Article pubs.acs.org/EF
Elevated Mango’s K1 Values Resulting from Thermochemical Sulfate Reduction within the Tazhong Oils, Tarim Basin Daofu Song,*,† Chunming Zhang,‡ Sumei Li,† T.-G. Wang,† and Meijun Li† †
State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, People’s Republic of China ‡ School of Earth Environment and Water Resources, Yangtze University, Wuhan, Hubei 434023, People’s Republic of China ABSTRACT: Mango proposed a significant invariance of four isoheptanes, in which a ratio of sums (K1) remains at approximately 1, regardless of the absolute concentrations. Gas chromatographic analyses of 70 oils from the Tarim Basin demonstrated that most of these oils share invariant K1 values close to 1.0, even if they are derived from different sources and have varying degrees of maturity and biodegradation. However, a few unexpectedly high K1 values occurred in certain oils from the Tazhong-4 Oilfield in the Tazhong Uplift, and these oils are characterized by abnormally high contents of dibenzothiophenes (DBTs). An excellent positive correlation between the elevated K1 values and the abnormally high DBT contents indicated that the thermochemical sulfate reduction (TSR) alteration, which contributed to the increased DBT contents, likely resulted in the elevated K1 values. Methylhexanes (MHs) are assumed to be more susceptible to TSR than dimethylpentanes (DMPs), and the terminal methyl groups of C7 compounds are more resistant than their mid-chain isomers in the TSR-altered oils. Therefore, in the oils affected by TSR alteration, a decrease in the 2,3-DMP/2,4-DMP ratios will not be sufficient to offset the increased 2-MH/3-MH ratios, which results in elevated K1 values. A positive correlation between the increased 2-MH/3-MH ratios and the elevated K1 values has validated this hypothesis. consistent with the observations of Mango.12−16 However, exceptions occur in certain oils from the Tazhong Uplift, which are characterized by abnormally high K1 values (>1.20). The exceptions have attracted extensive attention; however, the reason for the elevated values remains ambiguous.14,16 Zhang and Zhang reported that oils with normal and abnormal K1 values from the Tazhong Uplift were derived from the same kerogen type and attributed the variant K1 values to potential variation in source rock depositional conditions.14 Zhang et al. assumed that significant variation of the K1 values in Tazhong oils likely reflects a variety of homologous oil sets.16 However, source rocks that can generate oils with abnormally high K1 values have not been found. Wang speculated that mixing with biodegraded oils is probably responsible for the elevated K1 values.11 Thus, the objective of this paper is to investigate the variant K1 values in certain Tazhong oils and analyze possible reasons for this variation.
1. INTRODUCTION The light hydrocarbons (LHs) are the principal ingredient of crude oils, especially for light oil/condensate, in which they consist up to 90% of the whole oil.1 Their compositional characteristics are usually used in petroleum geochemistry to determine oil families, predict thermal maturity, and assess alteration effects, such as water washing, evaporative fractionation, and incipient biodegradation.2−9 Mango proposed a significant invariance of four isoheptanes, in which a ratio of sums [K1 = (2-methylhexane + 2,3-dimethylpentane)/(3-methylhexane + 2,4-dimethylpentane)] remains at approximately 1, regardless of the absolute concentrations.10 Because this ratio remains constant throughout the course of oil generation, Mango assumed that LHs in petroleums were generated from a steady-state catalysis rather than thermolysis.9,10 The invariant ratio is particularly remarkable because most other LH concentration ratios exhibit a considerable variance. Therefore, the invariant ratio has been widely investigated and tested. ten Have presented a mean K1 value of 1.07 for 500 Southeast Asia oils that cover the most important stratigraphic units from the Paleozoic to the Cenozoic.5 Wang reported an average K1 value of 1.08 for 339 Chinese oils, sampled from almost all of the oilfields in China, including marine and non-marine oils.11 These case studies show that most oils from all over the world, even with differences in depositional environment, kerogen type, maturity, and reservoir depths, share a similar K1 value of about 1.0, although slight variance likely occurs among different homologous oils (i.e., petroleum generated and expelled from the same source rock). The K1 values of the oils from the Tarim Basin have been widely reported, and most of them are near 1.0, which is © 2017 American Chemical Society
2. GEOLOGICAL SETTING The Tarim Basin is the largest inland oil-producing basin in China (Figure 1), with commercially significant amounts of petroleum derived from marine and terrigenous source rocks. It is constituted by a Paleozoic marine cratonic basin and two Meso-Cenozoic continental foreland depressions (the Kuqa Depression in the north and the Southwest Depression in the south).17 The Tarim Basin is a typical superimposed sedimentary basin developed on the Archaean and Proterozoic crystalline basement, with a sedimentary sequence from marine Received: September 28, 2016 Revised: December 22, 2016 Published: January 4, 2017 1250
DOI: 10.1021/acs.energyfuels.6b02503 Energy Fuels 2017, 31, 1250−1258
Article
Energy & Fuels
Figure 1. (a) Tectonic units of the Tazhong Uplift and locations of oil samples and (b) schematic structural cross section of the Tazhong-4 Oilfield (numbers represent K1 values).
