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Abstract

For the sake of explaining the organic geochemical characteristics of the hydrocarbon source rocks (SRs) around Karamay, such as thermal maturity (TM) (evidences from the temperature of maximum pyrolysis yield (Tmax), the Carbon Preference Index (CPI) and Odd to Even Preference (OEP) values, the C₃₀ M/H versus Tm/Ts, the C₂₉ ββ/(ββ + αα) versus C₂₉ 20 S/(20 S + 20 R)), hydrocarbon potential (HP) (evidences from total organic carbon (TOC) and rock pyrolysis (RP)), organic matter (OM) source input (evidences from n-Alkane distribution, the relative quantity of C₂₇-C₂₈-C₂₉ steranes, the (C₁₉ + C₂₀)/C₂₃ tricyclic terpane (TT), and the Terrigenous/aquatic ratio (TAR)) as well as paleoenvironment (evidences from the pristane/phytane (Pr/Ph) and the Gammacerane index), and in order to analyze the biodegradation degree and source of oil sand (OS) oil, 55 core samples of SRs in all were selected to measure TOC and RP. 14 core samples of SRs and 3 OS samples were selected for gas chromatography–mass spectrometry (GC-MS) analysis. 11 core samples of SRs and 3 OS samples were used for organic carbon isotope (OCI) analysis. The results indicate that except the SR from Fengcheng formation (P₁f) which is in the stage of high or over evolution, the other SRs are in mature stage. OM type of Carboniferous (C) SRs are type Ⅱ kerogen, with the largest abundance. OM type of the SRs in Wuerhe formation (P₂w) and Jiamuhe formation (P₁j) are type Ⅲ kerogen with good and medium OM abundances. Almost all SRs were formed in the reduction environment of high salinity, and the main OM source input is the lower aquatic plants. Karamay formation (T₂k) and Badaowan formation (J₁b) oil sands (OSs) around Karamay have undergone serious biodegradation. It is suggested that OS oil are mainly from P₁f SRs (evidences from the tricyclic terpanes (TT), triarysteranes, pregnane, homopregnane and OCI).

Keywords

Oil-source correlation biodegradation organic geochemical characteristics oil sands source rocks surroundings of Karamay

Introduction

Junggar basin is the basin with the most abundant OS resources in China. OS is mainly distributed in the northwest edge of the basin from Kebai fault zone to Wuxia fault zone. The OSs, which are mainly found in Middle Triassic T₂k, Upper Jurassic J₁b and Cretaceous Tugulu group, have such characteristics: there are many oil-bearing sand layers (3–14 layers); the thicknesses are large (1.5–26.15 m), and the oil contents are high (3%–18%). Among them, the geological resources of OS with burial depth of 0–100 m are 5.1 × 10⁸t, and the recoverable resources are 3.43 × 10⁸t; the geological resources of OS with burial depth of 100–500 m are 9.2 × 10⁸t, and the recoverable resources are 5.52 × 10⁸t. However, exploring and developing OS resources in this area has not been paid enough attention to. It was not until 2012 that China's first barrel of OS oil was processed and produced from the OS in Wuerhe area in the north of Karamay. In 2013, PetroChina and the local government jointly established Xinjiang Jingobi OS Mine Development Company and built development project of OS mine in Wuerhe area.

Analyzing the geochemical characteristics of OS oil can solve the source problem of OS oil and provide help for exploring and developing OS resources efficiently. However, the distribution of OSs in the surface and near surface environment is inevitably affected by biodegradation, and then some commonly used oil source contrast biomarkers are consumed, which makes it difficult to study the source of OS oil. Firstly, the degree of biodegradation of OS oil should be judged according to the geochemical characteristics. Volkman et al. (1983), Connan (1984), Williams et al. (1986), Wenger et al. (2002), Bao et al. (2007), Larter et al. (2012) sequenced the biomarkers of anti-biodegradation ability from weak to strong based on the experiment and crude oil sample analysis: n-Alkane, i-Alkane, isoprenoid alkane, bicyclic sesquiterpene alkane, hopane (25 - norhopanes formed after degradation), sterane, hopane (25 - norhopanes not formed after degradation), rearranged sterane, aromatic sterane and porphyrin. Peters and Moldowan (1993) classified the degree of biodegradation into 10 levels. Secondly, different oil source comparison methods are selected according to the degree of biodegradation. For crude oil subject to slight biodegradation, the method of comparison of chromatographic characteristics of saturated hydrocarbon in crude oil and SR extract can be applied, which includes the distribution features of n-Alkane and i-Alkane, the main peak carbon number of n-Alkane, odd and even dominant values, pristane/phytane (Pr/Ph), etc. For crude oil with medium biodegradation degree, such as n-Alkane, i-Alkane and isoprene alkanes, which have been modified by microorganisms, the geochemical characteristics of steranes and hopanes can be used for comparison (Peters and Moldowan, 1993). The commonly used parameters include: the Gammacerane index (Ga/C₃₀H = Gammacerane/17αβ C₃₀hopane) reflecting the sedimentary environment of the SRs, the relative contents of C₂₇-C₂₈-C₂₉ steranes reflecting the type of the SRs, the C29 20 S/(20 S + 20 R) (C29(ααα20 S + αββ20 S)/(ααα20 S + αββ20 S + ααα20 R + αββ20 R) steranes), the C29 ββ/(αα + ββ) (C29(αββ20 R + αββ20 S)/(ααα20 S + αββ20 S + ααα20 R + αββ20 R) steranes), the C₂₉M/C₂₉H (C₂₉ moretane/norhopane) and the C₃₁ 22 S/(22 S + 22 R) representing the maturity, etc. For crude oil which have suffered from serious biodegradation and whose steroids and hopane have been damaged to different degrees, aromatic steroids (such as triarysteranes) with stronger biodegradation ability can be used for oil source comparison (Seifert et al., 1984). TT and pregnane + homopregnane have strong anti-biodegradation ability, which is also a good method for oil source comparison of severely biodegradable crude oil (Anders and Robinson, 1971; Li et al., 2012; Peters et al., 2007; Reed, 1977; Wang et al., 2008). In addition, OCI is often used for oil source comparison. For light biodegradation crude oil, the carbon isotopes of n-Alkane can be used for oil source comparison. For medium biodegradation crude oil, the carbon isotopes of terpenes can be used for oil source comparison. For serious biodegradation crude oil, the carbon isotopes of the whole oil are generally used for oil source comparison (Atlas, 1981; Kennicutt, 1988; Stahl, 1980).

Geological setting

Karamay is located near the Kebai fault zone and the southwest edge of Mahu Sag. The faults and stratigraphic unconformities have been developed a lot (Figure 1

Figure 1.

Locations of the study wells where the SRs come from (The relationship between the samples and the wells is shown in Tables 1 to 3) and the sections.

). Through these faults and unconformities, the oil and gas generated by the SRs in Mahu Sag migrated to the sand of Triassic, Jurassic and Cretaceous near Karamay, and then was thickened and degraded by microorganisms, finally forming OSs.

There are four sets of strata that can be used as SRs in Mahu Sag (Figure 2

Figure 2.

