Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
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The Resource Evaluation of Jurassic Shale in North Fuling Area, Eastern Sichuan Basin, China Xiao Wang,† Sheng He,*,† Xiaowen Guo,† Baiqiao Zhang,‡ and Xuehui Chen‡ †
Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan 430074, China ‡ Research Institute of Exploration and Development, Jianghan Oilfield Branch Company, Sinopec, Wuhan 430223, China ABSTRACT: Light oil and natural gas are commonly found in Jurassic shales in the North Fuling area of Sichuan Bain in China. The main source rocks of the study area are shales from Lower−Middle Jurassic lacustrine layers including the Lianggaoshan Formation (J2l) and the Ziliujing Formation (J1z), which are further divided into the Da’anzhai (J1zD), Ma’anshan (J1zM), and Dongyuemiao (J1zDY) members. Multiple geochemical approaches and tests including kerogen elements, kerogen composition, vitrinite reflectance (VR), total organic carbon (TOC) content test, Rock-Eval pyrolysis, gas chromatography and gas chromatography−mass spectrometry, methane concentration, gas composition, and stable isotopes were employed to determine the geochemical characteristics of the source, oil and gas samples. J1zD shale demonstrates the best abundance and type of organic matter among all four strate and tends to be the most favorable source of hydrocarbons in this area. Crude oil in Jurassic strata is a waxy light oil. Oil samples and shale extracts are nonbiodegraded, and dominated by short-chain n-alkanes, maximizing around C12−C15. Natural gas in Jurassic shales can be categorized as wet gas. δ13C values in different components indicate the gas samples are oil-associated gas which probably originated from the cracking of crude oil. Source correlations suggest the oil and gas are probably generated from J1zD shale. The analysis leads to the general conclusion that the Jurassic shale is more favorable for conventional resources than shale gas.
1. INTRODUCTION The exploitation of shale gas resources in recent years has been great progress in the petroleum exploration and development in Sichuan Basin, China.1−3 The role shale gas plays in the marine organic-rich Lower Silurian Longmaxi and Upper Ordovician Wufeng shale has been highly successful in the Fuling gas field (also reported as Jiaoshiba area, Jiaoshiba shale gas field; for location, see Figure 1) of Eastern Sichuan Basin.4−11 Shale in the Fuling gas field features by high organic matter abundance (total organic carbon of greater than 2%), high maturity (VR 2.2−3.0%Ro), and thick effective layers (38−45 m organic-rich section).12−15 Gases in this field are typically dry gas, with a CH4 content range of 97.9−98.9%, and δ13C1 values range from −30.7‰ to −28.4‰, which are generally heavier than those of typical shale gas fields in China and worldwide.16−20 Oil and gas are found in lacustrine shales of Lower−Middle Jurassic strata in North Fuling area, which lies to the north of the Fuling gas field. Because the Fuling gas field is a great success for shale gas exploration in the Sichuan Basin, the appearance of oil and gas in North Fuling gives rise to the possibility of shallow shale gas accumulations in neighboring areas.21−27 Thorough investigations of the stratigraphy, lithology, petrography, and geochemistry were launched to study the properties of source rocks and hydrocarbons, aiming to determine the types of petroleum accumulation and evaluate the resource potential of Lower−Middle Jurassic lacustrine strata in theNorth Fuling area. In this study, 42 shale samples of two wells, six oil samples from four wells, and seven gas samples from two wells were selected to determine the source rock and hydrocarbon characteristics in the North Fuling area. Multiple geochemical © XXXX American Chemical Society
approaches including the measurements of kerogen elements and composition, vitrinite reflectance, total organic carbon, oil and gas composition, stable isotopes, and gas chromatography and analysis were carried out to evaluate the resource characteristics of Jurassic shales in the North Fuling area.
