Article pubs.acs.org/EF
Source Rocks for Gases from Gas Hydrate and Their Burial Depth in the Qilian Mountain Permafrost, Qinghai: Results from Thermal Stimulation Zhengquan Lu,*,†,‡ Xiaohua Xue,‡ Zewen Liao,§ and Hui Liu† †
Oil & Gas Survey, China Geological Survey, Beijing 100029, China Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 10037, China § Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China ‡
ABSTRACT: The composition and isotope ratio of gases from gas hydrate and organic geochemistry of mudstone, oil shale, and coal within a gas hydrate-bearing interval in the Qilian Mountain permafrost suggest that gases from gas hydrate are derived from the deep, but the lithology and burial depth of source rocks for gases from gas hydrate are not yet known. In this study, samples of mudstone, oil shale, and coal were analyzed with thermal stimulation within the gas hydrate-bearing interval in the Qilian Mountain permafrost. Results showed that gas composition and carbon isotopes of hydrocarbon gases thermally stimulated from mudstone at temperatures of 350−400 °C and from oil shale at temperatures of 380−400 °C are similar to those of gases from gas hydrate, indicating that the deep mudstone and oil shale are potential source rocks for gases from gas hydrate. The gas composition of hydrocarbon gases thermally stimulated from coal was similar to that of gas hydrate, but their carbon isotope values were very different, suggesting that coal was not necessarily related to a gas source for gas hydrate. The burial depths of source rocks for gases from gas hydrate were further estimated to be 1500−2000 m on the basis of the relationship among thermally stimulated temperature, vitrinite reflectance, and burial depth in the study area.
1. INTRODUCTION
Importantly, the origin of the gases in gas hydrate is a decisive factor not only for gas hydrate formation but also for gas hydrate prediction or exploration in the Qilian Mountain permafrost. Analyses of the composition and isotope ratio of gases from gas hydrate indicate that hydrocarbon gases are thermogenic in the Qilian Mountain permafrost.12 The organic geochemistry (parameters for the abundance, type, and thermal evolution of organic matter) of mudstone, oil shale, and coal in the depth interval where gas hydrate can be found suggests that these strata, especially within the gas hydrate stability zone, play little role in gas sources for gas hydrate, so that gases from gas hydrate are likely to be derived from deep in the Qilian Mountain permafrost.15 At present, the kind(s) of lithology (mudstone, oil shale, or even coal) and the burial depth at which gas sources for gas hydrate exist are not yet known. In this paper, core samples of mudstone, oil shale, and coal in the gas hydrate-bearing interval were analyzed with thermal stimulation in vacuum glass tubes heated step by step at various temperatures in the Qilian Mountain permafrost. The aim is to determine equivalent gas source rocks by comparing the composition and carbon isotope fraction of gases from gas hydrate and those of gases discharged from thermal stimulation at different temperatures. Furthermore, the aim is to ascertain their potential burial depth by analyzing the vitrinite reflectance (RO) of samples before and after thermal stimulation at specific stimulation temperatures.
Gas hydrate is formed from water and gas molecules under lowtemperature and high-pressure conditions.1−3 It is generally believed to represent approximately 0.1−2.1 × 1016 m3 of methane, 2 times greater than the volume of conventional fossil fuels (coal, oil, and natural gas) around the world.4 Gas hydrate has been reported to be found in 132 locations worldwide, including 123 in subsurface sediments and nine in permafrost.5 In 2007, gas hydrate was first discovered in the Shenhu area of the northern South China Sea.6,7 In 2008 and 2009, gas hydrate was sampled in the Muli area of the Qilian Mountain permafrost in China.8 Drilling results showed that gas hydrate occurred mainly in fissures and secondarily in pores within mudstone, oil shale, siltstone, fine sandstone, etc., within the depth interval of 133− 396 m. It was found that gas hydrate was vertically discontinuously distributed in different drilling wells and horizontally irregularly arranged and appeared to be uncorrelated with lithology and strongly controlled by fissures in the Qilian Mountain permafrost.9−11 Meanwhile, the gas composition of gas hydrate was very complex, consisting not only of methane but also of ethane, propane, carbon dioxide, and even butane in the Qilian Mountain permafrost.10−12 Accordingly, this kind of gas hydrate was very complicated and peculiar. Unlike gas hydrate found in the Mackenzie delta in Canada,13 the North Slope of Alaska in the United States,14 and the Messoyakha gas field in Russia,5 it occurred in low-latitude (∼38° N), high-altitude (∼4000 m), relatively thin permafrost or frozen rock (∼100 m) or consolidated and lithified rock (Jurassic) with a complex gas composition. © 2013 American Chemical Society
Received: June 8, 2013 Revised: September 30, 2013 Published: October 7, 2013 7233
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Figure 1. Geology in the Muli coal field of the Qilian Mountain permafrost (modified from refs 17 and 20).
