K., Wood, W., Pecher, I., Hamdan, L., Henrys, S., Jegen, M., Coffin, R., 2010. Preliminary interpretation of electromagnetic, heat flow, seismic, and geochemical data for gas hydrate distribution across the Porangahau Ridge, New Zealand. Marine Geology 272 (1e4), 89e98. Shankar, U., Riedel, M., 2011. Gas hydrate saturation in the Krishna-Godavari basin from P-wave velocity and electrical resistivity logs. Marine and Petroleum Geology 28 (10), 1768e1778. Shen, P., Xu, Y., 1991. The isotopic composition of nature gases from continental sediments in China. Geochimica 20 (2), 144e151 (in Chinese with English abstract).
Shen, P., Xu, Y., 1993. Isotopic compositional characteristics of terrigenous natural gases in China. Chinese Journal of Geochemistry 12 (1), 14e24. Shi, D., Zheng, J., 1999. The status and prospects of research and exploration of nature gas hydrate in the world. Advances in Earth Science 14 (4), 330e339 (in Chinese with English abstract). Sloan, E.D., 1998. Clathrate Hydrates of Natural Gases, second ed. Marcel Dekker, New York, pp. 1e585. Sloan, E.D., 2003. Fundamental principles and applications of natural gas hydrates. Nature 426, 353e359. Sloan, E.D., 2004. Introductory overview: hydrate knowledge development. American Mineralogist 89, 1155e1161. Tissot, B.P., Welter, D.H., 1989. Formation and Distribution of Petroleum. Petroleum Industry Press, pp. 1e469 (in Chinese). van Krevelen, D.W., 1982. Development of coal research e a review. Fuel 61 (9), 786e790. Wen, H., Shao, L., Li, Y., Lu, J., Zhang, S., Wang, W., Huang, M., 2011. Structure and stratigraphy of the Juhugeng coal district at Muli, Tianjun County, Qinghai Province. Geological Bulletin of China 30 (12), 1823e1828. Whiticar, M.J., Faber, E., Schoell, M., 1986. Biogenic methane formation in marine and freshwater environments: CO2 reduction vs. acetate fermentation e isotope evidence. Geochimica et Cosmochimica Acta 50, 693e709. Whiticar, M.J., 1999. Carbon and hydrogen isotope systematics of bacterial formation and oxidation of methane. Chemical Geology 161, 291e314. Wu, L., 1986. Fast Pyrolysis Quantitative Evaluation of Source Rock. Science Press, pp. 1e198 (in Chinese). Wu, N., Yang, S., Wang, H., Liang, J., Gong, Y., Lu, Z., Wu, D., Guan, H., 2009. Gasbearing fluid influx sub-system for gas hydrate geological system in Shenhu Area, Northern South China Sea. Chinese Journal of Geophysics 52 (6), 1641e 1650 (in Chinese with English abstract). Zhang, X., Yang, S., 2007. Study on Plate Tectonics in Qinghai Province e a Handbook for 1: 1000000 Geo-tectonic Map in Qinghai Province. Geological Publishing House, pp. 1e221. Zhou, Y., Guo, D., Qiu, G., 2000. Geocryology in China. Science Press, pp. 329e353 (in Chinese). Zhu, Y., Zhang, Y., Wen, H., Lu, Z., Jia, Z., Li, Y., Li, Q., Liu, C., Wang, P., Guo, X., 2009. Gas hydrates in the Qilian Mountain permafrost, Qinghai, Northwest China. Acta Geologica Sinica 83 (11), 1762e1771 (in Chinese with English abstract). Zhu, Y., Zhang, Y., Wen, H., Lu, Z., Wang, P., 2010. Gas hydrates in the Qilian Mountain permafrost and their basic characteristics. Acta Geoscientica Sinica 31 (1), 7e16 (in Chinese with English abstract).