Tangguzibasi Depression in the south, the Awati Sag in the west, and the Tadong Sag in the east (Figure 1). The Tazhong area is composed of sediments from Cambrian to Tertiary ages with an aggregate thickness of 6500−9500 m that overlies on the basement of pre-Sinian continental crust. The uplift is an inherited paleo-structural high in the cratonic region, providing favorable conditions for hydrocarbon accumulation.31,32 In recent decades, numerous petroleum accumulations have been found in the region, with most of them along the number 1 fault zone on the north side of Tazhong Uplift.33 However, deep exploration in sub-salt strata recently resulted in a considerable breakthrough in the ZS1C well and has attracted extensive attention.34,35
carbonate−clastic rocks, marine−terrigenous clastic−carbonate rocks, to terrigenous clastic rocks.18 The geological characteristics of the basin have been investigated in previous studies.17,19−21 The cratonic area of the basin (includes the Tazhong Uplift, Manjiaer Depression, and Tabei Uplift) is composed of gently deformed strata from Sinian to Quaternary ages, with 16 km of sedimentary strata preserved in the Manjiaer Depression.22−24 Two sets of marine source rocks were identified for the origin of the marine oils in the cratonic region, including the Cambrian−Lower Ordovician and Middle−Upper Ordovician rocks.25−27 The Kuqa Depression is situated in the northern margin of the basin, with Meso-Cenozoic terrigenous rocks in dominance. The potential hydrocarbon source rocks are Triassic−Jurassic coal measures and lacustrine mudstones.28−30 The Southwest Depression is located in the southern margin of the basin, and there are five potential source beds. The Tazhong Uplift is situated in the central part of the Tarim Basin, extends in a northwest−southeast direction and has an area of approximately 30 000 km2. The uplift is surrounded by the Manjaer Depression in the north, the
3. METHODS Gas chromatography (GC) of the oil was performed using an Agilent 6890A gas chromatograph fitted with a fused silica column (HP-PONA, 50 m × 0.20 mm internal diameter × 0.5 mm film thickness). The sample (0.5 mL) was injected in a split mode (split ratio was 100:1), with an injector temperature at 300 °C. The oven temperature program was from 35 °C with a 5 min initial isotherm and 1251
DOI: 10.1021/acs.energyfuels.6b02503 Energy Fuels 2017, 31, 1250−1258
Article
1252
6834.1−6850 XK4 14
O2y
0.84
light oil
1.04
0.86
2.90
0.08
1.60
907
0.15
0.46
1.02 ± 0.033 1.07
0.49 0.16
0.25 1829
1128 1.51
1.27 0.09
0.09 3.24
5.34 0.63
0.84 1.04
0.95 medium oil
light oil
6557.89−6618
0.91
6643.3−6679.1 O2y Ha8
Ha701
12
Tabei Uplift
Halahatang Oilfield
13
O2yj
0.85
0.10 0.02 93 0.50 0.59 1.45 0.94 condensate 1.03 5039.5−5041.5 YL2 11
K1bs
0.76
0.07 0.03 395 0.38 1.81 1.92 0.97 condensate 1.12 5026−5042 S3-1 10
K1b
0.80
0.23
0.06 0.04
0.02 126
563 0.36
0.56 1.06
3.89 3.26