2. GEOLOGICAL SETTINGS OF THE NORTH FULING AREA 2.1. Structural Settings. The Sichuan Basin is an irregular rhombus shaped sedimentary basin in southwest China. It covers an area over 2.3 × 105 km2 and is generally divided into eight structural units: the Songpan-Ganzi Fold Belt, the Longmenshan Nappe, the Northern Sichuan Fault-Fold Belt, the Central Sichuan Block, the Eastern Sichuan Fold Belt, the Micang Shan Fold Belt, the Emei-Washan Block-faulted Belt, and the Loushan Fold Belt.28,29 The North Fuling area is in the Eastern Sichuan Fold Belt (see the structural location in Figure 1). As illustrated in the section AB in Figure 1, the thickness of the strata is stable across the area and no major fault is involved, indicating the relatively steady tectonic environment from the Jurassic until the present. 2.2. Stratigraphy and Lithology. The targetd strata of the study area consist of Lower−Middle Jurassic lacustrine rocks, which include from bottom to top of the Ziliujing Formation (J1z) and the Lianggaoshan Formation (J2l). The Ziliujing Formation is divided from bottom to top into the Dongyuemiao Member (J1zDY), the Ma’anshan Member Received: October 12, 2017 Revised: December 27, 2017 Published: January 23, 2018 A
DOI: 10.1021/acs.energyfuels.7b03097 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
Figure 1. Structural location of North Fuling area (top), details of wells and Da’anzhai shale thickness in this area (middle), and a section crossing Well 101, Well 3-2, and Well FS1, illustrating the stratigraphy (bottom).
the maximum buried depth of −4500 m reached at −97 Ma, J1z became thermally late mature for oil. Afterward, the strata experienced steady but mild uplifting in the late Cretaceous, and some of the Cretaceous formations were eroded. The uplifting became substantial, and in the Eocene and the Cretaceous formations were almost eroded up in North Fuling and its adjacent area, forming J1z lifted to 2500−2700 m in depth and held the temperature of −80 °C in the present (Figure 3). The thermal maturity of J1z has remained 1.0−1.3% because of 100 Ma.30
(J1zM), and the Da’anzhai Member (J1zD). The formation thickness of J1zDY, J1zM, and J1zD ranges are 45−70 m, 35−65 m, and 70−90 m, respectively. The lithological composition of these strata is mainly shale, siltstone, and a small portion of limestone (Figure 2). Shales are revealed in both J2l and J1z, among which J1zD shale has the largest thickness, ranging from 15 to 57 m. The thickness of J1zD shale exhibits a decreasing trend from east to west and ranges from 35 to 45 m in the well area (see the thickness contour map in Figure 1). The largest thickness of a single layer shale appears in Well XL2, which reaches 22 m. 2.3. Burial and Thermal History. Sediments in the North Fuling area deposited rapidly during the Jurassic era. The depositional speed slowed down in the Early−Middle Cretaceous, and at the end of this period (approximately 97 Ma) the bottom of the Jurassic strata reached the highest paleo temperature of over 140 °C. The Lower Jurassic Ziliujing Formation (J1z) entered the early mature stage from −149 Ma at the depth of top surface of −2400 m; then the mid-mature stage started from −132 Ma with the depth of −3300 m. After
3. SAMPLING AND ANALYTICAL METHODS A series of approaches have been applied to Jurassic core samples and oil/gas samples to evaluate the geochemical characteristics and hydrocarbon potential in the North Fuling area. The 42 shale samples of J1zD and J1zDY from cores of two wells (Well 3-2 and Well 101), six oil samples of J1zD from four wells (Well FS1, Well HF-1, Well 2-2, and Well 101), and seven gas samples from two wells (Well 2-2, Well 6-2) were selected to determine the source rock and hydrocarbon characteristics. Total organic carbon content B
DOI: 10.1021/acs.energyfuels.7b03097 Energy Fuels XXXX, XXX, XXX−XXX
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following the Chinese Industry Standard SY/T 5124-2012.34 Saturated fractions of shale extracts and oil samples were analyzed using an Agilent 7890A/5975 GCMSD following the Chinese Technical Standard GB/T 13610-2003.35 Stable isotopes (δ13C ‰ VPDB, δD ‰ SMOW, and δ18O ‰ SMOW) of shale extracts (group component), oil (group component), and gas (molecular component) samples and the composition measurements of gas samples were analyzed using a MAT 253 Plus stable isotope ratio mass spectrometer and a Thermo Scientific Delta V isotope ratio mass spectrometer following the Chinese Technical Standard GB/T 18340.2-2010.36