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Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo
Gas source for gas hydrate and its significance in the Qilian Mountain permafrost, Qinghai Zhengquan Lu a, b, *, Youhai Zhu a, b, Hui Liu a, b, Yongqin Zhang c, Chunshuang Jin b, Xia Huang a, Pingkang Wang a a b c
Institute of Mineral Resources, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing 100037, China Oil & Gas Survey, China Geological Survey, Beijing 100029, China Institute of Exploration Techniques, Chinese Academy of Geological Sciences, Langfang, Hebei 065000, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 June 2012 Received in revised form 28 December 2012 Accepted 14 January 2013 Available online 1 February 2013
Gas source for gas hydrate is not clear yet in the Muli of Qilian Mountain permafrost. In this paper a case is illustrated in the hole of DK-2 during gas hydrate drilling; gas composition and isotopes of gas hydrate and its associated gases are analyzed; organic geochemistry on mudstone, oily shale, coal, oil & gas indications are correlated within the interval of gas hydrate occurrences; the aim is to discuss the source of gases from gas hydrate and its implication to gas hydrate exploration in the study area. Results from gas composition and isotopes of gas hydrate and its associated gases reveal that the origin of gases from gas hydrate is mainly concomitant with deep oil or crude oil in the study area. Parameters for the abundance, type and thermal evolution of organic matter in mudstone, oil shale, coal in the same interval of gas hydrate occurrence suggest that these strata, especially within gas hydrate stability zone, play little role in gas sources for gas hydrate. Reservoir pyrolysis results for oil & gas indication-bearing cores reveal that oil & gas indications are closely associated with gas hydrate within its interval, indicating that they may serve as a sign of gas hydrate in the study area. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Gas hydrate Gas source Implication Qilian Mountain permafrost Qinghai
1. Introduction Gas hydrate is a crystal material formed from water and light gases (such as CH4, C2H6, C3H8, i-C4H10, H2S, CO2, etc.) under low temperature and high pressure conditions (typically around 273.15 K and 3e5 MPa) when gas concentration is greater than its solubility (Sloan, 1998, 2003, 2004). In nature, gas hydrate occurs widely in marine sediments at water depth of more than 300 m (about 3 MPa) (Kvenvolden et al., 1993) and in permafrost in greater than 130 m below ground (about 3.5 MPa) (Shi and Zheng, 1999). Due to great significance in energy resources (Kvenvolden et al., 1993), latent feedback to environment (Dickens, 2004), and possible submarine hazards (Mienert et al., 2005) or impact on safety of drilling platform (McConnell et al., 2012), gas hydrate becomes one of hot fields for survey and research. After gas hydrate was first discovered in the Messoyakha gas field in the western Siberia permafrost, Russia, in 1960s (Makogon, 2010), some relevant gas hydrates were continuously found in the
* Corresponding author. Oil & Gas Survey, China Geological Survey, Beijing 100029, China. Tel.: þ86 10 64697550; fax: þ86 10 64697599. E-mail address: [email protected] (Z. Lu). 0264-8172/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marpetgeo.2013.01.003
Mackenzie delta permafrost, Canada (Bily and Dick, 1974; Dallimore and Collett, 2005) and in the Mount Elbert permafrost of Alaska, USA (Collett et al., 2011), and other marine gas hydrates were also found offshore East Korea (Kim et al., 2011), India (Shankar and Riedel, 2011) and New Zealand (Schwalenberg et al., 2010) in latest years. At present, about 230 locations of gas hydrate were mapped out around the world (Makogon, 2010). In China, gas hydrate was successfully sampled by drilling in the Qilian Mountain permafrost in 2008 (Lu et al., 2011; Zhu et al., 2009) after it was discovered in the northern continental slope of the South China Sea in 2007 (Wu et al., 2009). In total, eight scientific drilling wells, namely DK-1, DK-2, DK-3, DK-4, DK-5, DK-6, DK-7 and DK-8, were completed in the Qilian Mountain permafrost, among which gas hydrates were encountered in wells of DK-1, DK2, DK-3, DK-7 and DK-8, where their distance was no more than 30 m. On the contrary, only some anomalous phenomena associated with gas hydrate, indicated by (1) release of flammable gas from the well when gas hydrate bearing layers were penetrated, (2) a large amount of gas release when gas hydrate bearing cores were placed within airtight conditions, (3) anomalously low core temperatures measured by a Flir infrared camera, etc., were observed in wells of DK-4, DK-5 and DK-6, although there were several hundred meters to 1000 m away from any one of DK-1, DK-2, DK-3, DK-7 and
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Figure 1. A sketch map of tectonics in the Qilian Mountain permafrost (modified from Zhang and Yang, 2007) I1-the Alashan continental block; I2-the North Qilian suture in Neoproterozoic to Early Paleozoic era; I2-1-the Qilian-Menyuan magmatic arc belt in middle to late Early Paleozoic era (OeS); I3-the Middle Qilian continental block; I4-the South Shule Mt-Laji Mt suture in Early Paleozoic era; I5-the South Qilian continental block; I6-the Zongwulong Mt-South Qinghai Mt fault-depression trough in Late Paleozoic to Early Mesozoic era ( D-T2); I6-1-the Zongwulong Mt-Xinghai aulacogen (DeP); I6-2-the Zeku back-arc foreland basin (T1-2); I7-the Oulongbuluke continental block; I7-1-the Ebo Mt marginal craton basin; I7-2-the Dingzikou-Amunike Mt-Maoniu Mt magmatic arc belt in Neoproterozoic to late Early Paleozoic era (Pt3-S); I8-the marginal North Qaidam suture; I9-the Qaidam block; I10-the Qimantage-Doulan suture; I11-the middle East Kunlun continental block; I11-1-the middle East Kunlun magmatic arc belt (Pt3-J).
