Organic Geochemistry and Petrology of Mudrocks from the Upper

Aug 28, 2017 - However, the main hydrocarbon source rocks in the Wulungu Depression are ..... Petroleum exploration has proven that the Carboniferous ...
0 downloads 0 Views 7MB Size
Download PDF
Article pubs.acs.org/EF

Organic Geochemistry and Petrology of Mudrocks from the Upper Carboniferous Batamayineishan Formation, Wulungu Area, Junggar Basin, China: Implications for Petroleum Exploration Qingyong Luo,*,† Yansheng Qu,‡ Quan Chen,† and Zhengrong Xiong‡ †

State Key Laboratory of Petroleum Resources and Engineering, China University of Petroleum, Beijing 102249, China Western Branch Institute, Exploration and Development Institute, Shengli Oilfield Branch Company, SINOPEC, Dongying, Shandong 257061, China

‡

ABSTRACT: The hydrocarbon generation potential of the Carboniferous Batamayineishan mudrocks has been overlooked for a long time. In this study, organic petrology and geochemistry were used to determine the sedimentary environments, organic matter (OM) type, and organic maturity of the Carboniferous Batamayineishan source rocks, collected from outcrops, Wulungu area, Junggar Basin, China. The maceral composition of the studied samples is dominated by vitrinite and inertinite, ranging from 40 vol % to 100 vol % and from 0 vol % to 60 vol % on a mineral matter free basis, respectively. Liptinite macerals, mainly as lamalginites, are only present in trace amounts in several sediments. Vitrinite reflectance values are 0.67% to 2.78%. TOC values are high in these samples, whereas the S1, S2, and HI are very low. Pr/Ph ratios, the plot of Pr/n-C17 versus Ph/n-C18, DBT/P versus Pr/Ph, and ternary of fluorene, dibenzothiophene, and dibenzofuran indicate that anoxic saline environments prevailed during deposition of the Batamayineishan Formation. This is consistent with the presence of β-carotane and high abundances of gammacerane in the studied samples. The major biological source is from vascular plants, and the kerogen type is humic (type III) as indicated by the maceral composition and rock pyrolysis data. The samples from the No. 4 and No. 6 sections occur in the oil window, whereas the other samples are overmature in terms of hydrocarbon generation. The abundant organic richness, humic kerogen, and high thermal maturity demonstrate that these rocks are effective gas source rocks. The volcanics and mudrocks in the Carboniferous Batamayineishan Formation collectively constitute a self-generation and self-storage petroleum system, and represent a main exploration target for gas discovery for the Carboniferous in the Wulungu area.

1. INTRODUCTION The Junggar Basin, located in northwestern China, is surrounded by the Zhayier mountain to the northwest, Yilinheibiergen-Bogeda mountains to the south, and Qinggedili-Kelameili mountains to the northeast (Figure 1). The Junggar Basin is one of the most important oil-producing districts in China, and significant commercial tight oil and gas have been found in many wells.1,2 The Middle Permian Lucaogou Formation was generally regarded as containing the most prolific hydrocarbon source rocks in the Junggar Basin.3 Nevertheless, the Lucaogou Formation is not present in the Wulungu area, north of Junggar Basin (Figure 2), due to the uplift caused by Hercynian movement.4 The distribution and the organic geochemical characteristics of the hydrocarbon source rocks are very important to determine the direction of petroleum exploration in a basin. However, the main hydrocarbon source rocks in the Wulungu Depression are poorly understood.5 The Triassic Huangshanjie Formation and Jurassic Badaowan Formation were previously thought to be the most important hydrocarbon source rocks in the Wulungu Depression (Figure 2), but their thermal maturity is immature to early mature.6 Carboniferous age rocks, which mainly consist of volcanic rocks and tufaceous mudstones (Figure 3), were generally thought to be the basement in the northern Junggar Basin and their hydrocarbon generation potential has been ignored for a long time.5 In 2008, the first large gas field (Kelameili gas field) was discovered in the Carboniferous volcanic rocks from the Ludong-Wucaiwan area, Luliang © XXXX American Chemical Society