Stratigraphic columns (a and b), sections and OS samples, by referring Tao et al. (2016), Liu et al. (2016), Mao et al. (2020), Wang et al. (2020) and Feng et al. (2019, 2020a).

(a)): the C SRs, the P₁j SRs, the P₁f SRs and the P₂w SRs (Chen et al., 2016; Feng et al., 2019, 2020a, 2020b; Tao et al., 2016). Under the tectonic background of continuous compression, Mahu Sag in period C was transformed from ocean to land environment, and its SRs were mainly tuffs and mudstones (Zhou et al., 2019). The SRs of Permian were all formed in lacustrine environment, in which the P₁j of Early Permian was volcanic lacustrine environment, with the SRs being mainly mudstones, argillaceous siltstones and tuffs; the P₁f of Early Permian was in profundal and limited lacustrine environment, and the SRs were mudstones and argillaceous dolomites; in the period of P₂w of Middle Permian, the lake water became shallow, and the SRs were mainly mudstones deposited under the shallow lake environment. Tao et al. (2016), Yu et al. (2018a, 2018b), Zhang et al. (2018), Feng et al. (2020a, 2020b) thought that the P₁f SRs were deposited in the alkali lake environment, with a wide distribution area, good type and large abundance of organic matter. Now they are primarily in the stage of mature evolution and are the best SRs in the region. For example, the crude oil in the sand conglomerate reservoir of Baikouquan formation of Triassic in Mahu Sag primarily came from the P₁f SRs (Feng et al., 2019; Liu et al., 2016). Did the OS oil around Karamay mainly come from the P₁f SRs? At present, there is no research work in this area.

In order to analyze the source of OS oil around Karamay, it is necessary to analyze the geochemical characteristics of OS oil. OS samples come from the top of T₂k and the bottom of J₁b in Tuziakne Ditch, northwest Karamay (Figures 1 and 2(b)). The T₂k and J₁b in this area are in unconformity contact. Their sedimentary environments are all braided river delta, and the OSs are mainly coarse sandstones (Figure 2(b)). Due to long-term exposure to the surface, the sandstones have been subject to weathering and the densities are reduced (Figure 2). Among them, the OS oil must have undergone a strong biodegradation, which brings difficulties to the study of the source of OS oil. It is necessary to comprehensively analyze the geochemical characteristics of OS oil and make a comparative study of oil sources on the basis of evaluating the degree of biodegradation.

Sampling and methodology

A total of 55 core samples of SRs (8 in the study area and 47 in the north of the study area) were selected for TOC and RP measurement (Figure 1 and Table 1

Table 1.

TOC and RP results of the SRs in the study area and the northern part of the study area.

Region	Well	Sample No.	Depth (m)	Fmation	Lithology	TOC (%)	S₁ (mg/g)	S₂ (mg/g)	S₁ + S₂ (mg/g)	HI (mg/g)	T_max (℃)
Other areas of Mahu Sag	Ac1	2	4026	P₂w	Mudstone	0.28	0.08	0.18	0.26	63.25	566
	Ac1	3	4151	P₂w	Mudstone	0.52	0.04	0.06	0.1	11.6	444
	Ac1	5	4359	P₂w	Mudstone	0.84	0.08	0.19	0.27	22.61	292
	Ac1	6	4404	P₂w	Mudstone	0.19	0.04	0.08	0.12	42.11	329
	Ac1	7	4573	P₂w	Mudstone	1.06	0.08	0.06	0.14	5.68	567
	Ac1	9	4700	P₂w	Mudstone	7.9	0.12	0.18	0.3	2.28	298
	Ac1	10	4862	P₂w	Mudstone	1.62	0.05	0.07	0.12	4.32	474
	Ac1	11	4938	P₂w	Mudstone	1.7	0.42	0.35	0.77	20.64	446
	Ac1	12	5004	P₂w	Mudstone	1.19	0.1	0.14	0.24	11.81	424
	Ac1	13	5050	P₂w	Mudstone	1.92	0.18	0.22	0.4	11.48	532
	Ac1	14	5163	P₂w	Mudstone	1.52	0.03	0.02	0.05	1.32	412
	Ac1	15	5269	P₂w	Mudstone	1.72	0.09	0.13	0.22	7.57	413
	Ac1	16	5319	P₂w	Argillaceous siltstone	1.62	0.69	0.51	1.2	31.41	386
	Fc1	17	5456	P₁j	Argillaceous siltstone	0.44	0.05	0.06	0.11	13.73	424
	Fc1	18	5458	P₁j	Argillaceous siltstone	0.64	0.03	0.04	0.07	6.26	307
	Fc1	19	5459	P₁j	Argillaceous siltstone	2.27	0.25	1.08	1.33	47.49	441
	Fc1	20	5461	P₁j	Argillaceous siltstone	0.79	0.08	0.13	0.21	16.41	382
	Fc1	21	5462	P₁j	Argillaceous siltstone	0.68	0.05	0.09	0.14	13.22	472
	Fc1	22	5958	P₁j	Mudstone	1.64	0.06	0.21	0.27	12.8	515
	X76	25	4040	P₁j	Argillaceous siltstone	0.49	0.16	0.15	0.31	30.69	424
	X76	26	4041	P₁j	Argillaceous siltstone	0.92	0.15	0.3	0.45	32.67	454
	X76	27	4042	P₁j	Mudstone	0.52	0.09	0.14	0.23	27.07	523
	X76	28	4043	P₁j	Mudstone	0.3	0.12	0.15	0.27	50.27	311
	Fn1	30	4099	P₁f	Argillaceous dolomite	3.41	0.36	8.37	8.73	245.2	449
	Fn1	32	4124	P₁f	Mudstone	1.85	0.31	3.46	3.77	186.9	449
	Fn1	33	4154	P₁f	Argillaceous dolomite	2.07	0.39	2.06	2.45	99.45	446
	Fn1	34	4183	P₁f	Mudstone	1.09	0.3	0.61	0.91	55.73	443
	Fn1	37	4198	P₁f	Mudstone	1.45	0.37	1.66	2.03	114.8	447
	Fn1	39	4212	P₁f	Mudstone	2.56	0.59	7.29	7.88	284.8	460
	Fn1	41	4231	P₁f	Mudstone	0.87	0.91	0.93	1.84	106.8	436
	Fn1	43	4237	P₁f	Argillaceous dolomite	0.82	0.46	0.53	0.99	64.62	394
	Fn1	45	4252	P₁f	Mudstone	0.85	0.49	0.53	1.02	62.67	386
	Fn1	46	4320	P₁f	Argillaceous dolomite	3.35	0.27	2.33	2.6	69.56	449
	Fn1	47	4327	P₁f	Argillaceous dolomite	9.57	0.56	1.06	1.62	11.08	449
	Fn1	48	4338	P₁f	Argillaceous dolomite	7.39	0.3	1.91	2.21	25.84	446
	Fn1	49	4340	P₁f	Argillaceous dolomite	3.78	0.18	5.37	5.55	142.1	452
	Fn1	50	4358	P₁f	Argillaceous dolomite	3.13	1.02	1.42	2.44	45.4	433
	Bz7	51	745	C	Tuff	19.4	0.31	0.31	0.62	1.6	414
	Bz7	52	913	C	Tuff	20.7	0.02	0.04	0.06	0.19	546
	Bz7	53	1097	C	Tuff	1.24	0.14	0.18	0.32	14.49	521
	Bz7	54	1108	C	Tuff	1.3	0.07	0.12	0.19	9.24	395
	Bz7	55	1289	C	Tuff	3.88	0.12	0.27	0.39	6.96	436
	Bz7	56	1421	C	Tuff	0.91	0.06	0.15	0.21	16.4	433
	Bz7	57	1433	C	Tuff	1.49	0.08	0.16	0.24	10.77	448
	Bz7	58	1997	C	Tuff	3.62	0.17	0.24	0.41	6.64	389
	Bz7	59	2269	C	Tuff	2.82	0.11	0.3	0.41	10.63	289
	Bz7	60	2336	C	Tuff	1.57	0.1	0.18	0.28	11.49	400
Surroundings of Karamay	K80	67	4202	P₂w	Silty mudstone	2.19	0.08	0.19	0.27	8.67	439
	K80	68	4216	P₂w	Mudstone	3.47	0.31	0.98	1.29	28.23	455
	Mh16	70	4382	P₁f	Mudstone	0.84	0.03	0.06	0.09	7.17	512
	K302	76	4156	P₁j	Silty mudstone	1.56	0.07	0.1	0.17	6.4	444
	K302	79	4311	P₁j	Mudstone	1.76	0.01	0.01	0.02	0.57	398
	K022	80	2661	C	Tuffaceous mudstone	2.39	0.72	3	3.72	125.6	446
	Jl24	81	2720	C	Tuffaceous mudstone	4.9	0.11	3.38	3.49	68.91	441
	Jl24	82	2778	C	Tuffaceous mudstone	1.44	0.09	0.14	0.23	9.76	317