DK-8 (Lu et al., 2010a). On the one hand, it shows that gas hydrate distribution is complex (Lu et al., 2010b); on the other hand, it reveals that controlling factors for gas hydrate accumulation are unknown. In theory, gas hydrate is formed under suitable temperature and pressure conditions when gas source is sufficient (Sloan, 1998). In the Qilian Mountain permafrost, gas source may be a decisive factor for gas hydrate formation. Analyses on composition and isotope ratio of gases from gas hydrate indicate that hydrocarbon gases are thermogenic in the Qilian Mountain permafrost (Lu et al., 2010c), whereas it is thought of being from coalbed methane (Zhu et al., 2010). There is a remarkable fact that intervals of gas hydrate occurrences are well developed with coalbearing strata, oily shale, and oil & gas indications (such as oil stain, oil patch, oil immersion) which are often accompanied by gas hydrate in the Qilian Mountain permafrost (Lu et al., 2010b). In this paper the connection of gas source for gas hydrate with coal, mudstone, oily shale, and oil & gas indications is discussed with the help of conventional natural gas diagram, organic petrology and geochemistry.
2. Geological setting in the Muli of Qilian Mountain permafrost Wells of DK-1, DK-2, DK-3, DK-4, DK-5, DK-6, DK-7 and DK-8 were located in the Muli of Qilian Mountain permafrost, which belongs to the Sanlutian mining area of the Muli coal field. It is tectonically situated in the western Middle Qilian block formed in Caledonian Movement (513e386 Ma), adjacent to the South Qilian structural zone (Fig. 1; Feng, 1997), and it is also situated in the Muli Depression of the South Qilian Bain (Fu and Zhou, 1998, 2000). The Muli coal field is the biggest coal field in Qinghai province. Its destination layer is Jurassic lacustrine coal-bearing stratum, including Jiangcang Formation (J2j) and Muli Formation (J2m) of middle Jurassic (Wen et al., 2011). It is nowadays alpine-typed permafrost in the Qilian Mountain area, and is about 1 104 km2 in area in total, among which continuous permafrost has about 2.4 to 1.5 C and island permafrost has about 1.5 to 0 C on the yearly average in the atmosphere. The permafrost is about 50e139 m in thickness (Zhou et al., 2000).
Figure 2. Structural pattern in the study area (modified from Wen et al., 2011).
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Table 1 Composition of gases from gas hydrate in DK-1 (v/v, %). Sample
Depth/m
C1
C2
C3
i-C4
n-C4
i-C5
n-C5
Cþ 6
N2
CO2
Remark
G-5-1-2 G-6-1-2 S-1-2
134 143
42.90 10.47 59.01
5.40 1.62 6.23
5.68 3.38 9.43
0.70 0.35 0.93
3.48 0.53 1.01
0.45 0.05 0.13
0.70 0.05 0.12
2.55 0.50 1.71
35.98 76.76 19.27
2.16 6.28 2.16
* * **
Note: * e samples of gas dissociated from gas hydrate; ** e samples of gas collected from the drilling mud.
Table 2 Isotopic compositions of gases from gas hydrate in DK-1 (&). Sample
Type
Depth/mbs
d13C1
d13C2
d13C3
d13i-C4
d13n-C4
d13 CCO2
dD C1
dD C2
G-5-1-1 G-6-1-1 S-1-2
* * **
134 143
50.5 39.5 47.4
35.8 32.7 35.0
31.9 30.8 31.8
31.9 31.1 31.8
31.0 30.4 30.9
18.0 18.0 17.0
262 266 268
240 254
Note: * e samples of gas dissociated from gas hydrate; ** e samples of gas collected from the drilling mud.