Superuplift (Figure 2), Xinjiang Oil Field, and the gas source rocks were thought to be Carboniferous sediments.1,2 Thus, the Carboniferous was interpreted to be the self-generation and self-storage reservoir in this area, and the Carboniferous source rocks have attracted more and more attention.1,2 The Carboniferous contains two major organic-rich mudstones: the Dishuiquan (equivalent to the Jiangbasitao Formation in Wulungu area) and Batamayineishan formations, from oldest to youngest, in the Ludong-Wucaiwan area (Figure 3). The Ludong-Wucaiwan area is adjacent to the Wulungu Depression (Figure 2). However, there is no direct evidence for the existence of the Carboniferous source rocks in the Wulungu Depression due to the low degree of exploration.5 Numerous studies of the geochemistry and chronology of the volcanics from the Junggar Basin have helped to explain the Paleozoic tectonic evolution and continental growth of East Junggar.7,8 Zhang et al.9 determined the hydrocarbon potential and sedimentary environments of sediments from the Dishuiquan Formation in the Wucaiwan area. However, the organic petrological and geochemical characteristics of organic-rich mudstones in the Carboniferous Batamayineishan Formation have not been well documented. The organic-rich tufaceous mudstones within the thick volcanic strata could provide Received: June 20, 2017 Revised: August 27, 2017 Published: August 28, 2017 A

DOI: 10.1021/acs.energyfuels.7b01754 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 1. Map of Junggar Basin, northern China. The study area was marked by the green box.

Figure 2. Distribution of the sampling outcrops, Wulungu area, northern China.

2. GEOLOGICAL SETTING

information as to the paleoenvironments that existed during the Late Carboniferous. In the present study, tufaceous mudstones of the Batamayineishan Formation, collected from the Wulungu area (Figure 2), were characterized by organic petrology and organic geochemistry. The results presented here demonstrate new petrographical and geochemical information from a poorly explored part of the Junggar basin in order to interpret their depositional environments, thermal maturity, kerogen type, and oil/gas proneness of the Batamayineishan mudstones.

The Wulungu Depression, located in northern Junggar Basin, is surrounded by the Mountains of Qinggelidi to the east, Kelameili to the southeast, and Delun to the northeast (Figure 2). This Depression can be divided into Suosuoquan Sag and Hongyan Fault Zone, bounded by the Tusituoyila and Wulungudong faults, which covers an area of about 1.6 × 104 square kilometers (Figure 2). The stratigraphy in the Wulungu area ranges from the Carboniferous to the Palaeogene, excluding Permian and Lower Triassic. This area contains several sets of potential hydrocarbon source rocks of the Carboniferous, Triassic, and Jurassic age, and two sets of B

DOI: 10.1021/acs.energyfuels.7b01754 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 3. Generalized stratigraphy and the main source rocks, reservoirs, caprocks in the northern Junggar Basin.

a stable basin system.10 The Carboniferous source rocks in the northern Junggar Basin are mainly mudstones of the Jiangbasitao and Batamayineishan Formations. The Jiangbasitao

reservoirs of the Carboniferous and Jurassic age (Figure 3).1,2,5,6 The Carboniferous is a critical period when the Junggar area changed from an active basin-mountain system to C

DOI: 10.1021/acs.energyfuels.7b01754 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

system equipped with an HP-5 ms capillary column (60 m × 0.25 mm i.d., 0.25 μm film thickness) using ultrahigh purity He as the carrier gas at a flow rate of 1 mL/min (70 eV ionization energy). The temperature program for saturated hydrocarbons was 50 °C for 1 min, 50−120 °C at 20 °C/min, 120−310 °C at 3 °C/min, and finally held for 25 min at 310 °C. The temperature program for aromatic hydrocarbons was 80 °C for 1 min, 80−300 °C at 3 °C/min, and finally held for 25 min at 310 °C. The temperature of the injector was 300 °C. The MS was scanned from m/z 50 to 550 to confirm the distributions of the aliphatic and aromatic hydrocarbon.