). 14 core samples of SRs and 3 OS samples were selected for the GC-MS analysis of aliphatic hydrocarbons and aromatic hydrocarbons (Figure 1 and Table 2

Table 2.

The SR samples’ aliphatic ratios in the study area.

Well	Sample no.	Depth (m)	Fm.	Lithology	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
Well	Sample no.	Depth (m)	Fm.	Lithology	n-Alkanes							Tepanes (m/z 191)						Steranes (m/z 217)
Mh6	62	3772	P₂w	Mudstone	1.69	0.77	1.50	1.23	4.04	2.43	0.69	0.96	0.19	0.06	0.30	0.30	0.57	20.59	17.27	62.14	0.41	0.31
Mh6	63	3773	P₂w	Mudstone	0.60	0.43	1.28	1.12	1.07	0.51	0.63	1.93	0.46	0.27	0.19	0.18	0.56	36.80	33.38	29.83	0.42	0.35
K80	66	4084	P₂w	Silty mudstone	1.00	0.52	1.13	1.08	0.97	0.53	0.54	1.00	0.71	0.25	0.15	0.15	0.53	45.69	31.15	23.16	0.39	0.41
K80	67	4201.5	P₂w	Silty mudstone	0.23	0.08	1.11	1.18	0.93	0.32	0.41	1.84	0.35	0.29	0.14	0.19	0.63	3.41	46.72	49.87	0.48	0.41
K80	68	4215.8	P₂w	Mudstone	0.13	0.04	1.50	1.24	0.98	0.37	0.48	0.97	0.05	0.36	0.13	0.20	0.62	2.26	48.17	49.57	0.46	0.45
Mh16	70	4382	P₁f	Mudstone	3.08	1.49	1.04	1.07	0.55	0.69	1.03	0.29	0.53	0.53	0.13	0.12	0.58	17.53	45.35	37.12	0.43	0.47
K82	71	4084.3	P₁f	Tuffaceous mudstone	0.48	0.21	1.12	1.07	0.77	0.62	0.97	2.36	0.67	0.24	0.15	0.15	0.55	43.88	27.17	28.95	0.43	0.38
K302	76	4156	P₁j	Silty mudstone	0.15	0.03	1.33	1.28	0.94	0.49	0.70	4.15	0.57	0.58	0.43	0.08	0.42	47.72	31.40	20.88	0.38	0.41
K302	78	4232.6	P₁j	Mudstone	0.88	0.50	1.30	1.10	0.91	0.59	0.61	1.07	0.31	0.28	0.14	0.17	0.55	38.08	31.98	29.94	0.43	0.37
K302	79	4310.6	P₁j	Mudstone	0.36	0.14	1.20	1.07	1.16	0.51	0.73	1.81	0.48	0.30	0.15	0.15	0.65	8.13	42.55	49.32	0.50	0.38
K022	80	2661	C	Tuffaceous mudstone	1.77	0.60	1.09	1.07	0.72	0.35	0.43	0.75	0.16	0.33	0.14	0.24	0.65	4.85	46.48	48.67	0.46	0.43
Jl24	81	2720	C	Tuffaceous mudstone	3.08	1.52	3.12	3.14	0.55	0.92	2.03	0.56	0.32	0.11	0.85	0.10	0.23	86.78	10.79	2.43	0.16	0.36
Jl24	82	2778	C	Tuffaceous mudstone	0.21	0.11	1.18	1.11	1.31	0.51	0.71	2.92	0.70	0.22	0.16	0.12	0.61	18.66	40.80	40.54	0.47	0.37
Jl10	83	3002.5	C	Tuffaceous mudstone	1.52	0.55	1.11	1.06	0.72	0.32	0.39	0.80	0.46	0.40	0.14	0.20	0.48	13.54	41.55	44.91	0.48	0.49
1. Waxiness index =∑n-C_21–30/∑n-C_15–20
2. TAR = (n-C₂₇ + n-C₂₉ + n-C₃₁)/(n-C₁₅ + n-C₁₇ + n-C₁₉)
3. OEP = (n-C₂₇ + 6 × n-C₂₉ + n-C₃₁)/(4 × n-C₂₈ + 4 × n-C₃₀)
4. CPI = 0.5 × [(n-C₂₁ + n-C₂₃ + n-C₂₅ + n-C₂₇ + n-C₂₉) + (n-C₂₃ + n-C₂₅ + n-C₂₇ + n-C₂₉ + n-C₃₁)]/(n-C₂₂ + n-C₂₄ + n-C₂₆ + n-C₂₈ + n-C₃₀)
5. Pr/Ph = pristine/phytane
6. Pr/n-C₁₇ = pristine/n-C₁₇
7. Ph/n-C₁₈ = phytane/n-C₁₈
8. (C₁₉ + C₂₀)/C₂₃ TT
9. Ts/Tm
10. Ga/C₃₀H = Gammacerane/17αβ C₃₀ hopane
11. C₂₉M/C₂₉H = C₂₉ moretane/norhopane
12. C₃₀M/C₃₀H = C₃₀ moretane/hopane
13. C₃₁ 22 S/(22 S + 22 R)
14. C₂₇% = C₂₇ααα/C₂₇-C₂₉ααα20 R steranes
15. C₂₈% = C₂₈ααα/C₂₇-C₂₉ααα20 R steranes
16. C₂₉% = C₂₉ααα/C₂₇-C₂₉ααα20 R steranes
17. C₂₉ 20 S/(20 S + 20 R) = C₂₉(ααα20 S + αββ20 S)/(ααα20 S + αββ20 S + ααα20 R + αββ20 R) steranes
18. C₂₉ ββ/(αα + ββ) = C₂₉(αββ20 R + αββ20 S)/(ααα20 S + αββ20 S + ααα20 R + αββ20 R) steranes

). 11 core samples of SRs and 3 OS samples were selected for OCI analysis (Figure 1 and Table 3

Table 3.