In the study area, the middle part is an anticline composed of Triassic stratum, and the northern and southern flanks are two synclines composed of Jurassic lacustrine coal-bearing stratum. Large thrust faults were developed on the northern and southern flanks of the anticline and synclines. In two synclines, thrust faults caused a series of further NE direction shear fractures to develop, which cut depressions into interrupted blocks. Therefore, the study area is presented with north to south belts and west to east zones (Fig. 2). Gas hydrate and its associated anomalies such as voluminous gas release from wells or from tightened core samples, relatively low temperature record within cores, etc., occur within intervals of about 133e396 mbs (meter below surface) below permafrost in the study area. The base of permafrost is around 120 m revealed by drilling, during which at that depth ice flakes were observed upon cutting fresh cores. Some samples appear visible in fissures and are white ice-like lamina in smoky gray when mixed with drilling mud. Others are rarely visible and microscopically fillings in pores. Intervals with gas hydrate flakes were encountered in cores mainly composed of mudstone, oily shale, siltstone, and fine sandstone etc. However, gas hydrate was little obtained in medium sandstone and coarse sandstone. It is deemed that gas hydrate occurrences are in close relation with fractures rather than sandstone; in the meanwhile it is constrained by gas hydrate stability zone, especially by permafrost basic features (Lu et al., 2010b).
3. Samples and analyses Gas hydrate samples were individually collected at 134 m and 143 m beneath the ground in DK-1. Upon recovery, samples were wrapped up by tin paper and placed in cloth bag, then stored in liquid nitrogen. Gas samples released from drilling mud due to gas hydrate dissociation were also collected in DK-1, which were obtained by water-gas displacement and linked to a 20 ml vacuum vial. These gas samples were then stored in a refrigerator under low temperature conditions (<5 C). After gas hydrate samples were taken into laboratory, they were put in a vacuum container with a rubber stopper and gas hydrate was then naturally dissociated under laboratory conditions. Gas samples from gas hydrate were subsequently transported into a vacuum vial by water-gas displacement. In addition, 99 mudstone and oily shale samples together with 8 coal samples were also obtained at different intervals in DK-2. Gas samples were measured with gas composition and isotope ratios, which were analyzed in the state key laboratory of gas geochemistry, Lanzhou Center for Oil and Gas Resources, Institute of Geology and Geophysics, CAS. Rock samples were measured with microscopic components, vitrinite reflectance (RO), total organic carbon content (TOC), Rock-Eval of source rock and carbon isotope of organic matter. Microscopic components observations and vitrinite reflectance measurements were based on GB/T 8899-1998 and GB/T6948-2008 of China National Standard with a microscope of LEICA DM 2500P and a photometer of MSP200 under conditions
13
Figure 3. Plot of CH4/(C2H6 þ C3H8) to d CCH4 of gases from gas hydrate in DK-1. I1: bacterial gas, I2: mixed gas of bacterial and sub-bacterial gas, I3: sub-bacterial gas, II1: oil-concomitant gas, II2: oil-cracking gas, III1. mixed gas of oil-cracking gas and coal-genetic gas, III2: mixed gas of condensate oil-concomitant gas and coal-genetic gas, IV: coal-genetic gas, V1: abiogenic gas, V2: mixed gas of coal-genetic gas and abiogenic gas.
2
13
Figure 4. Plot of d HCH4 to d CCH4 of gases from gas hydrate in DK-1. A: bacterial gas, B: transition gas from bacterial gas to thermal gas, C: oil-concomitant gas, D: condensate oil-concomitant gas, E: coal-typed gas, F: marine over-matured gas.
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Table 3 Statistics of various indicators for organic matter abundance in oily shale in the DK-2 hole. Parameter H/C TOC (%) “A” (%) THC (106) THC/TOC (%)
Extremely good >3.5
Percent 87.50%
Good
Percent
Medium
Percent
Bad
Percent
No potentials
Percent
1.7e1.3 3.5e1.0 >0.12 >500 >6
87.50% 12.50% 100.00% 100.00% 25.00%
1.3e1.0 1.0e0.6 0.12e0.06 500e250 6e3
0.00% 0.00% 0.00% 0.00% 62.50%
1.0e0.5 0.6e0.4 0.06e0.01 250e100 3e1
12.50% 0.00% 0.00% 0.00% 12.50%
1.0e0.5 <0.4 <0.01 <100 <1
12.50% 0.00% 0.00% 0.00% 0.00%
Note: TOC e total organic carbon, THC e total hydrocarbon content.