Table 1. Descriptions and Geochemical Features of the Samples sample
well
M-1 M-2 OS-3 OS-4 OS-5 OS-6 OS-7 C-8 C-9
DK-2
a
DK-3
DK-3
depth (m) 276 476.9 145 238 250 374 447 469.8 497.1
lithology
TOC (%)
tape of organic matter
maturity of organic matter
relationship with gas hydratea within GHBZ below GHBZ within GHBZ within GHBZ below GHBZ within GHBZ below GHBZ below GHBZ below GHBZ
black gray to black mudstone
2.56−8.11
I−II1
0.35−0.78
brownish dark to brown oil shale
2.08−5.76
II1−II2
0.55−0.84
black coal
−
III1−III2
0.86−1.13
GHBZ, gas hydrate-bearing zone.
Jurassic.20 The alpine-type permafrost covers ∼1 × 104 km2 in the Qilian Mountain area; its continuous permafrost has a yearly average atmospheric temperature of approximately −2.4 to −1.5 °C, and the island permafrost has a yearly average atmospheric temperature of approximately −1.5 to 0 °C. The thickness of the permafrost is approximately 50−139 m.21 The study area is located in the Sanlutian sag of the Muli coal field. In the study area, the middle part is an anticline composed of a Triassic stratum, and the northern and southern flanks are two synclines composed of a Jurassic lacustrine coal-bearing
2. GEOLOGICAL SETTING The study area is located in the Muli coal field of the Qilian Mountain permafrost. It is tectonically situated in the western Middle Qilian block formed in the Caledonian Movement (513−386 Ma), adjacent to the South Qilian structural zone,16,17 and it is also situated in the Muli Depression of the South Qilian Basin.18,19 The Muli coal field is the biggest coal field in Qinghai province. Its destination layer is a Jurassic lacustrine coal-bearing stratum, including the Jiangcang Formation (J2j) and the Muli Formation (J2m) of the middle 7234
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Figure 2. Sampling locations in the column of DK-2 and DK-3. 156.3−156.6, 235.0−291.3, and 377.3−387.5 m beneath the surface) and three gas hydrate-bearing intervals in DK-3 (133.0−156.0, 225.1− 240.0, and 367.7−396.0 m beneath the surface).9 Mudstone and oil shale samples were obtained from 276 to 476.8 m and from 145 to 447 m, respectively, located within and beneath the gas hydrate-bearing intervals. Coal samples were taken from 469.8 to 497.1 m beneath the gas hydrate-bearing intervals. The organic geochemistry of samples shows that the TOCs of mudstone and oil shale are 2.56−8.11 and 2.08−5.76%, respectively, suggesting that their organic matter is abundant, their types of organic matter are I−II1 and II1−II2, respectively, and their maturities of organic matter are both low to middle. The type of organic matter for coal samples is III1−III2, and its maturity is low to middle.15 3.2. Experiment. Thermal stimulation of samples was conducted in the State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. Its main theoretical bases are the thermal degradation principle of kerogen and the time−temperature compensation principle of thermal evolution.22−27 First, samples were placed in vacuum glass tubes; then the glass tubes were put into a muffle furnace and heated to a certain
stratum (Figure 1). 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 (Figure 1). Gas hydrate drilling wells DK-1−DK-8 are situated in the west of the Sanlutian sag of the Muli coal field.