Formation is mainly composed of volcanics interbedded with mudstones, and the thickness of the source rocks is 200−1500 m (Figure 3). The Jiangbasitao samples are characterized by Type I−II kerogen and low-high thermal maturity (0.61−1.80% Ro).11 The Upper Carboniferous Batamayineishan Formation is mainly composed of volcanics, black mudstones, and carbonaceous mudstones. Thickness ranges from 0 to 2500 m, and they are primarily distributed in the Jimusaer and Wucaiwan sags, and Wulungu Depression (Figures 2 and 3).12 Moreover, the Batamayineishan Formation experienced different degrees of denudation due to the variable uplift in Northern Junggar during late Carboniferous to late Permian. Thus, the distribution of the mudstones has a very strong heterogeneity, and their thickness ranges from 100 to 300 m in this region. The mudstones were deposited in shallow, near-shore lacustrine environments. Volcanic rocks in the Batamayneishan Formation are the main reservoirs. These volcanics and mudstones in the Batamayneishan Formation form a very good combination of source rocks and reservoirs in northern Junggar (Figure 3).10

4. RESULTS 4.1. Organic Petrology. Maceral compositions of the studied samples are shown in Table 2. Vitrinite particles are the predominant maceral and are present as collinite, telinite, and vitrodetrinite. The vitrinite color gradually changes from gray to gray white with increasing thermal maturity under reflected light, with percentages ranging from 40 vol % to 100 vol % on a mineral matter free basis (mmf) (Figure 4). Inertinite particles are the second most abundant maceral component in the studied samples, and their concentrations fall between 0 vol % and 60 vol % (mmf). Inertinite contents exceed vitrinite contents only in sample B0501 (Table 2). The liptinite contents are insignificant in the present study, and only trace amounts of lamalginites have been observed in several samples with low thermal maturity as discussed below (Figure 4b and Table 2). The lamalginites display a distinctive lamellar shape with little recognizable structure and were thought to be sourced from phytoplankton or benthic algae. They display strong yellow fluorescence under fluorescent light (Figure 4b). The Ro in the samples from No. 4 and No. 6 sections are low, ranging from 0.84% to 0.91% and from 0.67% to 0.70%, respectively, as listed in Table 2. However, the Ro values in other sections are higher than 1.30%, with the maximum (around 2.70%) determined in the samples from No. 8 section. 4.2. Organic Geochemistry. 4.2.1. TOC and Rock Pyrolysis. The studied samples contain variable TOC concentrations, falling between 0.30% and 10.82% (average = 2.55%) (Figure 5), indicating that these mudstones are mostly good to excellent source rocks.16 The amount of free hydrocarbons (S1) and hydrocarbons generated from pyrolysis (S2) of the kerogen are low in these samples, ranging from 0.04 to 0.08 mg HC/g rock and from 0.04 to 1.94 mg HC/g rock, respectively. The highest S2 values (∼1.63−1.94 mg HC/g rock) were observed in the samples with lowest thermal maturity from No. 6 section as discussed below. The HI values are also extremely low, spanning from 1 to 34 mg HC/g TOC. Tmax values fall between 436 and 566 °C (Table 3). 4.2.2. Aliphatic Hydrocarbons. N-Alkane distribution in the studied samples extends from C14 to C38 and is generally unimodal with a maximum at C18−C21, C23, C25, C27, or C31 (Figure 6a). There is a very strong predominance of high molecular weight (HMW) homologues (≥C21+) with a low nC20‑/n-C21+ ratios, ranging from 0.08 to 0.57 (Table 4). Acyclic isoprenoids occur in significant amounts (Figure 6a), and phytane occurs in higher concentrations than pristane, as indicated by low pristane/phytane (Pr/Ph) ratios of 0.09−0.69 (average = 0.37). Pr/n-C17 and Ph/n-C18 values occur from 0.19 to 1.26 and 0.14 to 1.82, respectively (Table 4). The visible β-carotane peaks were observed in the m/z 125 mass chromatograms (Figure 6b). All samples contain pentacyclic terpanes, including Ts, Tm, C 29 and C 30 hopanes and moretanes, C30 diahopane, gammacerane, and C31−C35 homohopanes (Figure 6c). C29

3. SAMPLING AND METHODS A total of 71 gray black tufaceous mudstones of the Batamayineishan Formation collected from eight sections, Wulungu area, Junggar Basin, were chosen for geochemical and petrological analyses (Figure 2). Detailed information on the studied sections and samples are listed in Table 1. The determination of TOC has been demonstrated in detail

Table 1. Detailed Information of the Studied Sections and Samples section no.

section name

sample no.