Table 4 Statistics of various indicators for kerogen types in mudstone and oily shale in DK-2. Type (Hu et al., 1991)
S1 þ S2 (kg/t)
Percentage
IH (mg/g)
Percentage
S2/S3
Percentage
D (%)
Percentage
I II1 II2 III
>20 5e20 2e5 <2
20.20% 31.31% 24.24% 24.24%
>600 250e600 120e250 <120
15.15% 19.19% 33.33% 32.32%
>20 5e20 2.5e5 <2.5
5.05% 23.23% 26.26% 45.45%
>50 20e50 10e20 <10
18.18% 24.24% 28.28% 29.29%
Type (Tissot and Welter, 1989)
S1 þ S2 (kg/t)
Percentage
IH (mg/g)
Percentage
IO (mg/g)
Percentage
D (%)
Percentage
I II III IV
>6 6e4 <2 <2
49.49% 12.12% 24.24% 24.24%
>800 800e500 <150
5.05% 19.19% 40.40%
<40 60e40 150e50
22.22% 13.13% 53.54%
>50 10e50 <10 <10
18.18% 52.53% 29.29% 29.29%
Type (van Krevelen, 1982)
S1 þ S2 (kg/t)
Percentage
IH (mg/g)
Percentage
S2/S3
Percentage
D (%)
Percentage
I II III
>20 2e20 <2
20.20% 55.56% 24.24%
>600 120e600 <2.5
15.15% 52.53% 45.45%
>5 2.5e5 <10
28.28% 26.26% 29.29%
>50 10e50 <120
18.18% 52.53% 32.32%
of oil immersion and reflection, and completed in National Coal Quality Supervision and Inspection Center. Other measurements were completed in labs of Exploration and Development Research Institute of CNPC, among which total organic carbon analysis is based on GB/T19145-2003 with a carbon & sulfur analyzer of LECO CS-400 under normal temperature and pressure conditions; RockEval analysis is based on GB/ T18602-2001 with an instrument of Rock-Eval 2 Plus imported from France (Wu, 1986); carbon isotope of organic matter follows analysis based on SY/T5238-2008 with an instrument of Finngan MAT-252 and reference gas of GBW04405.
Figure 5. Plot of IH to IO in mudstone and oily shale in DK-2.
4. Results and discussion 4.1. Gas composition and isotope ratios Gas composition of gas hydrate and that of gas samples from drilling mud due to gas hydrate dissociation are presented in Table 1, and their isotope ratios are listed in Table 2. It is shown that 13 values of CCH4 =ðCC2 H6 þ CC3 H8 Þ are less than 10 and values of d CCH4 (PDB standard, &) are more positive (relative to 55& or 50&). 13 According to plot of CCH4 =ðCC2 H6 þ CC3 H8 Þ with respective to d CCH4 , gases from gas hydrate can be divided into bacterial gas, thermogenic gas and mixed gas. In the study area, it accordingly seems that gas from gas hydrate is mainly thermogenic gas (Fig. 3). For conventional nature gas, gas source is divided into bacterial gas, mixed gas of bacterial and sub-bacterial gas, sub-bacterial gas,
Figure 6. Plot of degradation rate (D) to Tmax in mudstone and oily shale in DK-2.
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gases from gas hydrate are mainly composed of oil-concomitant gas in the study area (Fig. 4). 4.2. Gas source from mudstone, oily shale and coal
Figure 7. Plot of S2/S3 to Tmax in mudstone and oily shale in DK-2.
4.2.1. Abundance of organic matter Based on TOC data of 99 samples from mudstone and oily shale at different depths in DK-2, all samples have TOC values greater than 0.4%. According to classification by Hu et al. (1991) and Chen (1994), samples with TOC ranging from 0.4% to 0.6% account for 1.01% in total samples; those ranging from 0.6% to 1.0% account for 10.10%; those from 1.0% to 2.0% account for 23.23%; and those greater than 2.0% account for 64.65%. It shows that gas source conditions of mudstones and oily shale are mainly of high-quality in the Muli of Qilian Mountain permafrost. Certainly coal is undoubtedly a good kind of gas source rock if judged from TOC. Particularly, statistics of indicators for abundance of organic matter from 8 samples of oily shale at different depths in DK-2, is presented in Table 3. H/C ratio, TOC, chloroform “A” content, total hydrocarbon content (THC), and THC/TOC ratio suggest that most of these samples are in conditions of very good and good source rock (Hu et al., 1991; Chen, 1994).