1.95 0.92
0.88 condensate 1.01
condensate 1.08 0.80
K1bs
0.79
4997−5003.5
5045.5−5063
E3s DLK8
S3
8
Luntai Oilfield
9
0.21
0.22 0.03
0.03 78
110 0.56
0.66 0.79
0.85 2.17
1.91 0.96
0.88 condensate 1.06
condensate 1.10
4972−5075
0.79
4995−4998
DLK7 7
E3s
DLK6 6
E3s
0.80
0.24
0.20 0.03
0.03 53
117 0.58
0.69 0.74
0.99 1.93
1.79 0.98
0.93 condensate 1.08
condensate 1.11 0.80
4995.5−5003
0.80
5133−5143
E3s DLK5 5
K1bs DLK4 4
5129−5137 DLK3 3
K1bs
0.79
condensate 1.10
0.97
1.85
0.84
0.66
52
0.04
0.23
1.05 ± 0.059
1.09 ± 0.019 0.23
0.25 0.02
0.03 49
80 0.63
0.71 0.70
0.84 1.86
1.73 1.00
0.97 condensate 1.10
condensate 1.12
5126−5132
0.782
5143−5151 K1bs DLK1
DLK2
1
2
K1bs
0.78
K1 type density (g/cm3) depth (m) well
strata
Dalaoba Oilfiled
invariance is observed in most oils (except for those from the Tazhong-4 Oilfield), with a correlation coefficient of over 0.96, although considerable concentration changes of various compounds used in the plots is observed. The average ratios of [(2-MH + 2,3-DMP)/(3-MH + 2,4-DMP), K1] for these oils is 1.05, and the standard deviation is only 0.045. The data suggest that most Tarim Basin oils, even driving from different kerogen
Kuqa Depression
Figure 2. Cross-plot of the concentrations of 2-MH + 2,3-DMP versus the concentrations of 3-MH + 2,4-DMP of 70 oils from the Tarim Basin.
number
Table 1. Basic Geochemical Parameters of the Oils Analyzed in the Tarim Basina
4. RESULTS AND DISCUSSION 4.1. Invariance of K1 Values. To test whether the invariant K1 values observed by Mango also occur in the Tarim Basin oils, 70 samples, representing nearly all petroleum types from biodegraded heavy oils to condensates, were selected and the source rocks were from Cambrian−Ordovician marine carbonates and shales (Tabei and Tazhong Uplift oils),22,25,26 lake mudstones (Dalaoba oils), and terrigenous coals (Luntai oils).30,38 A cross-plot of [3-methylhexane (3-MP) + 2,4-dimethylpentane (2,4-DMP)] versus [2-methylhexane (2-MH) + 2,3-dimethylpentane (2,3-DMP)] is shown in Figure 2. Indeed, a remarkable
location
then an initial heating rate program of 3 °C/min to 70 °C, after which the rate was increased to 4.5 °C/min to a final temperature of 300 °C with a 35 min hold time. Hydrogen carrier gas was used with a minimum purity of 99.999% (flow rate of 1.0 mL/min). The eluting compounds were detected using a flame ionization detector (FID, 300 °C). Quantification of all components was based on peak areas. The compounds were identified through analyses of known standards (such as PINAO mixed standards) and previously published data.36 The oils were deasphalted using n-hexane and fractionated using column chromatography into saturate, aromatic, and NSO fractions by sequential elution with n-hexane, toluene, and chloroform. Gas chromatography−mass spectrometry (GC−MS) analysis of aromatic fractions was then performed using an Agilent model 6890 gas chromatograph fitted with a HP-5MS capillary. The GC temperature operating condition was from 80 °C (1 min) to 300 °C (held 15 min) at 3 °C/min. To quantify the aromatic hydrocarbons, known amounts of dibenzothiophene-d8 were added to the oils before they were fractionated. Peak identifications and quantitative methods are reported in the study by Jiang et al.37
