4.1. Rock-Eval pyrolysis
Rock-Eval parameters obtained from pyrolysis of Nayband and Ab-e-Haji samples (Table 1) indicate minor potential for generation of hydrocarbons from the associated organic matter. This is consistent with previous findings about the quality of organic matter preserved in the studied formations (Alizadeh et al. 2011; Yousefi and Behbahani 2017; Zamansani et al. 2019).
Table 1 Rock-Eval pyrolysis data for samples from Nayband and Ab-e-Haji formations.
|
Sample
|
Rock-Eval data
|
Calculated ratios
|
TOC
(wt%)
|
S1
(mg HC/g rock)
|
S2
(mg HC/g rock)
|
S3
(mg CO2/g rock)
|
Tmax
(˚C)
|
HI
(mg HC/g TOC)
|
OI
(mg HC/g TOC)
|
S2/S3
|
PI
|
Ab-e-Haji Formation
|
A-41
|
0.46
|
0.05
|
0.21
|
0.13
|
380
|
45.65
|
28.26
|
1.62
|
0.19
|
A-40
|
0.47
|
0.03
|
0.15
|
0.19
|
385
|
31.91
|
40.43
|
0.79
|
0.17
|
A-39
|
0.55
|
0.03
|
0.13
|
0.25
|
484
|
23.64
|
45.45
|
0.52
|
0.19
|
A-38
|
0.51
|
0.04
|
0.2
|
0.23
|
610
|
39.22
|
45.10
|
0.87
|
0.17
|
A-37
|
0.59
|
0.03
|
0.43
|
0.28
|
612
|
72.88
|
47.46
|
1.54
|
0.07
|
A-36
|
0.75
|
0.07
|
0.37
|
0.16
|
444
|
49.33
|
21.33
|
2.31
|
0.16
|
A-35
|
0.31
|
0.03
|
0.2
|
0.44
|
381
|
64.52
|
141.94
|
0.45
|
0.13
|
A-34
|
0.32
|
0.03
|
0.07
|
0.1
|
472
|
21.88
|
31.25
|
0.70
|
0.30
|
A-33
|
0.69
|
0.14
|
0.47
|
0.09
|
612
|
68.12
|
13.04
|
5.22
|
0.23
|
A-32
|
0.28
|
0.03
|
0.09
|
0.13
|
389
|
32.14
|
46.43
|
0.69
|
0.25
|
A-31
|
0.32
|
0.05
|
0.29
|
0.15
|
612
|
90.63
|
46.88
|
1.93
|
0.15
|
A-30
|
0.56
|
0.01
|
0.15
|
0.24
|
461
|
26.79
|
42.86
|
0.63
|
0.06
|
A-29
|
0.86
|
0.02
|
0.07
|
0.29
|
601
|
8.14
|
33.72
|
0.24
|
0.22
|
A-28
|
0.59
|
0.03
|
0.22
|
0.14
|
473
|
37.29
|
23.73
|
1.57
|
0.12
|
A-27
|
0.57
|
0.03
|
0.3
|
0.26
|
383
|
52.63
|
45.61
|
1.15
|
0.09
|
A-26
|
0.26
|
0.01
|
0.02
|
0.24
|
604
|
7.69
|
92.31
|
0.08
|
0.33
|
A-25
|
0.57
|
0.01
|
0.03
|
0.31
|
531
|
5.26
|
54.39
|
0.10
|
0.25
|
A-24
|
0.19
|
0.04
|
0.04
|
0.1
|
288
|
21.05
|
52.63
|
0.40
|
0.50
|
A-23
|
0.2
|
0.02
|
0.53
|
0.08
|
612
|
26.50
|
40.00
|
6.63
|
0.04
|
A-22
|
0.69
|
0.02
|
0.12
|
0.3
|
512
|
17.39
|
43.48
|
0.40
|
0.14
|
A-21
|
0.58
|
0.03
|
0.18
|
0.13
|
513
|
31.03
|
22.41
|
1.38
|
0.14
|
A-20
|
0.94
|
0.02
|
0.23
|
0.27
|
517
|
24.47
|
28.72
|
0.85
|
0.08
|
A-19
|
0.61
|
0.04
|
0.08
|
11
|
605
|
13.11
|
1803.28
|
0.01
|
0.33
|
A-18
|
0.52
|
0.03
|
0.13
|
0.22
|
490
|
25.00
|
42.31
|
0.59
|
0.19
|
A-17
|
1.92
|
0.14
|
0.61
|
0.45
|
461
|
31.77
|
23.44
|
1.36
|
0.19
|
A-16
|
0.91
|
0.04
|
0.25
|
0.25
|
506
|
27.47
|
27.47
|
1.00
|
0.14
|
A-15
|
0.68
|
0.02
|
0.26
|
0.12
|
508
|
38.24
|
17.65
|
2.17
|
0.07
|
Nayband Formation
|
N-14
|
0.67
|
0.02
|
0.13
|
0.31
|
514
|
19.40
|
46.27
|
0.42
|
0.13
|
N-13
|
0.91
|
0.03
|
0.29
|
0.15
|
489
|
31.87
|
16.48
|
1.93
|
0.09
|
N-12
|
1.27
|
0.02
|
0.32
|
0.35
|
526
|
25.20
|
27.56
|
0.91
|
0.06
|
N-11
|
0.42
|
0.01
|
0.01
|
0.18
|
454
|
2.38
|
42.86
|
0.06
|
0.50
|
N-10
|
0.56
|
0.01
|
0.18
|
0.46
|
496
|
32.14
|
82.14
|
0.39
|
0.05
|
N-9
|
0.59
|
0.01
|
0.05
|
0.4
|
518
|
8.47
|
67.80
|
0.13
|
0.17
|
N-8
|
0.52
|
0.01
|
0.02
|
0.4
|
515
|
3.85
|
76.92
|
0.05
|
0.33
|
N-7
|
0.58
|
0.01
|
0.4
|
0.3
|
523
|
68.97
|
51.72
|
1.33
|
0.02
|
N-6
|
0.57
|
0.01
|
0.02
|
0.49
|
525
|
3.51
|
85.96
|
0.04
|
0.33
|
N-5
|
0.62
|
0.01
|
0.08
|
0.34
|
609
|
12.90
|
54.84
|
0.24
|
0.11
|
N-4
|
0.57
|
0.01
|
0.04
|
0.31
|
520
|
7.02
|
54.39
|
0.13
|
0.20
|
N-3
|
0.51
|
0.01
|
0.03
|
0.39
|
516
|
5.88
|
76.47
|
0.08
|
0.25
|
N-2
|
0.47
|
0.01
|
0.02
|
0.31
|
522
|
4.26
|
65.96
|
0.06
|
0.33
|
N-1
|
0.54
|
0.01
|
0.02
|
0.42
|
521
|
3.70
|
77.78
|
0.05