The δ¹³C values of the representative SR and OS samples.

Well	Sample no.	Depth	Fm.	Lithology	δ¹³C (‰)
Mh6	61	3700.3	P₂w	Mudstone	−28.45
Mh6	64	3838.4	P₂w	Mudstone	−28.69
Mh6	65	3847	P₂w	Mudstone	−25.59
K80	68	4215.8	P₂w	Mudstone	−29.71
K82	69	3856	P₂w	Mudstone	−28.46
K82	72	4088.5	P₁f	Tuffaceous mudstone	−30.05
Jl17	73	3786	P₁j	Silty mudstone	−28.80
K302	74	3993	P₁j	Mudstone	−29.85
K302	75	4154.6	P₁j	Silty mudstone	−29.42
K302	77	4231.8	P₁j	Silty mudstone	−28.71
K302	79	4310.6	P₁j	Mudstone	−28.18
Oil sand	YS-1	/	T₂k	Sandstone	−29.97
Oil sand	YS-2	/	J₁b	Sandstone	−30.89
Oil sand	YS-3	/	J₁b	Sandstone	−30.28

TOC was tested by the LECO cs-230 analyzer. The OGE–II oil evaluation station was used for RP experiment. Soxhlet apparatus was used to separate aliphatic hydrocarbons and aromatic hydrocarbons, and then GC-MS analysis was carried out. See Feng et al. (2020a) for the process of these experiments.

The carbon isotopic compositions of SR extracts and OS oil were measured by the Finngen-MAT252 Mass Spectrometer. The process was as follows: firstly, CCl₄ was used to dissolve the sample; then the sample was burned at 800°C; then it was reduced at 250°C; and finally, it was measured after being cooled by liquid nitrogen. The error range of stable carbon isotope composition is ±0.1 ‰, and the output standard of δ¹³C value is V-PDB.

Results

Characteristics of SRs

TOC and RP results of SRs can reflect the abundance, type and maturity of organic matter. Of the SR samples for TOC and RP experiments, 8 samples from the study area and 47 samples from the north of the study area are selected for supplement and comparison. Among these SR samples, the TOC from C is the highest (average TOC = 2.91%, standard deviation (SD) = 1.46%), followed by the TOC of P₂w (average TOC = 2.83%, SD = 0.46%) and P₁j (average TOC = 1.66%, SD = 0.1%), and the TOC of P₁f is the lowest, with the TOC value being 0.84% (Table 1 and Figure 3

Figure 3.

TOC and S₁ + S₂ values of the SRs in the study area and the north of the study area.

). The average values of S₁ + S₂ from large to small are: the C SRs (average S₁ + S₂ = 2.48 mg HC/g rock, SD = 1.59 mg HC/g rock), the P₂w SRs (average S₁ + S₂ = 0.78 mg HC/g rock, SD = 0.51 mg HC/g rock), the P₁j SRs (average S₁ + S₂ = 0.10 mg HC/g rock, SD = 0.08 mg HC/g rock) and the S₁ + S₂ of P₁f SRs is 0.09 mg HC/g rock (Table 1 and Figure 3). By comparing the TOC and S₁ + S₂ of the SRs of different strata in the study area, it is found that the trend of the two are the same. Comparing the TOC and S₁ + S₂ values of 47 samples in the north of the study area, it is found that: for the C SRs, the TOC values of the SR samples from the study area and the north of the study area are similar, while the S₁ + S₂ values of the SR samples from the study area are slightly larger; for the P₁j SRs, the TOC values of the SR samples from the study area are slightly larger, while the S₁ + S₂ values are relatively smaller; for the P₁f SRs, the TOC values and S₁ + S₂ values of the SR samples from the study area are significantly smaller; for the P₂w SRs, the TOC and S₁ + S₂ values of the SR samples from the study area are slightly higher.

Hydrogen index (HI) and temperature of maximum pyrolysis yield (Tmax) are of great significance to determine the type and maturity of organic matter in SRs. Among the SR samples in the study area, the average value of HI of samples from C is the largest, which is 68.10 mg HC/g TOC, and its range is also large, being between 9.76 and 125.64 mg HC/g TOC; the average value of HI of the samples from P₂w is the second largest, being 18.45 mg HC/g TOC; the average HI value from P₁f is 7.17 mg HC/g TOC; the lowest average value of HI is 3.49 mg HC/g TOC, which is from P₁j (Table 1). The highest Tmax is 512°C from P₁f; the second highest is 447°C from P₂w (ranging from 439°C to 455°C); the Tmax from P₁j is ranging from 398°C to 444°C (average = 421°C); the Tmax from C is ranging from 317°C to 446°C (average = 401°C) (Table 1).

Aliphatic hydrocarbons characteristics

The m/z 85 mass chromatograms (MCs) can reflect the n-Alkane distributions. The n-Alkane distribution of C SRs includes two types (Table 2 and Figure 4

Figure 4.

The m/z 85 MCs of the representative SR and OS samples.

): the first type is pre unimodal, and the main peak carbon is n-C₁₆, for example, Sample 82. Low waxiness index (WI) (0.21) and Terrigenous/aquatic ratio (TAR) (0.11) indicate that the low molecular weight (MW) n-Alkane is dominant, and low Odd to Even Preference (OEP) (1.18) as well as Carbon Preference Index (CPI) (1.11) values indicate that there is no obvious odd carbon number predominance. The second type is post unimodal, and the main peak carbon is n-C₂₃. For example, Samples 80 and 83. High waxiness index (1.77 and 1.52) and TAR (0.60 and 0.55) indicate that the high molecular weight n-Alkane is dominant, and low OEP (1.09 and 1.11) and CPI (1.07 and 1.06) values indicate that there is no obvious odd carbon number predominance (OCNP).

The n-Alkane distribution of P₁j SRs includes two types (Table 2 and Figure 4): the first type is pre unimodal, and the main peak carbon is n-C₁₆ or n-C₁₇, for example, Samples 76 and 79. Low WI (0.15 and 0.36) and TAR (0.03 and 0.14) indicate that the low MW n-Alkane is dominant, and low OEP (1.33 and 1.20) and CPI (1.28 and 1.07) values indicate that there is no obvious OCNP. The second type is bimodal, and the main peak carbons are n-C₁₈ and n-C₂₅. For example, Sample 78. Low WI (0.88) and TAR (0.50) indicate that the low MW n-Alkane is dominant, and low OEP (1.30) and CPI (1.10) value indicates that there is no obvious OCNP.