DBTs 2-MH/3-MH 2,3-DMP/2,4-DMP tol/n-C7 n-C7/MCH (μg/g of oil) DBTs/Ars DBT/P mean (K1) ± SD
Energy & Fuels
DOI: 10.1021/acs.energyfuels.6b02503 Energy Fuels 2017, 31, 1250−1258
Tazhong Uplift
Tazhong-4 Oilfield
Tahe Oilfield
location
Table 1. continued
0.86 0.96 0.95 0.94 0.88 0.83
O1−2y 5830−5824 O1−2y 5458.8−5525.7 O1−2y 5459−5521 5592−5602 5930−6000 5907.5−5911.6 4227−4234 6020−6150 4878−4986
C1b O2yj O3 D3d O3l S1k
1253
TZ4
TZ4
TZ401
47
48
ZS1
40
46
TZ11
39
TZ421
TZ40
38
45
TZ82
37
TZ4
TZ12
36
TZ421
TZ47
35
44
ZG162
34
43
ZG43
33
TZ401
ZG26
32
TZ411
Z1
31
41
TZ62
30
42
TZ47
S106
26
29
S108
25
TZ45
T707
24
28
TK213
23
TZ10
TK217
22
27
T901
21
CIII
CIII
CIII
CII
CII
CI
CI
CI
3614−3649
3712−3720
3597−3604
3478−3494.5
3532−3548
3221−3283
3439−3450
3244−3247
4301−4307 6426−6458
ϵ2a
4317−4324
5430−5487
4695.5−4777.5
S
C
O
O
4390.5−4402
0.92
0.80
0.75
0.85
0.82
0.83
0.79
O3l C
light oil
4980.1−5334.1 6094.8−6780
O1−3
heavy oil
light oil
light oil
light oil
light oil
light oil
light oil
1.32
1.39
1.36
1.34
1.26
1.30
1.24
1.22
1.07
1.04
1.06
1.10
1.03
1.07
condensate 1.10
1.05
condensate 1.07
6085.5−6295
1.03
1.06
1.05
1.07
1.07
0.97
0.99
1.07
0.99
0.96
1.01
1.10
1.02
1.03
1.02
O3l
light oil
light oil
light oil
light oil
medium oil
heavy oil
heavy oil
heavy oil
light oil
light oil
heavy oil
light oil
light oil
1.04
1.04
K1
light oil 0.80
0.86
0.81
0.86
0.82
0.83
light oil
light oil
type
O1−2y 5273−5349
4700.5−4758
0.86
O1−2y 5294.8−5384
S14
20
O
0.92
6692.1−6980 4720.25−4921
O3l
TK113H T2a
Rp7
19
18
6797−6936.8
XK9
17
0.80
0.84
O3l
XK5
RP1C
15 6629−6988.6
density (g/cm3)
6910−6920
depth (m)
O2y
strata O3l
well
16
number
1.26
1.38
1.39
1.21
1.35
1.21
1.20
0.93
0.86
0.89
0.99
0.84
0.90
0.95
1.04
0.91
0.81
0.93
0.85
0.91
0.94
0.78
0.78
0.80
0.77
0.74
0.83
0.97
0.86
0.85
0.84
0.86
0.87
1.71
1.82
1.16
1.59
1.06
1.33
2.59
3.32
3.00
2.34
2.97
2.82
2.42
2.11
2.51
4.06
2.27
3.99
2.61
2.37
2.58
2.58
3.65
3.71
3.49
3.38
2.51
3.10
3.02
4.05
3.09
3.17
0.16
0.25
0.23
0.11
0.20
0.11
0.01
0.25
0.14
0.03
0.31
0.26
0.32
0.35
0.41
0.22
0.18
0.04
0.45
0.01
0.01
0.11
0.11
0.02
0.12
0.12
0.13
0.18
0.16
0.10
0.21
0.10
0.10
2.17
1.95
2.17
2.76
2.12
2.81
4.31
1.93
2.23
2.02
1.87
1.18
1.16
1.91
1.45
1.85
1.39
1.93
1.57
1.92
1.90
1.72
1.24
1.58
1.55
1.61
1.50
1.73
1.45
1.64
1.43
1.61
1.62
2887
4681
3522
3363
3323
2684
3131
2450
1033
780
795
1043
699
2180
2136
2310
2224
1460
1603
1189
1082
674
1792
1753
1051
581
656
916
1213
1152
947
1245
1193
881
0.42
0.52
0.50
0.45
0.30
0.42
0.42
0.48
0.19
0.15
0.28
0.22
0.24
0.23
0.28
0.27
0.28
0.26
0.25
0.23
0.26
0.28
0.26
0.17
0.23
0.23