4. DATA AND RESULTS The physical property tests of the crude oil samples reveal that the samples are white to light yellow in color, have a wax content of 9.52−10.24%, and solidify at around 24−26 °C. The density of oil samples ranges from 0.80 to 0.82 g/cm3 at 20 °C. The light components (C1−C10) in the oil samples take portions of 5−16%. The molecular compositions of the gas samples reveal the samples are wet gas, as the concentrations of methane in hydrocarbon components range from 36.89% to 94.27% (details, see Gao et al.30). Other test results are summarized in the following: 4.1. TOC, Rock-Eval Pyrolysis, and Kerogen Tests of Shale Samples. The TOC values of the samples range from 0.1% to 2.28%, with an average of 1.02%. More than 50% of the TOC values from the samples of J1l, J1zD, and J1zDY are above 1.0%, while those of J1zM are relatively lower. Over 80% of J1zD samples have the TOC value greater than 1.0%, and 13% of the values is greater than 2%. The S1 data of the samples ranges from 0.04 to 1.90 mg/g, with an averaged value of 0.74 mg/g. The S2 data of the samples ranges from 0.08 to 3.55 mg/ g and averaged at 1.50 mg/g. The sum of these two parameters (S1+S2) can represent the hydrocarbon generation potentials in source rocks. Over 85% of J1zD samples have an (S1+S2) value greater than 2.0 mg/g, and following is the J1zDY, 50% of which are above 2.0%, and J1l has the lowest (S1+S2) value (Table 1). Both in J2l and J1zD source rock samples, the major components of kerogen are liptinite and vitrinite, which takes up 50−70% and 30−50% of the total weight, respectively (Figure 4). Vitrinite reflectance (VR) data in the Jurassic source rock samples ranges from 1.0 to 1.3%Ro. 4.2. GC-MS Analysis of Shale and Oil Samples. The oils and shale extracts are nonbiodegraded, and dominated by short-chain n-alkanes, maximizing around C12−C15. Pr and Ph
Figure 2. Summary stratigraphy column of the aimed strata in North Fuling area. (TOC, % w/w) was analyzed through a Leco CS-200 carbon/sulfur analyzer following the Chinese Technical Standard GB/T 191452003.31 Rock-Eval pyrolysis was carried out through a OCE-II oil−gas evaluation workstation following Chinese Technical Standard GB/T 18602-2012.32 Kerogen maceral composition (sapropelic, vitrinite, sapropelinite, exinite, % vol/vol) was determined using a Leica DM4500P high-end polarization microscope following the Chinese Industry Standard SY/T 5125-2014.33 Vitrinite reflectance (VR, %Ro) analysis was obtained using a Leica MSP200 microspectrophotometer
Figure 3. Burial and thermal history concentrating Jurassic strata of Well 3-2 in the North Fuling area (modified from Gao et al.30 Well location see Figure 1). C
DOI: 10.1021/acs.energyfuels.7b03097 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 1. TOC and Rock-Eval Pyrolysis Data of the Shale Samples well
depth (m)
stratum
TOC (%)
Tmax (°C)
S1 (mg/g)
S2 (mg/g)
S1+S2 (mg/g)
HCI (mg/g)
HI (mg/g)
3-2
2157.4 2160.2 2181.2 2185.9 2203.2 2205.1 2208.4 2211.1 2213.0 2215.2 2221.5 2226.6 2227.1 2228.2 2231.1 2233.7 2235.7 2237.8 2242.7 2258.7 2260.4 2264.5
J2l J2l J2l J2l J1zD J1zD J1zD J1zD J1zD J1zD J1zD J1zD J1zD J1M J1M J1M J1M J1M J1M J1M J1M J1M
0.36 0.84 0.10 2.28 0.65 0.47 0.74 0.96 0.22 1.57 1.20 1.25 1.44 0.78 2.05 0.13 1.23 1.48 1.11 0.85 2.06 0.78
479 471 453 379 452 453 398 336 354 356 464 460 467 470 446 416 459 468 472 459 345 486
0.10 0.45 0.05 0.04 1.78 0.76 1.90 0.70 0.95 0.64 1.50 0.45 0.21 0.20 0.36 0.20 0.36 0.04 0.05 0.14 0.45 0.04
0.21 0.93 0.11 0.08 3.55 2.17 3.45 1.46 2.26 1.66 3.07 0.97 0.66 0.57 1.21 0.89 0.99 0.15 0.13 0.37 1.13 0.11
0.31 1.38 0.16 0.12 5.33 2.93 5.35 2.16 3.21 2.30 4.57 1.42 0.87 0.77 1.57 1.09 1.35 0.19 0.18 0.51 1.58 0.15
27.78 53.57 38.46 40.00 78.07 61.79 128.38 107.69 85.59 75.29 72.82 57.69 32.81 42.55 53.73 48.78 48.65 17.39 20.00 51.85 46.88 18.18
58 111 85 80 156 176 233 225 204 195 149 124 103 121 181 217 134 65 52 137 118 50
101