1

Wucaicheng

2

Xidagou

3

Zhangpenggou

4

Laoyinggou

5

Baijiangou

6

Zhaheba

7

Baierkuduke

8

Takeerbasitao

B0101− B0105 B0201− B0216 B0301− B0309 B0401− B0410 B0501− B0504 B0602− B0616 B0701− B0703 B0801− B0809

formation

lithology

C2 b

Gray black tufaceous mudstone Gray black tufaceous mudstone Gray black tufaceous mudstone Gray black tufaceous mudstone Gray black tufaceous mudstone Gray black tufaceous mudstone Gray black tufaceous mudstone Gray black tufaceous mudstone

C2 b C2 b C2 b C2 b C2 b C2 b C2 b

by Luo et al.13,14 Rock pyrolysis was conducted on an OGE-II oil and gas evaluation workstation according to a standard procedure (GB/T 18602-2001). The polished blocks (perpendicular to bedding) of the mudstones were prepared as described by Luo et al.15 The maceral characteristics were analyzed on a Leica microscope with the reflected white and fluorescent light. The vitrinite reflectance (Ro) measurements were conducted in oil immersion under reflected light at 546 nm (1.518 refractive index oil). The system was linearly calibrated with a standard of known reflectance (Gadolinium−gallium−garnet, Ro = 1.725%). At least 30 vitrinite particles were measured in each sample. Maceral percentages are based on visual estimates. The samples were crushed to 100 mesh and extracted for 24 h with dichloromethane by Soxhlet extraction. The extracts were separated into aliphatic hydrocarbon, aromatic hydrocarbon, nonhydrocarbon, and asphaltene by silica gel and alumina column chromatography. Gas chromatography mass-spectrometry (GC−MS) of the saturated and aromatic fractions was conducted using an Agilent 7890−5975c D

DOI: 10.1021/acs.energyfuels.7b01754 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Table 2. Maceral Compositions and Vitrinite Reflectance of the Batamayineishan Mudstonesa Ro

a

section no.

sample no.

V (%)

I (%)

L (%)

S (%)

TI

min (%)

max (%)

ave (%)

n

S.D.

1 1 1 2 2 2 2 2 2 3 3 3 3 3 3 4 4 4 5 6 6 6 7 7 8 8 8 8

B0102 B0103 B0105 B0203 B0204 B0207 B0209 B0212 B0213 B0301 B0302 B0303 B0304 B0305 B0307 B0403 B0404 B0410 B0501 B0608 B0612 B0616 B0701 B0702 B0806 B0807 B0808 B0809

95 100 95 95 90 100 100 100 100 90 85 95 95 80 90 80 90 100 40 80 70 75 100 100 100 100 100 100

5 0 5 5 10 0 0 0 0 10 15 5 5 20 10 20 10 tr 60 20 30 25 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 tr tr tr 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 tr tr tr 0 0 0 0 0 0

−76 −75 −76 −76 −78 −75 −75 −75 −75 −78 −79 −76 −76 −80 −78 −80 −78 −75 −90 −80 −83 −81 −75 −75 −75 −75 −75 −75

1.41 1.46 1.39 1.33 1.26 1.49 1.40 1.48 1.40 1.43 1.36 1.53 1.28 1.45 1.35 0.64 0.63 0.72 2.47 0.49 0.45 0.45 1.22 1.23 2.43 2.20 2.40 2.23

1.82 1.80 1.87 1.88 1.89 1.87 1.86 1.80 1.78 1.82 1.93 1.83 1.83 1.83 1.97 1.09 1.19 1.06 2.89 0.88 0.97 0.89 1.66 1.68 3.15 2.99 2.93 3.13

1.63 1.63 1.63 1.68 1.61 1.70 1.62 1.63 1.61 1.60 1.66 1.66 1.57 1.62 1.65 0.84 0.88 0.91 2.68 0.67 0.70 0.70 1.48 1.46 2.78 2.61 2.69 2.73

35 35 35 37 35 38 40 41 39 50 44 37 50 47 33 44 35 35 36 33 33 30 23 32 36 35 35 29

0.11 0.10 0.13 0.11 0.18 0.08 0.11 0.09 0.10 0.08 0.15 0.07 0.14 0.10 0.15 0.10 0.16 0.10 0.12 0.11 0.12 0.13 0.13 0.10 0.19 0.19 0.17 0.24

Abbreviations: V, Vitrinite; I, Inertinite (%); L, Lipitinite; S, Sapropelinite; TI, Type index; S.D.-Standard deviation; tr, trace.