0.22
0.20
0.23
0.20
0.15
0.18
0.22
0.17
1.67
2.47
2.62
2.13
1.57
2.16
1.93
1.94
0.26
0.69
1.34
0.62
0.47
0.28
1.74
1.55
1.57
0.80
0.81
0.70
0.62
1.10
0.50
0.60
0.50
0.49
0.48
0.55
0.48
0.47
0.43
0.55
0.61
051
1.39 ± 0.120
1.06 ± 0.022
1.01 ± 0.049
DBTs 2-MH/3-MH 2,3-DMP/2,4-DMP tol/n-C7 n-C7/MCH (μg/g of oil) DBTs/Ars DBT/P mean (K1) ± SD
Energy & Fuels Article
DOI: 10.1021/acs.energyfuels.6b02503 Energy Fuels 2017, 31, 1250−1258
Yubei Oilfield
1254
TZ62
Ma3
M4
T904
LK1
TD2
YN2
65
66
67
68
69
70
YB9
63
64
YB1−4
62
ZS1C
57
YB1-3H
TZ1
56
YB1-2X
TZ6
55
61
TZ422
54
60
TZ421
53
YB1-1X
TZ411
52
59
TZ402
51
YB1
TZ402
50
58
TZ401
well
49
number
0.93 0.92 0.93 0.95
O1−2y 5010−5190 O1−2y 5810−6382 O1−2y 5072 O1−2y 6552−7091
C
1414−1424 2044.4−2140
J
O1
J 3618−3627
4561.93−5040
4265.4−4305
O1−2y 5900−5939
O1
0.75
1.02
0.82
0.82
0.80
0.91
0.93
O1−2y 5956−6020
4053−4073.6
0.91
O1−2y 5550−5630
S
0.93
3586−3597 6861−6944
0.84
0.87
0.84
0.90
density (g/cm3)
ϵ1x
3710.9−3728.7
3604−3624
3570−3575
3720−3723
3613−3628
3510−3535
3685−3703
depth (m)
O
CIII
CIII
CIII
CIII
CIII
CIII
CIII
strata
1.47
1.52
1.39
1.35
1.52
1.40
1.36
1.43
K1
1.00
0.99
1.02
1.12
1.05
1.00
heavy oil light oil
1.06
1.08
0.94
condensate 1.00
condensate 1.03
condensate 0.98
medium oil
heavy oil
heavy oil
medium oi 1.01
heavy oil
heavy oil
medium oil
condensate 1.71
light oil
medium oil
light oil
medium oil
type
0.80
0.70
0.71
0.80
0.78
0.74
0.69
0.75
0.64
0.66
0.66
0.64
0.67
1.67
1.48
1.53
1.45
1.39
1.53
1.45
2.72
4.48
3.33
3.51
5.65
3.88
4.98
4.62
4.79
5.18
5.07
1.81
1.39
1.42
1.12
0.02
0
0.03
0.14
0.25
0.67
0.73
0.02
0.26
0.30
0.36
0.23
0.26
4.45
0.30
0.67
0.15
1.64
0.30
0.49
0.84
0.68
0.85
0.52
0.49
1.12
1.49
1.30
1.29
1.50
1.49
1.45
2.38
2.14
2.76
2.88
0.94
2.17
12
56
17
565
79
82
290
2215
1130
1012
752
710
888
32288
6105
5482
3809
4190
4844
4168
4058
5743
0.10
0.04
0.13
0.05
0.06
0.04
0.06
0.20
0.19
0.19
0.18
0.20
0.21
0.65
0.37
0.53
0.46
0.56
0.54
0.44
0.44
0.54
0.11
0.04
0.26
0.11
0.14
0.18
0.13
0.60
0.66
0.62
0.69
0.50
0.54
14.7
3.19
4.66
2.76
2.17
4.75
2.68
2.66
4.56
1.03 ± 0.062
1.01 ± 0.023
DBTs 2-MH/3-MH 2,3-DMP/2,4-DMP tol/n-C7 n-C7/MCH (μg/g of oil) DBTs/Ars DBT/P mean (K1) ± SD
K1, (2-methylhexane + 2,3-dimethylpentane)/(3-mehtylhexane + 2,4-dimethylpentane); 2-MH/3-MH, 2-methylhexane/3-methylhexane; 2,3-DMP/2,4-DMP, 2,3-dimethylpentane/2,4-dimethylpentane; tol/n-C7, toluene/n-heptane; n-C7/MCH, n-heptane/methylcyclohexane; DBTs (μg/g of oil), absolute abundance of dibenzothiophenes recognizable; DBTs/Ars, dibenzothiophenes/aromatic compounds recognizable; and DBT/P, dibenzothiophene/phenanthrene.
a
candidate group
Southwest Depression
location
Table 1. continued
Energy & Fuels Article
DOI: 10.1021/acs.energyfuels.6b02503 Energy Fuels 2017, 31, 1250−1258
Article
Energy & Fuels
Figure 3. Cross-plots of (a) K1 values versus DBT concentrations and (b) DBT/P versus DBTs/Ars ratios for the oils from the Tarim Basin.