3. SAMPLES AND EXPERIMENT 3.1. Samples. In this study, samples were collected from gas hydrate drilling wells DK-2 and DK-3 in the Qilian Mountain permafrost, Qinghai province. They are blackish gray or black mudstone (M-1 and M-2), brown or dark brown oil shale (OS-3− OS-7), and coal (C-8 and C-9), and their details are listed in Table 1. In profile, there are three or four gas hydrate-bearing intervals revealed by drilling in DK-2 and DK-3 (Figure 2). For example, four gas hydrate-bearing intervals were observed in DK-2 (144.4−152.0, 7235
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Table 2. Gas Compositions and Carbon Isotopes of Thermally Stimulated Gases from Mudstonea sample M-1
M-2
a
temp (°C)
C1 (mL/g)
∑C2−C5 (mL/g)
∑C1−C5 (mL/g)
CO2 (mL/g)
dry coefficient (%)
C1/(C2 + C3)
δ13C1 (‰)
δ13C2 (‰)
δ13C3 (‰)
250 300 350 380 400 450 500 250 300 350 380 400 450 500
0.00 0.02 0.33 1.11 2.53 4.94 7.70 0.00 0.01 0.09 0.36 0.69 1.58 1.89
0.00 0.03 0.38 1.37 3.10 3.79 2.74 0.00 0.01 0.07 0.19 0.32 0.40 0.24
0.01 0.05 0.71 2.48 5.63 8.73 10.44 0.01 0.02 0.15 0.56 1.01 1.97 2.12
0.77 0.93 0.73 1.08 1.71 1.82 5.14 0.36 0.73 0.97 1.21 1.67 3.36 4.75
53.57 48.15 46.47 44.73 45.00 56.58 73.76 35.82 40.20 56.78 65.13 67.99 80.01 88.83
2.21 1.62 1.26 1.09 1.06 1.41 2.86 2.41 1.82 1.89 2.26 2.71 4.25 8.13
\ −47.74 −44.72 −45.04 −47.01 −44.86 −39.44 \ −38.37 −38.12 −36.32 −35.63 −30.75 −29.25
\ −35.69 −37.26 −37.81 −39.48 −35.28 −30.63 \ −29.26 −29.12 −27.45 −26.55 −23.82 −22.3
\ −28.62 −35.56 −36.14 −36.87 −28.04 − \ −25.3 −27.45 −26.76 −26.3 − −
Legend: \, no data detected; , no reliable data because of interference.
Figure 3. Yields of methane and heavy hydrocarbons thermally stimulated from samples at various temperatures: (A) samples of mudstone, (B) samples of oil shale, and (C) samples of coal. temperature, and finally, thermally stimulated products were collected. For the thermally stimulated products, the gas composition and yield together with the carbon isotope fraction of the hydrocarbon gases were analyzed. Detailed experimental steps were as follows. (1) Weigh samples of approximately 0.5−5 g and place them into glass tubes; then evacuate and seal the glass tubes. The weight of the samples decreases with temperature. The grain size of coal samples ranges from 2 to 3 mm, and that of mudstone or oil shale goes through 60 meshes. (2) Place glass tubes into a muffle furnace, heat them to a programmed temperature, and then keep them at a constant temperature for a certain period of time (72 h at 250, 300, 350, and 380 °C and 24 h at 400, 450, and 500 °C). The time period for keeping samples at a given temperature is based on ref 22, and during that period, more than 95% of the gases are released from the samples, which are generally considered to have arrived at a gas-yielding balance. The last step is to cool them naturally. (3) Take them out and measure gas compositions in glass tubes using a gas component analyzer and the carbon isotope fraction of the gases using a gas chromatography isotope ratio mass spectrometer. Their yield is calculated according to peak time and peak area; processes are controlled by computers. The gas component analyzer is the HP6890/Wasson-ECE gas analyzer, and the gas
chromatography and isotope ratio mass spectrometer is a GV· Isoprime·GC-IPMS instrument.