2144.1 2146.5 2147.2 2147.5 2148.3 2150.4 2152.9 2153.4 2155.0 2156.2 2157.2 2158.0 2158.6 2240.5 2244.0 2246.0 2268.5 2269.9 2273.5 2275.0
J1zD J1zD J1zD J1zD J1zD J1zD J1zD J1zD J1zD J1zD J1zD J1zD J1zD J1M J1zM J1zM J1zDY J1zDY J1zDY J1DY
0.64 0.67 0.41 0.23 0.25 0.27 1.09 0.93 0.93 0.70 0.77 2.15 1.30 1.24 0.96 1.34 0.82 1.84 1.74 1.86
455 397 453 398 456 455 453 458 457 455 458 457 458 458 399 339 467 340 466 466
1.03 0.95 0.87 1.47 0.80 0.58 0.65 1.02 1.20 1.21 1.08 0.94 0.92 1.49 0.90 0.56 1.37 0.64 0.92 1.16
2.34 2.31 1.80 2.59 1.69 0.98 1.17 3.30 2.33 2.20 1.91 2.26 1.47 2.09 1.25 1.39 1.71 1.32 1.30 1.44
3.37 3.26 2.67 4.06 2.49 1.56 1.82 4.32 3.53 3.41 2.99 3.20 2.39 3.58 2.15 1.95 3.08 1.96 2.22 2.60
65.61 87.16 93.55 122.50 86.02 82.86 84.42 47.44 96.00 93.08 87.10 65.28 95.83 111.19 109.76 71.80 74.46 31.22 52.87 62.37
149 212 194 216 182 140 152 153 186 169 154 157 153 156 152 178 93 64 75 77
shale samples from four wells followed the same trend and yielded values for saturated hydrocarbons −28.65‰ to −29.54‰ and aromatic hydrocarbon between −27.16‰ and −28.64‰, −27.13‰ and −28.17‰, and −27.58‰ and −27.9‰, respectively (Table 2). For the gas samples, δ13C values vary from −49.42 ‰ to −44.16‰ in methane, −32.95‰ to −30.04‰ in ethane, −28.72‰ to −26.45‰ in propane, and −29.58‰ to −24.90‰ in n-butane.30
are present in all the samples but the abundance is poor. Calculated Pr/Ph ratios vary in a small range from 1.1 to 1.8 (averaged at 1.5) for the oil samples and from 0.9 to 2.0 (averaged at 1.5) for shale extracts, and the values of Pr/n-C17 are Ph/n-C18 are around 0.1. Terrigenous plant triterpanoids, C30 steranes, bisnorhopane, and gammacerane are absent (Figure 5). 4.3. Stable Isotopes in Shale, Oil, and Gas Samples. The carbon isotopic composition (δ13C) of organic fractions including alkane, aromatic, NSO (nitrogen, sulfur, and oxygencontaining compounds), and asphaltene were determined and evaluated in both the oil samples and the shale extracts. The δ13C value of alkane, aromatic, NSO, and asphaltene fractions in oil samples from three wells range between −27.49‰ to −30.27‰, −27.35‰ to −30.64‰, −28.05‰ to −28.67‰, and −27.76‰ to −28.91‰, respectively; The extracts of J1zD
5. DISCUSSION 5.1. Geochemical Properties of the Jurassic Shales. The geochemical properties of source rocks are commonly described in three aspects: abundance, types, and thermal maturity of organic matter. According to the TOC and pyrolysis results, J1zD samples have a TOC value greater than 1.0% and a (S1+S2) value greater than 2.0 mg/g, showing the D
DOI: 10.1021/acs.energyfuels.7b03097 Energy Fuels XXXX, XXX, XXX−XXX
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above 2.0% and a TOC above 1.0%. J2l and J1zM show low TOC and (S1+S2) values, implying they are not the favorable source rock in this area. Besides the vertical variation, the parameters distribute differently in horizontal. For J1zD shale, most of the wells reveal TOC ranging from 1.0 to 1.3% and (S1+S2) ranging from 2.0 to 2.5 mg/g and Well XL2 shows the best value. The contour map suggests organic matter tends to be more abundant in the northern part of study area (Figure 6). According to the microscopy maceral identification, the content of liptinite and vitrinite takes up 50−70% and 30−50% of the total weight of kerogen samples, respectively. This proportion corresponds to the organic matter type II−III, indicating that the source rocks of J2l and J1zD would generate oil principally and produce a relatively smaller amount of gas. The H/C atomic ratio of the kerogen samples ranges from 0.6 to 0.8, and the O/C atomic ratio ranges from 0.05 to 0.3. H/C and O/C atomic ratios showed on the van Krevelen diagram37 that the kerogen type in J1zD and J1zDY source rocks are type II, and in J2l the type is predominantly III (Figure 7a). A plot of whole rock hydrogen index (HI) and pyrolysis Tmax can be used to classify the type of organic matter and maturity38 and shows that the Safer shale samples generally plot in the early mature to mature zone of type II kerogen with contribution of type I and
Figure 4. Representative maceral microphotographics of Jurassic kerogen samples in North Fuling area. The macerals are dominantly amorphous liptinite and dark vitrinite.