Figure 4. Gasoline range gas chromatogram of ZS1C oils. nC5, n-pentane; 2,2-DMC4, 2,2-dimethylbutane; CyC5, cyclopentane; 2-MC5, 2-methylpentane; 3-MC5, 3-methylpentane; nC6, n-hexane; MCyC5, methylcyclopentane; Bz, benzene; CyC6, cyclohexane; 2-MC6, 2-methylhexane; 3-MC6, 3-methylhexane; 1c3-DMCyC5, cis-1,3-dimethylcyclopentane; 1t3- DMCyC5, trans-1,3-dimethylcyclopentane; 1t2-DMCyC5, trans-1,2-dimethylcyclopentane; nC7, n-heptane; MCyC6, methylcyclohexane; Tol, toluene; and 2-MC7, 2-methylheptane.
contents (Table 1). As shown in Figure 3a, the DBT contents for these oils with elevated K1 values are more than 2500 μg/g of oil and the value in the ZS1C condensate is up to 32 288 μg/g of oil. Accordingly, their dibenzothiophene/phenanthrene (DBT/P) and dibenzothiophenes/recognizable aromatic compounds (DBTs/Ars) ratios are greater than 2.0 and 0.3, respectively (Figure 3b). For a comparison, the DBT contents in other samples with invariant K1 values are much lower, less than 2500 μg/g of oil (Figure 3a). The oils from the Kuche Depression have DBT contents of less than 600 μg/g of oil, which is probably related to their non-marine origin.48,49 The DBTs in the marine oils with invariant K1 values form the Tabei, Tazhong, and Yubei areas are more abundant than those in the non-marine oils, and their contents are also less than 2500 μg/g of oil. The candidate group has much lower DBT content than most Tabei and Tazong oils, which was likely caused by different concentrations in their source rocks. The variability of DBT abundances in different homologous sets of oils can also be observed from the ratios of DBT/P as well as DBTs/Ars (Figure 3b and Table 1). Generally, the Tazhong-4 oils with elevated K1 values have much more abundant DBTs than any other samples characterized by invariant K1 values. In addition, an excellent positive correlation is observed between the elevated K1 values and the abnormally high DBT contents, indicating that the process that increased the DBT contents also resulted in the elevated K1 values (Figure 3a).
type and having different maturity and reservoir depths, share an invariant K1 value of about 1.0. 4.2. Abnormally High K1 Values. In addition to the invariance, another notable characteristic is the significant variability of K1 in the oils from the Tazhong-4 Oilfield along the eastern part of the Tazhong Fault Uplift. These K1 values are abnormally high, ranging from 1.22 to 1.71, and they deviate significantly from the general range (Figure 2). Although K1 is relatively constant in all oils, it can vary between oil sets from different source rocks.5,39 In the cratonic area of Tarim Basin, there are two different end member oils derived from the lower Paleozoic source rocks, one from the Cambrian−Lower Ordovician and the other from the Upper Ordovician.22,25,26,40 The oils from the Tabei Uplift and the candidate group in Table 1 have different geochemical characteristics and represent the two different end member oils in the cratonic area.26,33,41−45 As shown in Figure 2, both oils are characterized by invariant K1 values of approximately 1.0. The Tazhong oils, including the Tazhong-4 oils, may also be derived from the same Cambrian−Ordovician source rocks.25,46,47 Therefore, the deviated K1 values in the Tazhong-4 oils are apparently unrelated to their sources. In addition, source rocks that can generate oils with abnormally high K1 values have not been found. Interestingly, the oils with abnormally high K1 values are also characterized by abnormally high dibenzothiophene (DBT) 1255