4. RESULTS AND DISCUSSION 4.1. Gas Compositions of Thermally Stimulated Gases. The gas compositions and yields of generated gases from mudstone at different temperatures are listed in Table 2. The thermal stimulation experiment of mudstones (Figure 3A) reveals that little hydrocarbon gas is generated from mudstone at 250 °C. Yields of CH4, C2H6, C3H8, iC4H10, nC4H10, iC5H12, and nC5H12 are approximately zero, while only a small amount of CO2 is generated (average of ∼0.57 mL/g). At 300 °C, except for very low yields of CH4, C2H6, and C3H8 (averages of ∼0.01 or ∼0.02 mL/g), yields of iC4H10, nC4H10, iC5H12, and nC5H12 are still nearly zero, but the amount of CO2 gradually increases (its average yield increases to 0.83 mL/g). At 350 °C, yields of CH4, C2H6, C3H8, iC4H10, nC4H10, iC5H12, and nC5H12 begin to quickly increase but that of CO2 does not change much. At 380 and 400 °C, yields of CH4, C2H6, C3H8, iC4H10, nC4H10, iC5H12, and nC5H12 continuously increase and 7236
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Table 3. Gas Compositions and Carbon isotopes of Thermally Stimulated Gases from Oil Shalea sample OS-3
OS-4
OS-5
OS-6
OS-7
a
temp (°C)
C1 (mL/g)
∑C2−C5 (mL/g)
∑C1−C5 (mL/g)
CO2 (mL/g)
dry coefficient (%)
C1/(C2 + C3)
δ13C1 (‰)
δ13C2 (‰)
δ13C3 (‰)
250 300 350 380 400 450 500 250 300 350 380 400 450 500 250 300 350 380 400 450 500 250 300 350 380 400 450 500 250 300 350 380 400 450 500
0.00 0.01 0.14 0.50 0.74 1.98 2.60 0.00 0.03 0.35 1.33 2.79 6.15 9.52 0.01 0.03 0.54 1.62 2.23 7.78 11.88 0.00 0.03 0.43 1.32 2.04 4.57 6.34 0.00 0.02 0.26 0.89 0.96 3.35 4.60
0.00 0.02 0.18 0.47 0.57 0.84 0.66 0.00 0.03 0.44 1.95 4.33 5.24 4.48 0.00 0.03 0.59 2.07 3.13 6.39 5.30 0.00 0.03 0.31 0.81 1.06 1.34 1.04 0.00 0.02 0.20 0.55 0.55 0.99 0.82
0.00 0.03 0.32 0.97 1.32 2.82 3.26 0.01 0.06 0.79 3.28 7.12 11.39 14.00 0.01 0.07 1.12 3.69 5.37 14.17 17.17 0.01 0.06 0.74 2.13 3.10 5.91 7.39 0.00 0.04 0.46 1.44 1.51 4.34 5.42
2.24 2.92 7.30 8.45 7.90 7.54 8.34 1.46 1.94 1.57 1.95 2.85 1.67 6.58 1.71 1.67 1.51 1.93 1.57 1.59 5.54 0.92 1.13 1.32 1.46 1.34 2.59 3.91 1.67 1.31 2.68 3.58 3.77 8.59 15.81
51.16 41.13 43.91 51.31 56.50 70.29 79.62 55.56 47.70 43.97 40.55 39.19 54.02 68.00 63.75 49.55 47.86 44.02 41.57 54.88 69.12 62.07 55.05 58.51 61.94 65.78 77.33 85.86 57.45 53.19 57.03 61.74 63.62 77.10 84.83
2.05 1.63 1.29 1.46 1.73 2.66 4.06 2.86 1.78 1.20 0.97 0.90 1.32 2.21 3.33 1.84 1.32 1.06 0.97 1.35 2.31 3.69 2.18 1.81 1.98 2.28 3.57 6.16 3.33 1.99 1.76 1.96 2.12 3.61 5.71
\ \ −44.06 −42.43 −42.46 −37.42 −34.76 −37.78 −44.22 −47.91 −49.4 −48.41 −47.66 −41.14 \ −43.22 −46.36 −47.2 −48.27 −45.37 −42.48 \ −37.08 −37.39 −36.12 −34.95 −29.38 −27.09 \ −37.1 −37.93 −37.82 −36.55 −32.42 −29.87
\ \ −35.17 −33.6 −32.95 −30.74 −29.52 −36.35 −38.64 −39.79 −40.43 −40.06 −36.62 −34.06 \ −33.45 −38.3 −39.14 −40.5 −35.92 −34.38 \ −31.05 −29.66 −28.33 −28.3 −25.93 −24.74 \ −30.79 −29.72 −28.98 −27.17 −26.03 −24.8
\ \ −32.95 −32.27 −30.56 −27.24 − −35.74 −37.9 −38.19 −38.31 −36.79 −31.31 − \ −25.88 −36.84 −38.01 −37.4 −31.22 − \ −28.06 −28.51 −27.46 −27.34 − − \ −30.35 −28.73 −27.28 −28.61 −23.18 −
Legend: \, no data detected; , no reliable data because of interference.