The n-Alkane distribution of P₁f SRs also includes two types (Table 2 and Figure 4): the first type is pre unimodal, and the main peak carbon is n-C₁₇, for example, Sample 71. Low WI (0.48) and TAR (0.21) indicate that the low MW n-Alkane is dominant; low OEP (1.12) and CPI (1.07) values indicate that there is no obvious OCNP. The second type is post unimodal, and the main peak carbon is n-C₂₅, for example, Sample 70. High WI (3.08) and TAR (1.49) indicate that the high MW n-Alkane is dominant; low OEP (1.04) and CPI (1.07) values indicate that there is no obvious OCNP. Different from other SR samples, the m/z 85 MCs baseline of Sample 70 slightly uplifted, indicating that it suffered slight microbial degradation and some unresolved complex mixture (UCM) were formed.

The n-Alkane distribution of P₂w SRs also includes two types (Table 2 and Figure 4): the first type is pre unimodal, and the main peak carbon is n-C₁₇, for example, Samples 67 and 68. Low WI (0.23 and 0.13) and TAR (0.08 and 0.04) indicate that the low MW n-Alkane is dominant; low OEP (1.11 and 1.50) and CPI (1.18 and 1.24) values indicate that there is no obvious OCNP. The second type is bimodal, and the main peak carbon is n-C₁₆ or n-C₁₇ and n-C₂₅ or n-C₂₇, for example, Samples 62, 63 and 66, in which the low WI (0.60 and 1.00) and TAR (0.43 and 0.52) of Samples 63 and 66 show that the low MW n-Alkane is dominant; the high WI (1.69) and TAR (0.77) of Sample 62 show that the high MW n-Alkane is dominant. These samples all have low OEP (1.13–1.50) and CPI (1.08–1.23) values, indicating that there is no obvious OCNP.

The m/z 85 MCs of the three OS samples all showed obvious baseline uplift, indicating that they all suffered from strong biodegradation (Figure 4).

Pr/Ph ratio can reflect the redox conditions of sedimentary environment. The Pr/Ph ratios in the study area range from 0.55 to 4.04 (Table 2). There are four samples with Pr/Ph ratios greater than 1, including Samples 62, 63, 79 and 82. The Pr/Ph ratio of Sample 62 is the largest, which is 4.04. The Pr/n-C₁₇ ratio of Sample 62 is also the largest, which is 2.43. The Pr/n-C₁₇ ratios of the other samples are 0.32 – 0.92 (Table 2). The Ph/n-C₁₈ ratios of all the SR samples are 0.39 – 2.03 (Table 2).

The distribution of TT can be seen from m/z 191 MCs of aliphatic hydrocarbons (Figure 5

Figure 5.

The m/z 191 MCs of the representative SR and OS samples, displaying the distribution of tricyclic terpanes and hopanes.

). According to the relative content (RC), four types can be classified: for Type 1, the C₂₀ TT has the largest relative content (Figure 5), for example, Samples 63, 71, 76 and 82. The relative contents of C₂₃ TT are the highest for the other three types. Among them, the samples with the relative contents of C₂₀ TT greater than C₂₁ TT (such as Samples 68 and 78) are classified into Type 2 (Figure 5); the samples with the relative contents of C₂₀ TT less than C₂₁ TT and the relative contents of C₂₄ TT greater than C₂₁ TT (such as Samples 70 and YS-2) are classified into Type 3 (Figure 5); the samples with the relative contents of C₂₀ TT less than C₂₁ TT and the relative contents of C₂₄ TT less than C₂₁ TT (such as Sample 80) are classified into Type 4 (Figure 5). The (C₁₉ + C₂₀)/C₂₃ TT ratio can reflect the OM’s source and the depositional paleo-environment (Bohacs et al., 2000; Peters et al., 2008; Tao et al., 2015). The ratios from P₂w and P₁f samples are the most similar, 0.96–1.93 (average = 1.34, SD = 0.45) and 0.29–2.36 (average = 1.33, SD = 1.04), respectively; the ratios from P₁j samples are the largest, ranging from 1.07 to 4.15 (average = 2.35, SD = 1.31); the ratios from C samples are the smallest, ranging from 0.52 to 2.92 (average = 1.26, SD = 0.96) (Table 2).

Hopanes evolved from the bacteriohopanepolyols precursors which came from the bacterial membranes (Peters et al., 2005). The m/z 191 MCs of aliphatic hydrocarbons also show the distribution characteristics of hopane (Figure 5). C₃₀ αβ hopane (C₃₀H), C₂₉ αβ hopane (C₂₉H), C₃₁H to C₃₅H of all SR samples can be detected, and the relative contents gradually decrease. Almost all samples’ 22 S isomers of C₃₁H to C₃₅H are more enriched than 22 R isomers. Relevant parameters of hopane have also been calculated (Table 2), including the C₂₉M/C₂₉H ratio, the C₃₀M/C₃₀H ratio, the Gammacerane index, the Ts/Tm and the 17αβ C₃₁22S/(22 S + 22 R) value. It is difficult to distinguish the hopane in the OS samples, but C₂₉H can still be detected (Figure 5).

The distribution of steranes and diasteranes are shown in m/z 217 MCs. In this study, C₂₇ ααα 20 R cholestanes, C₂₈ ααα 20 R cholestanes and C₂₉ ααα 20 R cholestanes exhibit two types of distribution (Figure 6

Figure 6.

The m/z 217 MCs of the representative SR and OS samples, displaying the distribution of steranes, pregnanes and homopregnane.

): upward type distribution, including Samples 68 and 80; “V” type distribution, including Samples 63, 71, 70, 76, 78 and 82. The relative content of regular steranes in SRs is significantly higher than that of diasteranes. The average percentage of relative content of C₂₇-C₂₈-C₂₉ regular steranes in SR samples gradually increase. Among them, C₂₇ regular steranes are 3.41% - 86.78% (average = 27.71%, SD = 22.75%); C₂₈ regular steranes are 10.79% – 48.17% (average = 35.34%, SD = 10.96%); C₂₉ regular steranes are 2.43% - 62.14% (average = 36.95%, SD = 14.96%) (Table 2). For the SRs, the C₂₉ 20 S/(20 S + 20 R) sterane ratios are 0.16 – 0.50and the C₂₉ ββ/(ββ + αα) sterane ratios are 0.31 – 0.49 (Table 2). The relative content of regular steranes in OS samples is low, and some of them have been consumed (Figure 6). There are also two distribution characteristics of ββα pregnane, αβ pregnane and homopregnane in SR and OS samples (Figure 6): inverted “V” type distribution includes all SR samples except Sample 70, and descending type distribution includes all OS samples and SR Sample 70.

Characteristics of triarysteranes

Triarysteranes in aromatic hydrocarbons have strong anti-biodegradation ability, which is of great significance to the oil source comparison of severely degraded crude oil. The distribution characteristics of triarysteranes can be seen from the m/z 231 MCs of aromatic hydrocarbons (Figure 7

Figure 7.