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the alteration has been verified to have occurred in the deeper Cambrian carbonate reservoir.52,53 A typical sample of the deep TSR altered residual is the ZS1C oil (with the highest K1 value of 1.71) located in the Lower Cambrian (ϵ1x, underlying the Cambrian salt strata), which has a DBT/P ratio as high as 14.7 and a DBTs/Ars ratio up to 0.65. Other signatures, such as abundant thiadiamondoids and high δ34S of DBTs, also support a severe alteration of ZS1C oil by TSR.35,53,62 The mixing of DBT-enriched deep oils/condensates altered by TSR with shallow normal oils in various proportions is responsible for the elevated DBT concentration in the Tazhong-4 Carboniferous reservoired oils and also resulted in elevated K1 values.52,53 A significant variation of K1 values can be clearly observed for the Tazhong-4 oils occurring at different reservoir depths (Figure 1b). Generally, K1 values increase with reservoir depth as observed in well TZ411, which presents a K1 of 1.24 in CI and 1.52 in CIII. The increased mixing proportion of TSR-altered deep oils from the CI to CIII reservoir probably resulted in the K1 values increasing with reservoir depth. 4.3. Hypothetical Mechanisms. Mango proposed that the invariance in the isoheptane ratio resulted from a steady-state kinetic scheme in which the MHs originated from a common n-heptane precursor and the DMPs were daughter products of the MHs (Figure 5).9,39 These reactions occurred via a transition metal catalyst forming a three-ring (cyclopropyl) intermediate. Under constant conditions, 2-MH/3-MH and 2,3-DMP/2,4-DMP ratios remain invariant over time; thus, the ratio of sums, (2-MH + 2,3-DMP)/(3-MH + 2,4-DMP), remains constant. Under changing temperature (or pressure), the daughter ratios (2-MH/3-MH and 2,3-DMP/2,4-DMP) change but in opposite directions; thus, they tend to offset each other and balance K1. However, TSR has been shown to change the composition of gasoline-range hydrocarbons and to affect the Mango parameter.5,63 As exemplified for crude oils generated from the Upper Devonian Duvernay Formation in Western Canada, the TSR-affected Peco samples and samples from Brazeau River field all plot above the K1 = 1 line.5 Data from other oil families also suggest a tendency for all TSR-affected oils to plot above the K1 = 1 line.51,63 Moreover, Mango stated that oils affected
The DBT content of oils is determined by its kerogen type and source depositional environment and can be altered by secondary processes, such as water washing, biodegradation, thermochemical sulfate reduction (TSR), gas invasion, and maturity.49−51 The abnormally high DBT contents of the Tazhong-4 oils have been confirmed to be related to TSR alteration.52−56 Therefore, it is deduced that the abnormally high K1 values in the Tazhong-4 oils likely also resulted from TSR alteration. TSR has been shown to significantly affect the composition of LHs before completely destroying them. As shown in Figure 4, the unexpectedly high abundance of toluene in the ZS1C oils (toluene/n-heptane = 4.45) is also a typical signature of severe TSR alteration.57 As indicated by Mango and Peters et al., TSR can also affect K1 values; oils severely altered by TSR yield K1 values appreciable greater than unaltered oils from the same source rocks.3,58 TSR is a chemical process in which petroleum hydrocarbons react with inorganic sulfate at high temperatures, and it mainly occurs in carbonate reservoirs associated with evaporate successions.59−61 Therefore, it seems to be no possibility for the Carboniferous oils in the Tazhong-4 Oilfield to have been altered by TSR in situ in the present clastic reservoir. In fact,
Figure 5. Metal-catalyzed steady-state kinetic reaction scheme for the formation of C7 hydrocarbons.39 P1, n-heptane; P2, 2-methylhexane + 3-methylhexane; P3, 3-ethylpentane + 3,3-dimethylpentane + 2,3dimethylpentane + 2,4-dimethylpentane + 2,2-dimethylpentane + 2,2,3-trimethylbutane; N15, ethylcyclopentane + 1,2-dimethylcyclopentane (cis + trans); N16, toluene + methylcyclohexane; and N25, 1,1dimethylcyclopentane + 1,3-dimethylcyclopentane (cis + trans). The subscripts for the rate constants represent the parents (P1 and P2), and the superscripts denote the number of ring carbon atoms in the respective ring-closure steps.
Figure 6. Diagrams showing the (a) variant 2-MH/3-MH and 2,3-DMP/2,4-DMP ratios and (b) relative percentages of 2-MH, 3-MH, 2,3-DMP, and 2,4-DMP in the Tazhong oils. 1256
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contribute to a highly polar environment that catalyzes Mango’s reaction and alters the kinetic pathway, so that 2-MH and 3-MH are generated in a way that favors 2-MH over 3-MH. The process may further enhance the K1 values.