Table 4. Gas Compositions and Carbon Isotopes of Thermally Stimulated Gases from Coala sample C-8
C-9
a
temp (°C)
C1 (mL/g)
∑C2−C5 (mL/g)
∑C1−C5 (mL/g)
CO2 (mL/g)
dry coefficient (%)
C1/(C2 + C3)
δ13C1 (‰)
δ13C2 (‰)
δ13C3 (‰)
250 300 350 380 400 450 500 250 300 350 380 400 450 500
0.01 0.51 6.45 15.84 24.63 58.61 101.59 0.06 1.26 6.88 17.28 33.32 60.25 118.48
0.02 0.39 3.83 9.11 10.69 18.59 8.47 0.40 1.12 4.88 10.65 18.01 19.10 11.70
0.03 0.90 10.29 24.95 35.32 77.20 110.05 0.46 2.38 11.76 27.93 51.33 79.35 130.18
0.37 2.13 3.50 4.42 5.33 7.26 11.20 0.89 1.42 2.91 3.87 4.94 6.49 12.23
29.77 56.91 62.73 63.48 69.74 75.92 92.30 13.55 53.03 58.51 61.86 64.91 75.93 91.02
0.46 1.57 1.97 2.06 2.73 3.30 12.03 0.23 1.54 1.71 1.94 2.19 3.30 10.17
−31.39 −34.32 −36.83 −39.22 −36.33 −34.11 −28.93 \ −33.71 −36.85 −37.84 −35.15 −33.41 −29.29
−21.13 −24.66 −27.15 −28.34 −27.1 −24.17 \ −26.58 −27.33 −27.3 −26.41 −23.11 \
−24.64 −25.67 −26.61 −25.34 \ \ −18.36 −24.14 −24.34 −24.31 \
Legend: \, no data detected; , no reliable data because of interference.
1.14 mL/g, respectively. At 400 °C, their averages are 1.61, 0.78, 0.54, 0.07, 0.20, 0.03, 0.05, and 1.69 mL/g, respectively. At 450 and 500 °C, yields of CH4, C2H6, C3H8, iC4H10,
that of CO2 also gradually increases. For example, at 380 °C, the average yields of CH4, C2H6, C3H8, iC4H10, nC4H10, iC5H12, nC5H12, and CO2 are 0.74, 0.36, 0.23, 0.03, 0.08, 0.01, 0.02, and 7237
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Figure 4. Plots of δ13C1, δ13C2, and δ13C3 in thermally stimulated hydrocarbons at various temperatures.