best abundance of organic matter among the four strata; following is the J1zDY, 50% of which have a (S1+S2) value
Figure 5. Total ion chromatograms of four oil samples and two shale extracts in North Fuling area. E
DOI: 10.1021/acs.energyfuels.7b03097 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 2. Stable Isotopes in Shale Extracts and Oil Samples δ13C (‰ VPDB)
δ18O (‰ SMOW)
sample type
well
depth (m)
stratum
alkane
aromatic
NSO
asphaltene
alkane
aromatic
NSO
asphaltene
shale extracts
3-2
2157 2158 2160 2181 2203 2211 2228 2236 2260 2261
J2l J2l J2l J1z J1z J1z J1z J1z J1z J1M
−27.86 −26.75 −27.10 −28.55 −30.28 −30.65 −28.31 −27.72 −28.68 −28.45
−24.28 −24.31 −24.55 −26.68 −29.27 −29.78 −26.36 −25.56 −27.27 −26.84
−25.04 −25.74 −26.15 −27.40 −29.88 −29.99 −28.80 −28.57 −27.81 −27.68
−24.47 −24.57 −25.28 −25.09 −30.07 −29.60 −28.40 −28.73 −28.25 −27.23
32.14 27.96 25.38 25.92 22.59 26.72 22.18 25.67 26.75 21.95
53.48 51.11 34.70 44.75 41.93 42.95 41.70 34.90 43.47 55.44
25.00 21.79 22.96 23.57 23.07 24.32 24.60 24.90 24.99 24.78
59.24 53.00 50.25 59.55 46.14 48.85 49.26 42.78 44.25 54.43
101
2144 2148 2155 2158 2246 2270
J1z J1z J1z J1M J1DY J1DY
−29.95 −30.58 −30.44 −29.74 −33.03 −30.51
−28.92 −29.35 −29.16 −28.20 −31.45 −28.52
−28.93 −29.77 −28.99 −28.31 −31.33 −28.71
−29.06 −29.71 −29.63 −28.61 −32.15 −29.46
26.53 22.97 24.43 25.90 25.03 21.92
44.62 42.42 39.10 48.23 56.05 55.87
24.38 26.39 26.01 27.14 26.76 27.18
51.37 48.30 45.06 44.28 45.47 48.65
FS1
2178 2130
J1z J1z
−30.60 −30.17
−29.34 −30.51
−28.97 −28.21
24.43 27.06
43.19 34.76
28.22 27.96
HF-1
2321
J1z
−30.68
−29.38
−30.60
25.37
42.24
24.67
2-2
2250 2280
J1z J1z
−30.29 −30.53
−29.59 −29.76
−29.77 −29.97
26.93 25.21
38.90 43.79
25.66 24.61
101
2350
J1z
−30.17
−29.46
−29.89
26.69
44.84
26.41
oil
Vitrinite reflectance (VR) data in the Jurassic source rock samples ranges from 1.0 to 1.3%Ro, suggesting that all samples are mature for hydrocarbon generation. Tmax data corresponding to the VR values are generally over 460 °C, indicating that the organic matter reaches the “wet gas” window. 5.2. Physical and Geochemical Properties of Oil Samples. The physical properties define the oils as waxy light oil. The general shapes of peaks are similar in all the oil samples and shale extracts, suggesting the Jurassic shales are a possible source of the oils (see Figure 5). Specific acyclic isoprenoids including pristane (Pr) and phytane (Ph) can reflect the paleo-environmental conditions of source rocks.42,43 Calculated Pr/Ph ratios vary in a small range from 1.1 to 1.8 (averaged at 1.5) for the oil samples and from 0.9 to 2.0 (averaged at 1.5) for shale extracts, and the values of Pr/n-C17 and Ph/n-C18 are around 0.1, indicating the oils and shales contain high contribution of algal and bacterial input in a lacustrine-swamp environment.44 Because common biomarkers like triterpanoids, C30 steranes, bisnorhopane, and gammacerane were absent, we deployed a series of aromatic compounds together with δ13C to correlate the oil and shale extracts (Figure 8). The plot of phenanthrene, MP, DMP, and TMP shows that the oil and shale extracts are similar in a general distribution pattern, but the oil samples and J1zD shale extracts contain less DMP than J2l extracts. When looking at the concentrations of MBF, MDBF, fluorene, and methlyfluorene, the oil samples and J1zD shale extracts are higher than J2l shale extracts. For the dibenzothiophene series, the most significant difference is J2l shale extracts contain more DMDBT than the oil samples and J1zD shale extracts. δ13C values of the oil samples and J1zD shale extracts are smaller than J2l shale