|
0.33
|
According to the obtained geochemical parameters (Table 1 and Fig. 3), the organic matter contained within the Nayband formation is of terrestrial origin and has low hydrogen contents (e.g., HI values mostly below 70 mg HC/g TOC). This indicates that the organic matter suffered major oxidation during transport and/or sedimentation. Similarly, samples from the Ab-e-Haji Formation are consistent with predominantly hydrogen-poor terrestrial organic matter that was deposited under oxic conditions (Fig. 3). The very low HI values (mostly below 100 mg HC/g TOC) along with very low S2 readings (mostly below 1 mg HC/g rock) indicates that a major fraction of the associated TOC in both formations is inert and will not participate in the process of hydrocarbon generation (Table 1 and Fig. 3).
In terms of thermal maturity, the obtained Tmax values should be used with caution. Owing to very low S2 readings for our samples, Tmax values cannot provide an accurate measure of maturity. This is supported by the wide range of variation observed for the Tmax readings (Fig. 3). Therefore, we rely upon vitrinite reflectance measurements for maturity evaluation of the studied formations.
4.2. Organic petrography
Representative samples were selected from Nayband and Ab-e-Haji formations for detailed organic petrographic inspections and for vitrinite reflectance measurements (Table 2). In line with previous studies (Yousefi and Behbahani 2017), our readings indicate that the studied formations are thermally mature.
Table 2 List of samples selected for organic petrographic analyses from the studied formations.
Formation
|
Sample
|
Mean VRo (%)
|
Standard deviation
|
Number of readings
|
VRo range (%)
|
Ab-e-Haji
|
A-39
|
1.12
|
0.38
|
100
|
0.35-0.88
|
A-25
|
0.80
|
0.22
|
77
|
0.35-1.20
|
A-18
|
0.83
|
0.16
|
34
|
0.55-1.0
|
A-17
|
0.71
|
0.33
|
95
|
0.31-1.12
|
Nayband
|
N-13
|
1.02
|
0.27
|
39
|
0.52-1.27
|
N-12
|
0.96
|
0.20
|
33
|
0.62-1.27
|
N-7
|
0.88
|
0.18
|
38
|
0.52-1.22
|
N-4
|
0.97
|
0.21
|
11
|
0.62-1.22
|
Organic petrographic inspections provides more robust assessment of the maceral constitution of studied samples. This technique is used in combination with bulk-rock pyrolysis data to better understand the variations of organic matter type among the studied formations. Our results indicate that the samples from lower parts of the Nayband Formation (i.e. samples N-4 and N-7) are rich in vitrinite and inertinite macerals (Fig. 4). In addition, vitrinite macerals are in most cases characterized with micro-fractures and dark oxidation rims. These are strong indications of the natural oxidation during transport and/or sedimentation (Lo and Cardott 1995), and are fully supported by Rock-Eval pyrolysis result mentioned above.
Moving up within the Nayband Formation (i.e. samples N-12 and N-13), the relative abundance of semifusinite macerals increases (Fig. 5 a, b). In addition, a relative increase in the abundance of combustion chars is noticed towards the top of the Nayband Formation (Fig. 5 c, d).
In a similar fashion, semifusinite and combustion chars are abundant in the lowermost samples of the Ab-e-Haji Formation (i.e. samples A-17 and A-18, Fig. 6). However, their abundance decreases rapidly towards the top of the section (i.e. samples A-25 and A-39), and vitrinites with marked weathering characteristics become the dominant maceral types (Fig. 7). It is noteworthy that weathering features are relatively stronger in the Ab-e-Haji samples compared to those observed in the underlying Nayband Formation (Fig. 7).