nC4H10, and CO2 continue to increase but those of nC4H10, iC5H12, and nC5H12 gradually decrease. For example, at 500 °C, average yields of nC4H10, iC5H12, and nC5H12 decrease to approximately zero. The gas compositions and yields of generated gases from oil shale at different temperatures are listed in Table 3. The thermal stimulation experiment of oil shale (Figure 3B) shows that yields of hydrocarbon gases such as CH4, C2H6, C3H8, iC4H10, nC4H10, iC5H12, and nC5H12 are approximately zero, while a large amount of CO2 is generated (average of 1.60 mL/ g) at 250 °C. At 300 °C, yields of CH4, C2H6, and C3H8 gradually increase (averages of 0.02, 0.01, and 0.01 mL/g, respectively), the yield of CO2 does not change much (average of 1.79 mL/g), and the yields of iC4H10, nC4H10, iC5H12, and nC5H12 are small (nearly zero). At 350 °C, large yields of CH4, C2H6, C3H8, iC4H10, nC4H10, iC5H12, nC5H12, and CO2 are seen (averages of 0.34, 0.15, 0.09, 0.02, 0.03, 0.01, 0.01, and 2.88 mL/g, respectively). At 380 °C, yields of CH4, C2H6, C3H8, iC4H10, nC4H10, iC5H12, nC5H12, and CO2 quickly increase (averages of 1.13, 0.54, 0.34, 0.05, 0.11, 0.03, 0.03, and 3.47 mL/g, respectively). At 400 °C, yields of CH4, C2H6, C3H8, iC4H10, nC4H10, iC5H12, nC5H12, and CO2 change little while yields of CH4, C2H6, C3H8, iC4H10, nC4H10, and CO2 continue to increase greatly, though yields of iC5H12 and nC5H12 obviously decrease to approximately zero at 450 °C. At 500 °C, yields of CH4, C2H6, C3H8, iC4H10, and CO2 continuously increase while the yield of nC4H10 decreases to approximately zero, as do those of iC5H12 and nC5H12. The gas compositions and yields of generated gases from coal at different temperatures are listed in Table 4. The thermal stimulation experiment with coal (Figure 3C) shows yields of hydrocarbon gases of CH4, C2H6, C3H8, iC4H10, nC4H10, iC5H12, and nC5H12 together with CO2 occur, though their averages are relatively low at 250 °C. At 300 °C, yields of CH4, C2H6, C3H8, iC4H10, nC4H10, iC5H12, and nC5H12 begin to quickly increase (averages of 0.89, 0.37, 0.21, 0.08, 0.04, 0.04, and 0.01 mL/g, respectively). At 350 and 380 °C, yields of CH4, C2H6, C3H8, iC4H10, nC4H10, iC5H12, and nC5H12 greatly increase. For example, at 350 °C, average yields of CH4, C2H6, C3H8, iC4H10, nC4H10, iC5H12, and nC5H12 are 6.67, 2.34, 0.31, 0.21, 0.29, 0.08, and 0.06 mL/g, respectively, while at 380 °C, their average yields increase to 16.56, 5.31, 2.99, 0.40, 0.76, 0.18, and 0.18 mL/g, respectively. However, at 400 °C, yields of CH4, C2H6, C3H8, iC4H10, nC4H10, iC5H12, and nC5H12
obviously decrease (averages of 1.61, 7.86, 4.26, 0.55, 1.16, 0.20, and 0.28 mL/g, respectively). At 450 °C, yields of CH4, C2H6, C3H8, iC4H10, nC4H10, and iC5H12 dramatically rebound (e.g., averages reach 59.43, 12.76, 5.26, 0.54, 0.15, and 0.02 mL/g, respectively) while the yield of nC5H12 decreases to approximately zero. At 500 °C, yields of CH4, C2H6, and C3H8 continue to increase, to 110.03, 9.99, and 0.05 mL/g, respectively, but yields of iC4H10, nC4H10, iC5H12, and nC5H12 decrease to approximately zero. From low to high temperatures, yields of CO2 gradually increase to 11.71 mL/g. On the whole, gases generated from samples in the thermal stimulation experiment are mainly composed of CO2 and a small amount of hydrocarbon gases at low temperatures. As the temperature increases, contents of hydrocarbon gases increase, but amounts of CO2 change relatively little. The hydrocarbon gases are mainly methane and quite a few heavy hydrocarbon gases. Their drying coefficients are