The m/z 231 MCs of the representative SR and OS samples, displaying the distribution of triarysteranes.

). Triarysteranes in SRs are easy to identify, with two types: the first type has the largest relative content of C₂₆20R + C₂₇20S, and the representative samples include Samples 70, 76 and 80 from P₁f, P₁j and C; the second type has the largest relative content of C₂₈20S, and the representative sample is Sample 68 from P₂w. For the OS samples, triarysteranes of Sample YS-2 from J₁b can be identified, which is similar to the first type; only a small amount of triarysteranes of Sample YS-3 from J₁b can be identified; triarysteranes cannot be identified in Sample YS-1 fromT₂k (Figure 7).

Characteristics of OCI

The OCI of the extracts from the SRs in the study area are similar (Table 3). The distribution of δ¹³C is between −30.05‰ and -25.59‰. Among them, the OCI of the SRs from P₁f are the lightest, and δ¹³C is −30.05‰; the OCI of the SRs from P₁j are lighter, and δ¹³C is between −29.85‰ and −28.18‰ (average = −28.99‰, SD = 0.58‰); the OCI of the SRs from P₂w are the heaviest, and δ¹³C is from −29.71‰ to −25.59‰ (average = −28.18‰, SD = 1.38‰); the OCI of the SRs from C are not measured. The OCI of OSs is very light, and δ¹³C is between −30.89‰ and −29.97‰ (average = −30.38‰, SD = 0.38‰).

Discussion

Organic geochemical characteristics of SRs

TM of SRs

The TM of SRs can be estimated by some parameters from RP and aliphatic hydrocarbon (Flannery and George, 2014; George et al., 1994, 1997; Geršlová et al., 2015; Luo et al., 2016a, 2016b, 2017; Peters et al., 2005).

Hl and Tmax can reflect the TM of SRs (Peters and Cassa, 1994). It can be seen from Figure 8

Figure 8.

HI and Tmax of the SRs in the study area and the North.

that the OM of P₁f SR is in the stage of high - over maturity evolution, and that of P₂w and P₁j SRs are in the stage of maturity evolution(excluding Sample 79 from P₁j, because its S₂ value is very small, being only 0.01 mg HC/g Rock, and the corresponding Tmax is not credible), while most of the OM of C SRs are in maturity evolution stage (Samples 80 and 81), and a few are in immaturity stage (Sample 82). Comparing the SR samples in the north of the study area (Figure 8), it is found that the TM of OM in C SRs in the study area is higher, and the TM of OM in P₁j SRs is lower; the TM of OM in P₁f SR is significantly higher, and the TM of OM in P₂w SRs is concentrated in the mature stage. At the same time, the CPI and OEP values of almost all the samples are low (Table 2), which indicates that the TM is high.

With the increase of the TM, C₃₀M/C₃₀H in hopanes decreases gradually, while Ts/Tm increases gradually (Peters et al., 2005). As shown in Figure 9

Figure 9.

Plot of C₃₀M/C₃₀H versus Tm/Ts can determine the TM.

, the SR samples in the study area drop in the zone of pre material to early material. Compared with Tmax, the TM reflected by C₃₀M/C₃₀H and Ts/Tm is slightly lower.

The C₂₉ 20 S/(20 S + 20 R) and C₂₉ ββ/(ββ + αα) can reflect the TM of OM (Peters et al., 2005). With the increase of the TM, the C₂₉ 20 S has gradually increased compared with C₂₉ 20 R (equilibrium point C₂₉ 20S/(20S + 20 R) = 0.52 – 0.55) and the C₂₉ ββ has gradually increased compared with C₂₉ αα (equilibrium point C₂₉ ββ/(ββ + αα) = 0.67 – 0.71) (Seifert and Moldowan, 1978). From Figure 10

Figure 10.

Plot of C₂₉ ββ/(ββ + αα) versus C₂₉ 20 S/(20 S + 20 R) can determine the TM.

, all the SR samples are in the mature stage.

In conclusion, almost all the SR samples are in mature TM stage, while only 70 samples from P₁f are in high - over mature TM stage and 82 samples from C are in immature TM stage.

OM and sedimentary paleoenvironment of SRs

The relative content of Pr and Ph can reflect the redox conditions of the ancient water body when the SR was deposited (Powell and Mckirdy, 1973). Didyk et al. (1978), Peters et al. (2005) thought that if Pr/Ph is <1.0, the environment was in reduction condistions; if Pr/Ph is >3.0, the environment was in oxidation conditions. It can be seen from Figure 11

Figure 11.

Plot of Pr/Ph and Ga/C₃₀H can reflect the paleosedimentary environment of the SRs.

that only Pr/Ph of Sample 62 from P₂w is >3.0, which reflected oxidation environment, and all the other samples’ Pr/Ph values are around 1, indicating reduction environment. Consistent with the redox conditions of ancient water reflected by Pr/Ph (Figure 12

Figure 12.

Plot of Pr/n-C₁₇ versus Ph/n-C₁₈ can reflect the paleosedimentary environment of the SRs.

), Pr/n-C₁₇ and Ph/n-C₁₈ also indicate that Sample 62 belonged to oxidation environment and all the other samples belonged to reduction environment (Peters et al., 1999).

The SRs deposited in high salinity water have high Gammacerane content (Damsté et al., 1995). It can be seen from Table 2 and Figure 11 that almost all SR samples have the characteristics of high Gammacerane index, which indicates that they were formed in the environment of saliency qualified water column with reduction. Only Sample 81 from C and Sample 62 from P₂w have low Gammacerane indexes (Table 2 and Figure 11).

The relative contents of C₂₇, C₂₈ and C₂₉ steranes can indicate the input of OM. If the OM is from higher plants, the relative content of C₂₉ sterane is higher (Huang and Meinschein, 1979). Otherwise, the indicator OM mainly comes from the prokaryotic organizations (Isaken, 1991). According to the relative contents of C₂₇, C₂₈ and C₂₉ steranes, most of the SR samples in the study area can be divided into two types (Figure 13

Figure 13.

The relative contents of C₂₇-C₂₈-C₂₉ steranes.

): Type 1 has low relative content of C₂₇ sterane, while the relative contents of C₂₈ and C₂₉ steranes are similar, and this type of samples fall into the terrestrial zone; Type 2 has similar relative contents of C₂₇, C₂₈ and C₂₉ steranes, and this type of samples fall into the marine zone. Except for the C sedimentary period, the study area was a transitional environment of land and sea, and the P₂w, P₁f and P₁j sedimentary periods were all lake environments. Combined with the characteristics of high Ga content, it is considered that the lake was a salt water lake with high salinity. Therefore, the relative contents of C₂₇, C₂₈ and C₂₉ steranes in the SR formed in the salt water lake may be similar to that in the marine environment. Two samples are totally different from the other samples (Table 2 and Figure 13): one is Sample 62 from P₂w, with the relative content of C₂₉ sterane being significantly higher than the relative contents of C₂₇ and C₂₈ steranes, so it is speculated that a large proportion of its OM was terrigenous higher plants; the other one is Sample 81 from C, with the relative content of C₂₇ sterane being significantly higher than the relative contents of C₂₈ and C₂₉ steranes, and the OM is mainly aquatic plankton.