by TSR yield primesum sum >1 and are distinguished further by K1 > 1.3 Mango suggested that temperature controls preferential ring opening of cyclopropane (three-ring) intermediates to form isoheptanes.9 On the basis of this model, 2-MH/3-MH and 2,4-DMP/2,3-DMP should depend upon the temperature, with the latter more sensitive. In fact, 2-MH/3-MH and 2,4-DMP/ 2,3-DMP had been suggested earlier as temperature functions.10,63 It seems that the more thermally stable 2-MH and 2,4-DMP are also more resistant to TSR alteration, a process occurring at high temperatures.58−61,64,65 As shown in Figure 6a, relative to the normal oils from Tazhong Uplift, a notable increase in the 2-MH/3-MH ratio and a decrease in the 2,3-DMP/2,4-DMP ratio are observed for the oils from the Tazhong-4 Oilfield. Considering that the maturity of the Tazhong-4 oils is not evidently higher than that of the Tazhong normal oils (certain oils are also mixed with deep oils), TSR alteration is considered to be the main reason for the changes. Moreover, Figure 6b shows that the relative content of MHs changed marked with elevated K1 values, while the DMPs varied slightly, indicating that the MHs are more susceptible to TSR. This result indicates that 3-MH will deplete faster when affected by TSR. According to Mango, the isoheptane (K1) invariance among homologous oil suites is caused by the offsetting changes of 2-MH/3-MH and 2,3-DMP/2,4-DMP ratios.9 However, in the oils affected by TSR, the decreased 2,3-DMP/2,4-DMP ratio cannot sufficiently offset the increased 2-MH/3-MH ratios, which ultimately results in the elevated K1 values. This effect is also shown in Figure 7, in which the
5. CONCLUSION The Tazhong-4 oils with elevated K1 values (>1.0) are also characterized by abnormally high DBT contents, which were assumed to be related to TSR alteration. An excellent positive correlation between the abnormally high DBT content and the elevated K1 values indicates that TSR alteration is also responsible for the elevated K1 value. In addition to these TSR-altered oils, all of the other samples are characterized by invariant K1 values near 1.0, even though they originate from different source rocks and experience different secondary alteration. Therefore, the K1 value can be used as a simple and effective indicator of TSR alteration in the Tarim Basin. However, it must be noted that the biodegraded heavy oils in the paper are mixtures of early biodegraded oils and late filled fresh oils. The LHs in these heavy oils should come from the late filled oils, whose biodegradation degree is unclear. Therefore, the influence of biodegradation on the K1 value still needs to be further explored. As discussed above, the oils of Tazhong-4 Oilfield with elevated K1 values are a mix of deep oils with abnormally high K1 values altered by TSR and normal shallow oils with invariant K1 values. Therefore, the elevated K1 values in shallow oils can also be used to estimate the approximate mixing ratios. Recently, several oils with elevated K1 values were found in the shallow Mesozoic clastic reservoirs on the northern part of the Tazhong Uplift. Thus, it is reasonable to speculate that a potential deep oil pool (sub-salt) occurs in the area. Relative to DMPs, MHs seems to be more susceptible to TSR and the terminal methyl groups of C7 compounds are more resistant than their mid-chain isomers in the TSR-altered oils. Therefore, in the oils affected by TSR, the decreased 2,3-DMP/2,4-DMP ratios will not be sufficient to offset the increased 2-MH/3-MH ratios, which ultimately results in the elevated K1 values. On the basis of the mechanisms, the elevated K1 values in the oils affected by TSR should primarily depend upon the increased 2-MH/3-MH ratios.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-10-8973-1109. E-mail: songdaofu2008@163. com. Figure 7. Cross-plot showing the relationship between K1 values and 2-MH/3-MH ratios in the Tazhong-4 oils.
ORCID
Daofu Song: 0000-0003-2866-4880 Meijun Li: 0000-0002-7141-6068
2-MH/3-MH ratios show an excellent positive correlation with the elevated K1 values, indicating that the elevated K1 values mainly depend upon increased 2-MH/3-MH ratios. Although Mango’s steady-state metal-catalyzed reaction remains controversial, molecular models have been developed that supported Mango’s ring-closure reactions.66,67 According to Mango’s hypothesis, 2-MH and 3-MH are derived from a common parent and catalyst composition in source rocks represents the major variable that controls the reaction pathway. TSR has been found to generate a variety of polar compounds, including H2S, CO2, and organic sulfur compounds. Under appropriate conditions, these polar compounds may
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to Ryuzo Tanaka and four anonymous reviewers for the time and constructive comments that greatly improved the manuscript. The authors also thank Shengbao Shi and Guangli Wang for assistance with GC and GC−MS analyses. The work was funded by the National Natural Science Foundation of China (Grant 41503029). 1257
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