and C30 αβ/(αβ + βα) hopane ratios span from 0.79 to 0.88 (averaging 0.85) and 0.78 to 0.87 (averaging 0.85), respectively. The C31 and C32 22S/(22S + 22R) homohopane ratios range from 0.44 to 0.64 (averaging 0.53) and 0.49 to 0.62 (averaging 0.57), respectively. The samples contain low Ts/(Ts + Tm) ratios (0.13−0.38) and C29 Ts/(C29 Ts + C29 αβ hopane) (0.09−0.27). The moderate to high Gammacerane index (Gammacerane/C30 αβ hopane) has been determined in these samples, with a range between 5.19% and 30.10% (average = 17.80%). High quantities of steranes and diasteranes were observed in the samples (Figure 6d). C29 ααα 20S/(20S +20R) and C29 αββ/(αββ+ααα) ratios are very low in these samples, ranging from 0.34 to 0.42 and from 0.25 to 0.40, respectively (Table 4). The tricyclic terpanes, from C19 to C29 without C27, are present in significant amounts, with C23 member as the predominant homologue (Figure 6c). It is worth noting that the concentrations of the tricyclic terpanes are much higher than that of 17α-hopanes in the samples with high thermal maturity as indicated by the high tricyclics/17α-hopanes ratios (1.06−2.44, average = 1.71), whereas tricyclics/17α-hopanes ratios (0.07−0.15, average = 0.11) is extremely low in the samples with lower thermal maturity from No. 6 section as discussed below (Table 4). 4.2.3. Aromatic Hydrocarbons. Abundant alkylnaphthalenes, alkylphenanthrenes, dibenzothiophenes (DBT), fluorene (F), and dibenzofuran (DBF) were detected in these samples. DBT/phenanthrene (DBT/P) was thought to be related with redox conditions. This ratio is very low in the studied samples,

ranging from 0 to 0.21. The ternary plot of DBT, F, and DBF (Figure 7), proposed by Lin et al.,17 usually indicates paleoenvironment of the sediments. DBT is predominant, with a range of 50% to 100% for DBT/(DBT+F+DBF) ratio. F/(DBT+F+DBF) and DBF/(DBT+F+DBF) ratios are ranging from 0% to 21% and from 0% to 44%, respectively (Figure 7).

5. DISCUSSION 5.1. Sedimentary Environments. The marine shale, carbonate, and marl source rocks generally display higher C31 22R homohopane/C30 hopane (C31R/C30 > 0.25) than the lacustrine source rocks.18 The studied samples primarily display low C31R/C30 ratios, indicating a lacustrine depositional environment. A high value (>1.0) of C29/C30 hopane is generally indicative of carbonate or marl source rocks, but a lower value is indicative of lacustrine source rocks.18 According to the low C29/C30 hopane values (∼0.56−0.93), it can be concluded that the studied samples were deposited in lacustrine environments. The Pr/Ph ratios were used to evaluate the depositional environments.19 It is generally accepted that Pr/Ph values 3.0.19 The Pr/ Ph values (0.09−0.69) in the Batamayineishan Formation is lower than that of the lower Carboniferous Dishuiquan Formation (0.77−1.76).9 In this study, these low Pr/Ph values indicate that anoxic conditions prevailed during the deposition of the Batamayineishan Formation. Peters et al.20 defined E

DOI: 10.1021/acs.energyfuels.7b01754 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 4. Optical characteristics of the vitrinite, inertinite, and lamalginite in the Batamayineishan Formation: (a) vitrinite and inertinite, reflected light, tufaceous mudstone, B0608; (b) same field as (a), but in a fluorescence mode; (c) vitrinite and inertinite, reflected light, tufaceous mudstone, B0302; (d) vitrinite, reflected light, tufaceous mudstone, B0808.

Table 3. Rock Pyrolysis Data of the Batamayineishan Mudstones

Figure 5. Histograms of TOC in the tufaceous mudstones of the Batamayineishan Formation.

different redox fields in the plot of Pr/n-C17 versus Ph/n-C18: oxidizing and reducing (Figure 8). This plot further supports anoxic environments during deposition of these sediments. The plot of DBT/P versus Pr/Ph has been used to determine depositional environment and lithology.21 As shown in Figure 9, these mudstones are inferred to have been deposited in lacustrine hypersaline environments. β-Carotane is generally related with reducing hypersaline lacustrine or highly restricted marine depositional environments.18 β-Carotane has been widely detected in worldwide sedimentary rocks and crude oils.22−24 This compound has also been reported in the lower Carboniferous Dishuiquan Formation and the upper Permian Lucaogou shales and oils from Junggar Basin.3,9,25 Moderate amounts of β-carotane has been detected in the studied samples (Figure 6b), further

sample no.