The (C₁₉ + C₂₀)/C₂₃ TT and the TAR can also reflect the input of OM (Bourbonniere and Meyers, 1996; Meyers, 1997; Peters & Moldowan, 1993; Tao et al., 2015). The ratio of (C₁₉ + C₂₀)/C₂₃ TT and the TAR are small for most SRs in the study area, which also indicate that the amount of terrestrial OM is small (Table 2 and Figure 14

Figure 14.

TAR versus (C₁₉ + C₂₀)/C₂₃ TT shows the contributions of the OM input.

). In contrast, Sample 70 from P₁f and Sample 81 from C have the same feature of high TAR, indicating the input of higher plants (Table 2 and Figure 14). However, the degradation of Sample 70 may affect the distribution of n-Alkanes, and the OM input results reflected by the TAR and C₂₇-C₂₈-C₂₉ steranes of Sample 81 contradict each other.

In conclusion, almost all the samples are formed in water with high salinity and reduction environment, and terrestrial higher plants are not the main input of OM. Sample 62 from P₂w and Sample 81 from C are totally different from the other samples. Among them, Sample 62 has relatively high content of C₂₉ sterane, and its n-Alkanes have bimodal characteristics, with the high molecular weight of n-Alkanes content being large, which indicates that the OM has a large proportion of terrestrial higher plants, combined with the characteristics of low Ga content and high Pr/Ph. It is speculated that Sample 62 was deposited in the freshwater oxidation environment near the lake estuary. For Sample 81, the relative content of C₂₇ sterane is high, but the high molecular weight of n-Alkane is high (TAR value is large); the contribution of terrestrial higher plants may be large, and the content of Ga is also low, while Pr/Ph is less than 1, indicating that Sample 81 might be in an environment of freshwater deep-water lake.

Hydrocarbon generating capacity of SRs

According to the Tmax of RP test and some parameters from hopane and sterane (Table 2 and Figure 8 to 10), except for one sample of P₁f which is in the stage of high - over TM, the other SRs are in the mature stage. According to the RP results, the kerogen type of C SRs is the best, which is type Ⅱ; the kerogen type of P₁j and P₂w SRs is type Ⅲ; however, the thermal evolution of P₁f is too high to judge the kerogen type. According to the n-Alkane distribution (Figure 4) and the elative quantity of C₂₇-C₂₈-C₂₉ steranes (Figure 13), the SRs of all strata have the input of terrigenous higher plants, but terrigenous higher plants are not the main source of OM. According to the TAR (Figure 14), the terrestrial higher plants’ contribution to Sample 62 of P₂w and Sample 81 of C is obvious.

According to TOC and S₁ + S₂ (Figure 3), the OM abundance of C SRs is the largest, reaching good to excellent level; the OM abundance of P₂w SRs is large, reaching good level; the OM abundance of P₁j SRs is moderate level; the OM abundance of P₁f SR is the least, possibly affected by its high TM.

According to the Pr/Ph (Figure 11 and Table 2) value, the Gammacerane index (Figure 11 and Table 2) and the Pr/n-C₁₇ versus Ph/n-C₁₈ (Figure 12) value, it is believed that almost all the samples were formed in the water body of high salinity and reduction environment, while the Pr/Ph value of Sample 62 from P₂w is greater than 3, and the Gammacerane content is low, which was fresh water oxidation environment. The Gammacerane content of Sample 81 from C is also very low, which is speculated to be fresh water environment.

Finally, the SRs of C are considered the best; the P₂w SRs are good, and the P₁j SRs are medium. Although P₁f SR has a high TM and has been subjected to biodegradation, it is considered that the hydrocarbon generation capacity of P₁f SRs were also very good in the reservoir forming period, compared with the north of the study area.

Biodegradation degree of OS oil

Biodegradation of crude oil is a step-by-step process. Through experiments and analysis of crude oil samples, Volkman et al. (1983); Williams et al. (1986); Wenger et al. (2002); Larter et al. (2012) have sequenced the biomarkers of anti-biodegradation ability from weak to strong: n-Alkane, i-Alkane, isoprenoid alkane, bicyclic sesquiterpene alkane, hopane (25 - norhopanes formed after degradation), sterane, hopane (25 - norhopanes not formed after degradation), rearranged sterane, aromatic sterane and porphyrin.

The m/z 85 MCs of aliphatic hydrocarbon reflect the n-Alkane distributions. It can be seen that the baseline of m/z 85 MCs of the three OS oil samples has obvious uplift (Figure 4). It shows that n-Alkane has been strongly biodegraded and a large number of UCM has been formed.

From the m/z 191 MCs of aliphatic hydrocarbon, the distribution characteristics of hopane and TT can be seen (Figure 5). The baselines of m/z 191 MCs of the three OS oil samples in the distribution range of hopanes are significantly raised, and C₃₀H to C₃₅H and 25 - norhopanes could not be detected, but C₂₉H and Ga could be detected. On the one hand, it shows that the OS oil has experienced strong biodegradation, and hopanes are destroyed due to degradation (Bennett et al., 2006; Rullkötter and Wendisch, 1982; Seifert et al., 1984; Wang et al., 2010; Wenger et al., 2002); on the other hand, it shows that C₂₉H and Ga have stronger anti-biodegradation ability than C₃₀H to C₃₅H and 25 - norhopanes. C₁₉TT to C₂₅TT of Sample YS-2 can be detected, but that of Sample YS-3 and Sample YS-1 cannot be detected. This shows that the biodegradation of Sample YS-3 and Sample YS-1 is more serious than that of Sample YS-2.

The distribution characteristics of regular steranes, diasteranes and pregnanes in OS oil can be seen from m/z 217 MCs of aliphatic hydrocarbons (Figure 6). In the distribution range of regular steranes and diasteranes, the baseline has obvious uplift. Compared with the SR samples without biodegradation, regular steranes are more difficult to identify and their relative contents are significantly reduced, which indicates that regular steranes have been subject to obvious biodegradation. Compared with C₂₇-C₂₈-C₂₉ regular steranes, the relative contents of diasteranes in the three OS samples did not increase significantly, which indicates that diasteranes may also have been subject to biodegradation. The pregnane and homopregnane of the three OS samples can be detected well, which shows that the anti-biodegradation ability of pregnane and homopregnane is better than that of regular steranes and diasteranes. Compared with the TT of Sample YS-3 and Sample YS-1, it can be inferred that the anti-biodegradation ability of pregnane and homopregnane is better than that of TT.