oil-concomitant gas, oil-cracking gas, mixed gas of oil-cracking gas and coal-genetic gas, mixed gas of condensate oil-concomitant gas and coal-genetic gas, coal-genetic gas, abiogenic gas, mixed gas of coal-genetic gas and abiogenic gas (Schoell, 1980, 1983; Whiticar et al., 1986; Whiticar, 1999). Plot of CCH4 =ðCC2 H6 þ CC3 H8 Þ to d13 CCH4 indicates that gases from gas hydrate and those collected from drilling mud are nearly oil-concomitant gas, occasionally with a little mixed gas of condensate oil-concomitant gas and coalgenetic gas in the study area (Fig. 3). Based on features of hydrogen isotope of natural gas from 2 onshore sedimentary basins in China, it is believed that d HCH4 (SMOW standard, &) in natural gas generated from marine (saline lake) source rock are generally more positive (relative to 190&), whereas, those from freshwater environment are more negative 2 (relative to 190&) (Shen and Xu, 1993). On the basis of d HCH4 and 13 d CCH4 values, normal nature gases from onshore sedimentary basins in China can be divided into bacterial gas, transition gas from bacterial gas to thermal gas, oil-concomitant gas, condensate oilconcomitant gas, coal-typed gas, and marine over-matured gas 2 13 (Shen and Xu, 1991, 1993). Plot of d HCH4 to d CCH4 indicates that
4.2.2. Type of organic matter Rock-Eval data of source rocks from 99 samples of mudstone and oily shale at different depths in DK-2 are presented in Table 4. Parameters of hydrocarbon-generation potential (S1 þ S2), hydrogen index (IH), oxygen index (IO), type index (S2/S3), and degradation rate (D) of mudstone and oily shale indicate that types of organic matter range from type I, type II and type III at gas hydrate bearing intervals (van Krevelen, 1982; Tissot and Welter, 1989; Hunt, 1991), of which type II (II1 and II2) is slightly predominant; type III comes second, and type I has a small population. Plots of IH to IO, D to Tmax and S2/S3 ratio to Tmax in mudstone and oily shale clearly display this kind of features (Figs. 5e7). Carbon isotopes in kerogen of 38 samples from mudstone and oily shale in DK-2 (Table 5) indicate that organic matter (OM) is composed of standard sapropelic OM (type I1), humic sapropelic OM (type I2), intermediate or mixed OM (type II), sapropelic humic OM (type III1) and standard humic OM (type III2) (Huang et al., 1984). For organic microscopic component observations from samples of oily shale in DK-2 (Table 6), results indicate that all samples are mainly composed of exinite and secondly of sapropelinite, although
Table 5 Characteristics of carbon isotope in kerogen of mudstone and oily shale in DK-2. Type (Huang et al., 1984)
III2
III1
II
I2
I1
d13C in kerogen (&PDB)
22.5 to 24.5 28.95%
24.5 to 26.0& 10.53%
26.0 to 27.0& 10.53%
27.0 to 28.0& 18.42%
28.0 to 29.0 5.26%
Percentage
Table 6 Characteristics of microscopic components and RO in oily shale in DK-2. Sample
Sapropel
Exinite
Vitrinite
Inertinite
Type index
Type
RO (%)
Maturity
d13C in kerogen
DK-2-S-1 DK-2-S-2 DK-2-S-3 DK-2-S-4 DK-2-S-5 DK-2-S-6 DK-2-S-7 DK-2-S-8
10 5 10 5 20 5 30 5
83 87 83 85 70 83 60 80
4 5 5 6 8 9 7 10
3 3 2 4 2 3 3 5
46 42 46 39 47 37 57 33
II1 II1 II1 II2 II1 II2 II1 II2
0.35 0.62 0.78 0.58 0.70 0.55 0.66 0.60
Immure Lowly mature Mature Lowly mature Lowly mature to mature Lowly mature Lowly mature Lowly mature
32.7 32.0 30.2 31.1 27.7 30.3 32.8 30.4
(&PDB)
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Figure 8. Plot of H/C to O/C in kerogen of oily shale in DK-2.