Figure 6. Contour map of TOC and (S1+S2) of J1zD shale.
mixed types II and III kerogens (Figure 7b). Hydrogen Index (HI) and temperature of maximum pyrolysis output (Tmax) obtained from Rock-Eval pyrolysis suggest that organic matter in J2l shale is mainly types II and III. In J1zD shale, organic matter is primarily type II and a small portion of types II and III. Organic matter in J1zDY shale is of type II (Figure 6b). In summary, the organic matter in Jurassic lacustrine shales is generally type II and a small portion of type III. J1zD shale exhibits the most favorable organic type for hydrocarbon generation. The classical maturity zones for organic matter evolution were defined by Vassoyevich et al.39 and then renamed by Tissot and Welte40,41 in order to separate the main zones of petroleum formation in the van Krevelen diagram. F
DOI: 10.1021/acs.energyfuels.7b03097 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 7. Kerogen types of Jurassic source rock samples. (a) Determination of kerogen types by element ratios. (b) Determination of kerogen types by Rock-Eval parameters.
Figure 8. Oil−source correlation using concentrations of polycyclic aromatic hydrocarbons and stable carbon isotopes in oil samples and shale extracts.
narrow range from −49.72‰ to −44.16‰; all samples are plotted into the oil-associated gas field.30 Generally, δ13C in in methane, ethane, and propane demonstrates a rising trend (Figure 9, the δ13C become greater with increasing molecular weight), also known as normal order, indicating a uniform biogenic origin and slight secondary alteration.45,46 This distribution pattern is consistent with the gas samples from T3−J1 in the Western and Central Sichuan Basins. The δ13C values are greater in the study area (in Eastern Sichuan Basin) than in the central and west areas of the basin. This phenomenon is probably caused by the different origins of the gases in different area: the T3−J1 gases in the Central and
extracts. In summary, the characteristics of aromatic compounds and stable isotopes of the oil samples are similar to each of the J1zD shale extracts, suggesting the oils probably originated from J1zD shale. 5.3. Composition and Origin of Gas Samples. All of the gas samples can be categorized to wet gas as the concentrations of methane in hydrocarbon components range from 34.82% to 91.14%. The plot of δ13C in methane and the concentration of C2+ alkanes is commonly employed to determine the genetic types of the gas samples. In the plot of the gas samples from Jurassic shales in this area, the C2+ concentration scatters from 7% to 23%, while the δ13C in methane stays in a relatively G
DOI: 10.1021/acs.energyfuels.7b03097 Energy Fuels XXXX, XXX, XXX−XXX
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xiao Wang: 0000-0002-7354-5111 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by grant 41672139 from the National Natural Science Foundation of China, by grants 2017ZX05005001-008 and 2016ZX05034002-003 from the National Key Scientific Special Project of China and by grants DD20160185 and 12120114046901 from China Geological Survey. Additional supports were provided by the Fundamental Research Funds (no. CUGQYZX1707) for the Central Universities from China University of Geosciences (Wuhan) and by the Programme (no. B14031) of Introducing Talents of Discipline to Universities.