From the m/z 231 MCs of aromatic hydrocarbons, the distribution characteristics of triarysteranes can be seen (Figure 7). Triarysteranes can be identified well in Sample YS-2, which shows that the biodegradation degree of triarysteranes is light. Some triarysteranes can be identified in Sample YS-3, which indicates that triarysteranes have been biodegraded. Triarysteranes could not be detected in Sample YS-1, which indicates that all triarysteranes have been consumed by biodegradation. It can be seen that the biodegradation order of the three samples from weak to strong is Sample YS-2, Sample YS-3 and Sample YS-1. By comparing the pregnane, homopregnane and triarysteranes of Sample YS-1, it is concluded that the anti-biodegradation ability of pregnane and homopregnane is stronger. By comparing the TT and triarysteranes of Sample YS-2, it is concluded that triarysteranes have stronger anti-biodegradation ability.

In a word, regular steranes, hopanes (no 25 - norhopanes formed after degradation) and diasteranes of Sample YS-2 have been biodegraded obviously, while TT, triarylosanes, pregnane and homopregnane have no obvious biodegradation; regular steranes, hopanes (no 25 - norhopanes formed after degradation), diasteranes and TT of Sample YS-3 have been biodegraded obviously, while triarylosanes have suffered slight biodegradation, and pregnane as well as homopregnane had no obvious biodegradation; regular steranes, hopanes (no 25 - norhopanes formed after degradation), diasteranes, TT and tricyclic of Sample YS-1 have been biodegraded obviously, while pregnane and homopregnane have suffered slight biodegradation.

On the one hand, the recognition shows that the degree of biodegradation of all OSs are very serious, and the order of biodegradation can be further identified from weak to strong as Sample YS-2, Sample YS-3 and Sample YS-1. On the other hand, it shows that the order of biomarker compounds with complete anti-biodegradation ability from weak to strong is speculated as: n-Alkane, i-Alkane, isoprenoid alkane, bicyclic sesquiterpene alkane, hopane (25 - norhopanes formed after degradation), sterane, hopane (25 - norhopanes not formed after degradation), rearranged sterane, TT, aromatic sterane, pregnane and homopregnane.

Source of OS oil

According to the degree of biodegradation, the relative contents of TT, aromatic sterane, pregnane and homopregnane can be used to analyze the source of OS oil. Bost et al. (2001) thought that in TT, except C₂₀TT, the lower the carbon number, the easier the biodegradation. However, according to the distribution characteristics of TT in SRs and OS Sample YS-2 (Figure 5), the relative content of C₂₀TT is lower than that of C₂₁TT in severely biodegradable OS Sample YS-2, which is significantly different from most of the non-biodegradable SRs except for Samples 70 and 80, which shows that the TT distribution characteristics are mainly controlled by the source of crude oil and can be used for oil source comparison. The TT distribution of Sample YS-2 is similar to that of SR Sample 70 from P₁f. The OS oil may come from the SRs of P₁f. According to the distribution characteristics of pregnane and homopregnane in SRs and all OSs (Figure 6), the distribution characteristics of pregnane and homopregnane in Samples YS-2, YS-3 and YS-1 are similar, and they are only similar to those of SR Sample 70 from P₁f. According to the distribution characteristics of triarysteranes in SRs and OS Sample YS-2 (Figure 7), the distribution characteristics of triarysteranes in OS Sample YS-2 are similar to those of SR Sample 70 from P₁f, SR Sample 76 from P₁j and SR Sample 80 from C. Based on the above biomarkers, it is considered that the OS oil may mainly come from P₁f SRs.

The OCI can further determine the source of OS oil. The biodegradation experiment of crude oil has shown that (Atlas, 1981; Kennicutt, 1988; Stahl, 1980) light carbon isotope ¹²C was preferentially used by microorganisms, resulting in the enrichment of heavy carbon isotope ¹³C by residual saturated hydrocarbon. The addition of microbial degradation products resulted in the enrichment of light carbon isotope ¹²C by non-hydrocarbon and asphaltene, with almost no change in aromatics δ¹³C value, but little change in total oil δ¹³C value. In the process of biodegradation, the transfer of organic acid and carbon dioxide may be beneficial to the enrichment of heavy isotope ¹³C. Only the total oil carbon isotope δ¹³C values of the SR samples from P₂w, P₁f as well as P₁j and the three OS samples were measured (Table 3). Due to the lack of samples, the results of previous carbon isotope tests of the SRs in the adjacent area were supplemented (Chen et al., 2016) (Figure 15). It is found that the δ¹³C values of P₂w and P₁f in the study area are similar to those measured by predecessors, while the δ¹³C values of P₁j in the study area are obviously lighter. The δ¹³C values of the three OS samples are almost the same, indicating that they were from the same SR. These values are all distributed in the OCI range of P₁f SRs, and they are obviously lighter than the OCI values of P₂w and P₁j SRs. This further proves that the OS oil came from P₁f SRs. However, considering the carbon isotope fraction during the process of kerogen thermal degradation and hydrocarbon generation, this does not mean that P₂w and P₁f SRs have no contribution.

Figure 15.

The δ¹³C values of other region are from Chen et al. (2016).

Conclusions

According to the organic geochemical characteristics of the SR samples from P₂w, P₁f, P₁j, and C around Karamay: P₁f SR is in high - over mature stage now, and the others are in the mature stage; C SRs are of Type Ⅱ kerogen with the largest OM abundance, and the P₂w and P₁j SRs are of Type III kerogen with the good and medium OM abundances; besides, a few of P₂w and C SRs were formed in the fresh water oxidation and reduction environment respectively. All the SRs were formed in the water body of high salinity reduction environment.

According to the biomarker characteristics of OS samples from T₂k and J₁b around Karamay, it is concluded that all OSs have experienced a very serious biodegradation, and the biodegradation order from weak to strong is Sample YS-2, Sample YS-3 and Sample YS-1. It is speculated that the complete biomarker sequence of anti-biodegradation ability is: n-Alkane, i-Alkane, isoprene like alkane, bicyclic sesquiterpene alkane, hopane (25 - norhopanes formed after degradation), sterane, hopane (no 25 - norhopanes formed after degradation), diasterane, tricyclic terpanes, aromatic sterane, pregnane and homopregnane.

The biomarker characteristics of the OS oil samples in the surrounding area of Karamay are similar to those of the P₁f SR samples. The OCI of the OS oil samples, which is distributed in the OCI range of the P₁f SRs, is obviously lighter than the OCI of the SRs of P₂w and P₁j. It is speculated that the OS oil mainly comes from the P₁f SRs.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Xinjiang Uygur Autonomous Region Scientific Research Program in Colleges and Universities (XJEDU2017I011), and the Innovative Talent Project of Karamay City, Xinjiang Uygur Autonomous Region, China (2020CXRC0002).

ORCID iD

Chong Feng

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Organic geochemical characteristics and comparison of the source rocks and the severely biodegradable oil sands around Karamay,China

Abstract

Keywords

Introduction

Geological setting

Sampling and methodology

Results

Characteristics of SRs

Aliphatic hydrocarbons characteristics

Characteristics of triarysteranes

Characteristics of OCI

Discussion

Organic geochemical characteristics of SRs

TM of SRs

OM and sedimentary paleoenvironment of SRs

Hydrocarbon generating capacity of SRs

Biodegradation degree of OS oil

Source of OS oil

Conclusions

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References