they all include vitrinite and inertinite, suggesting that their organic matter corresponds to type II1 and type II2, with relatively major type II1. Values of carbon isotopes in kerogen from these oily shale samples are less than 30.0&, except for one with 27.7& (Table 6), indicating that their kerogen is superior to type I. Ratios of H/C and O/C in kerogen from oily shale samples also illustrate that kerogen is preferable type I (Fig. 8). Of course, different parameters do not produce the same judgment for type of organic matter. It is believed that ratios of H/C and O/C in kerogen and S2/S3 in sedimentary rocks are effective parameters in determining kerogen types while IH has a high correlation with ratio of H/C (Hou and Tian, 1990). Thus, type of organic matter from oily shale samples is at least between type I and type II. Organic microscopic components observations and carbon isotopes in kerogen from coal samples in DK-2 are listed in Table 7. Organic microscopic components of coal samples are mainly composed of vitrinite, whose contents are mostly more than 60%, and secondly of inertinite and exinite with general contents of no more than several percent. In addition, organic microscopic components have a certain amount of mineral matter. These features indicate that their type of coal samples is mainly type III. Carbon isotopes in kerogen from six coal samples are between 23.7& and 24.8&. Based on criterion by Huang et al. (1984), type of organic matter in coal samples is type III1 and III2. 4.2.3. Thermal evolution of organic matter Plot of IH to Tmax in mudstone and oily shale in DK-2 demonstrates that thermal evolution of organic matter is mostly in mature
Figure 9. Plot of IH to Tmax in mudstone and oily shale in DK-2.
stage and only several samples are in highly mature or over mature stage (Fig. 9). RO values of oily shale in DK-2 are between 0.35% and 0.78% (Table 6), showing that most of samples are lowly mature, except for few samples being immature or intermediately mature. RO values of coal samples in DK-2 are between 0.86% and 1.13% (Table 7), indicative of being in mature stage. Based on thermal evolution stages of organic matter, namely five stages of being immature, oil-generation, condensate oil, wet gas, and dry gas (van Krevelen, 1982; Huang et al., 1984; Tissot and Welter, 1989; Hu et al., 1991; Hunt, 1991; Chen, 1994), mudstone, oily shale and coal samples are at the most in condensate oil stage, without reaching wet gas and dry gas stages in the study area, suggesting they have not yet produced abundant hydrocarbon gases to meet gas source conditions of gas hydrate formation. It should be noted that mudstone and oily shale samples were restricted within gas hydrate stability zone due to drilling limitation in this study. Therefore, it is not clear whether or not samples of mudstone and oily shale in deeper strata meet for gas source conditions of gas hydrate formation. Given that deeper strata would experience higher thermal evolution, mudstone and oily shale in deeper strata might be a gas source for gas hydrate. In the study area, coal beds occurred in shallow strata (Wen et al., 2011), where it is nearly overlapped with gas hydrate stability zone; no coal beds are expected in deep strata; therefore, coal beds is difficult to serve as a major gas source for gas hydrate formation.
Table 7 Characteristics of microscopic components and carbon isotope in kerogen of coal in DK-2. Sample
DK-2-C-1 DK-2-C-2 DK-2-C-3 DK-2-C-4 DK-2-C-5 DK-2-C-6 DK-2-C-7 DK-2-C-8
d13C in kerogen
Microscopic components Mineral
Exinite
Vitrinite
Inertinite
31.3% 43.6% 4.0% 5.5% 15.5% 2.1% 1.4% 1.5%
0.3% 0.0% 5.2% 10.2% 3.6% 5.3% 0.3% 1.2%
65.6% 56.4% 89.5% 79.0% 62.8% 85.2% 66.7% 69.9%
2.8% 0.0% 1.2% 5.3% 18.1% 7.4% 31.6% 27.4%
Type
RO (%)
Maturity
24.8 24.8
III1 III1
24.2
III2
24 24.5 23.7
III2 III2 III2
0.96 0.94 0.86 0.89 1 0.96 1.13 1.06
Mature Mature Mature Mature Mature Mature Mature Mature
(&PDB)
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347
Figure 10. Oil and gas indications within gas hydrate intervals in DK-2. Note: left e oil stain seepage from fissures of carbonate lamina inter-bedded with mudstone at about 298.50 m; middle e oil stains from interfaces between carbonate lamina and mudstone at about 299.62 m; right e asphalt observed within fissure of silt mudstone at about 316.30 m.