Figure 9. Stable carbon isotopes in different molecules of the gas samples from Jurassic shales in North Fuling area.
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Western Sichuan Basins derived were characterized as coalformed gas, causing the δ13C values higher than in oil-type gas.47,48 Meanwhile, the distribution pattern of Jurassic gas is different from the full reversal order of the O3−S1 shale gas in the Fuling Gas field, which resulted from the multicharge and mixing of gases (early mature wet gas and postmature dry gas) from the same O3−S1 shales at different levels of thermal maturity.19 Maturity of gas samples is investigated primarily by analyzing the δ13C in methane, ethane, and propane. Berner and Faber’s49 isotope/maturity models for reservoir gases of type II and type III source rocks were applied in this study as well to determine the thermal maturity of gas from Jurassic shales in the North Fuling area. The plot of δ13C in ethane and propane concentrates in the maturity range of 0.8−1.1% on the model of type II source rock. The determinations of maturity from different models are consistent and suggest the gas from Jurassic shales demonstrates a medium thermal maturity range of approximately 0.8−1.2%.30
6. CONCLUSIONS The test results of TOC and (S1+S2) suggest J1zD shale demonstrates the greatest abundance of organic matter among the Jurassic lacustrine shales in North Fuling area, and the organic matter tends to be more abundant in the northern part of study area. Organic matter in Jurassic shales is generally type II and a small portion of type III in J2l. VR data in the Jurassic source rock samples ranges from 0.8 to 1.3%Ro, suggesting that most samples are mature for hydrocarbon generation. Crude oil in Jurassic strata is a nonbiodegraded waxy light oil, and the GC-MS and carbon isotope analysis suggest the possible source rock of the oil is J1zD shale. The natural gas in Jurassic shales is wet gas according to its CH4 content. The isotope analysis indicates the gases are oil-associated gas originating from the cracking of crude oil. The normal order of δ13C values with increasing molecular weight indicate that the gas has a uniform biogenic origin and slight secondary alteration unlike the shale gas in Fuling Gas Field. The comprehensive understandings of the source rock, oil, and gas lead to the general conclusion that the Jurassic shales demonstrate good conventional resources potential. H
ABBREVIATIONS J1zD = Da’anzhai member of Lower Jurassic Ziliujing Formation J1zM = Ma’anshan member of Lower Jurassic Ziliujing Formation J1zDY = Dongyuemiao member of Lower Jurassic Ziliujing Formation J2l = Middle Jurassic Lianggaoshan Formation T3−J1 = Upper Triassic to Lower Jurassic O3−S1 = Upper Ordovician to Lower Silurian TOC = total organic carbon content test, % w/w S1 = the first peak on the pyrolysis profile, indicating free hydrocarbons of the sample, mg/g S2 = the second peak on the pyrolysis profile, indicating hydrocarbons generated from kerogen decomposition, mg/g S1+S2 = the sum of S1 and S2, indicating the hydrocarbon generation potential of the sample, mg/g Tmax = maximum temperature during Rock-Eval pyrolysis, °C HCI = hydrocarbon index, calculated from S1/organic carbon, mg/g HI = hydrogen index, calculated from S2/organic carbon, mg/g VR = vitrinite reflectance, %Ro GC = gas chromatography GC-MS = gas chromatography−mass spectrometry NSO = heterocyclic aromatic compounds containing nitrogen, sulfur, or oxygen Pr = pristane Ph = phytane MP = methyl phenanthrene DMP = dimethyl phenanthrene TMP = trimethyl phenanthrene DBF = benzofuran MDBF = methyl benzofuran DBT = dibenzothiophene MDBT = methyl dibenzothiophene DMDBT = dimethyl dibenzothiophene TMDBT = trimethyl dibenzothiophene δ13C = carbon stable isotope, ‰ (VPDB) δD = hydrogen stable isotope, ‰ (SMOW) δ18O = oxygen stable isotope, ‰ (SMOW) DOI: 10.1021/acs.energyfuels.7b03097 Energy Fuels XXXX, XXX, XXX−XXX
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