S1 (mg HC/g rock)

S2 (mg HC/g rock)

Tmax (°C)

HI (mg HC/g TOC)

B0101 B0103 B0201 B0202 B0208 B0216 B0301 B0305 B0309 B0403 B0407 B0410 B0501 B0503 B0504 B0602 B0608 B0612 B0616 B0701 B0802 B0805 B0809

0.07 0.05 0.04 0.03 0.03 0.02 0.02 0.03 0.02 0.03 0.02 0.02 0.01 0.03 0.02 0.05 0.08 0.07 0.05 0.02 0.08 0.05 0.03

0.14 0.12 0.1 0.1 0.09 0.06 0.14 0.14 0.15 0.19 0.1 0.1 0.04 0.04 0.04 0.67 1.69 1.94 1.63 0.04 0.16 0.08 0.07

518 523 516 516 517 525 553 565 515 436 519 488 515 477 526 445 443 444 448 513 566 503 554

11 16 8 8 1 1 3 26 15 34 11 8 3 2 13 16 24 25 28 14 9 23 10

indicating highly anoxic hypersaline stratification during their deposition in the Wulungu area. Gammacerane is generally used as an indicator of a salinitystratified water column and is considered to have originated from tetrahymanol in planktonic bacterivorous ciliates living at F

DOI: 10.1021/acs.energyfuels.7b01754 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

Figure 6. Partial m/z 85 (a), m/z 125 (b), m/z 191 (c), and m/z 217 (d) mass chromatograms of the studied sediments, displaying the distribution of n-alkanes, isoprenoids, β-carotane, tricyclic terpanes, tetracyclic terpanes, hopanes, and steranes: (a) tufaceous mudstone, B0202; (b) tufaceous mudstone, B0503; (c) tufaceous mudstone, B0208; (d) tufaceous mudstone, B0701. In c, the numbers 19−29 refer to the carbon number of tricyclic terpanes; 24Tet = C24 tetracyclic terpane; Ts = C27 18α(H),22,29,30-trisnorneohopane; Tm = C27 17α(H),22,29,30-trisnorhopane; C29αβ = 17α(H),21β(H)-30-norhopane; C29Ts = 18α(H)-30-norneohopane; C30* = C30 17α(H)-diahopane; C30αβ = 17α(H),21β(H)-hopane; C31 αβ = 17α(H),21β(H)-homohopane (22S+22R); C32 αβ = 17α(H),21β(H)-bishomohopane (22S+22R); C33αβ = 17α(H),21β(H)-trishomohopane (22S +22R); C34αβ = 17α(H),21β(H)-tetrakishomohopane (22S+22R). In d, RS is the abbreviation of regular steranes.

the boundary between a lower saline water zone and an upper fresh water zone.26 Low Pr/Ph ratios often coexist with high gammacerane index values in the source rock extracts deposited under anoxic saline environments.18 This is the situation in our study, as suggested by the high gammacerane index (5.19− 30.10%) and low Pr/Ph ratios. F, DBT, and DBF were thought to be derived from the same precursor due to their similar molecular structure, and their relative compositions can be used to infer the primary sedimentary environment. Abundant DBT was generally related with anoxic environments, whereas the dominant DBF indicates an oxic environment.17,27 It may be concluded on the basis of the diagram in Figure 7 that all the studied samples were deposited in a strongly reducing depositional environment, which is consistent with the conclusions based on Pr/nC17 versus Ph/n-C18, DBT/P versus Pr/Ph, high gammacerane index, low Pr/Ph ratios, and moderate concentrations of βcarotane. 5.2. Organic Matter (OM) Type and Thermal Maturity. The predominant macerals are vitrinite and inertinite with only trace amounts of liptinite, mainly as lamalginite, indicating that the major biological source is vascular plant tissues. According to the maceral compositions, the organic type index (TI) can be calculated as following in order to determine the OM type and evaluate the oil/gas proneness:

Types I, II1, II2, and III kerogen are associated with TI of >80, 40−80, 0−40,