4.3. Connection of gas hydrate with oil & gas indications Composition and isotope of gases from gas hydrate and its associated gas in the Muli of Qilian Mountain permafrost indicate that gas source for gas hydrate is mainly from deep oil or crude oil concomitant gases. Abundance, type and thermal evolution of organic matter in mudstone, oily shale and coal indicate that they did not supply sufficient gases for gas hydrate within gas hydrate stability zone. Gas hydrate was encountered in DK-1, DK-2, DK-3, DK-7 and DK-8, where various gas hydrate-associated anomalies together with oil & gas indications (oil stain, oil patch, oil immersion) had been observed during the drilling. For example, oil stains seep from fissures or interfaces between carbonate lamina and mudstone at about 298.50 m and 299.62 m, and asphalt occurs within fissure of silt mudstone at about 316.30 m in DK-2 (Fig. 10). Based on pyrolysis analysis of reservoir rock with oil & gas indications (Fig. 11), it proves that oil & gas indications which are often associated with gas hydrate are common on the one hand; it also indicates that crude oil represented by these oil & gas indications are mainly composed of intermediate-weighted oil with a small amount of heavy oil, over-heavy oil and even asphalt. On the other hand, close association of oil & gas indications with gas hydrate provides a sound proof for gas source of gas hydrate coming from oil or crude oil associated gases, which was deduced by compositions and isotopes of gases from gas hydrate and its
Figure 11. Classification diagram for gas hydrate-associated oil & gas indications in DK-2. Note: O1 ¼ (S0 þ S1)/(S0 þ S1 þ S21 þ S22 þ S23), O2 ¼ (S21)/(S0 þ S1 þ S21 þ S22 þ S23), O3 þ 4 ¼ (S22 þ S23)/(S0 þ S1 þ S21 þ S22 þ S23).
associated gas. On the contrary, in DK-4, and especially in DK-5 and DK-6, gas hydrate samples had not been obtained although various anomalies had been observed; in the meanwhile, oil & gas indications were not watched. Therefore, it is deduced that gas source of gas hydrate has a close relationship with oil & gas indications in the Muli of Qilian Mountain permafrost. 5. Conclusions (1) Gas source for gas hydrate is mainly from deep or crude oilconcomitant gases in the Muli of Qilian Mountain permafrost. (2) Gas hydrate is closely connected with oil & gas indications in the study area. Acknowledgments This work is jointly financed by National Basic Research Program of China (973 Program, grant No.: 2009CB219501), National Natural Science Foundation of China (NSFC, grant No.: 41073040), National Special Program for Gas Hydrate Exploration and Test-production (grant No.: GZHL20110310), and Ministry of Finance (MOF, grant No.: K1003). References Bily, C., Dick, J.W.L., 1974. Naturally occurring gas hydrates in the Mackenzie Delta, N.W.T. Bulletin of Canadian Petroleum Geology 22, 340e353. Chen, R., 1994. Geology of Oil and Natural Gas. China University of Geosciences Press, pp. 1e262 (in Chinese). Collett, T.S., Lee, M.W., Agena, W.F., Miller, J.J., Lewis, K.A., Zyrianova, M.V., Boswell, R., Inks, T.L., 2011. Permafrost-associated natural gas hydrate occurrences on the Alaska North Slope. Marine and Petroleum Geology 28 (2), 279e 294. Dallimore, S.R., Collett, T.S., 2005. Scientific results from the Mallik 2002 gas hydrate production research well program, Mackenzie Delta, Northwest Territories, Canada. Geological Survey of Canada Bulletin 585, 1e140. Dickens, G.R., 2004. Global change: hydrocarbon-driven warming. Nature 429 (6991), 513e515. Feng, Y., 1997. Investigatory summary of the Qilian orogenic belt, China: history, presence and prospect. Advances in Earth Science 12 (4), 307e314 (in Chinese with English abstract). Fu, J., Zhou, L., 1998. Carboniferous-Jurassic stratigraphic provinces of the southern Qilian Basin and their petro-geological features. Northwest Geoscience 19 (2), 47e54 (in Chinese with English abstract). Fu, J., Zhou, L., 2000. Triassic stratigraphic provinces of the southern Qilian basin and their petro-geological features. Northwest Geoscience 21 (2), 64e72 (in Chinese with English abstract). Hou, D., Tian, S., 1990. A study of relationship between kerogen parameters and the validity in the classification of kerogen by multi-variate statistic analysis. Petroleum Exploration and Development 5, 33e37 (in Chinese with English abstract). Hu, J., Huang, D., Xu, S., Gan, K., Xue, S., Ying, F., 1991. Continental Petroleum Geology Theoretical Principle in China. Petroleum Industry Press, pp. 1e322 (in Chinese). Huang, D., Li, J., Zhang, D., 1984. Kerogen types and study on effectiveness, limitation and interrelation of their identification parameters. Acta Sedimentologica Sinica 2 (3), 18e32 (in Chinese with English abstract).
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