Geochemical Characteristics and Hydrocarbon Generation Modeling of the Paleocene Source Rocks in the Jiaojiang Sag, East China Sea Basin

0. INTRODUCTION

Low-exploration basins have attracted considerable attention to seek reserve replacement or exploration breakthrough. Jiaojiang sag in the East China Sea Basin (ECSB) is a new exploration area with no commercial oil and gas discovery. Limited exploration in the Jiaojiang sag started in 1980s and only 5 wells in the central uplift or the sag edge were drilled (Cai et al., 2016), including 1 oil-gas well (Well JJ4-1) and 1 oil-gas shows well (Well JJ6-1). These two wells showed the presence of oil and gas in the sandstone of the Paleocene E₁y and E₁m formations. In addition, a gas field called LS36-1 has been discovered in the Lishui sag, which is adjacent to the Jiaojiang sag. These findings strongly indicate the presence of significant exploration potential in the Jiaojiang sag. Nonetheless, the scarcity of drilling and geological data in the past resulted in uncertainties about the petroleum system in the Jiaojiang sag, and these uncertainties hampered oil-gas exploration.

Recently, several studies have focused on the basic geological issues such as tectonic evolution, sedimentary system evolution and provenance in the Jiaojiang sag. The achievements can be summarized as follows: (1) Yandang uplift in the east of Jiaojiang sag and Minzhe uplift in the west provided a continuous provenance supply during the Paleocene (Gao et al., 2019; Tian et al., 2012a); (2) the sand-rich sedimentary systems include fan delta facies, delta facies and gravity flow facies, and organic-rich sedimentary systems include lake facies and marine facies (Zhu et al., 2019; Chen et al., 2017); (3) three sets of reservoir-cap assemblages are presented in the Paleocene strata (Tian et al., 2012b); (4) mudstone and coal-measure mudstone sediments in the Paleocene E₁y, E₁l and E₁m formations are considered as potential source rocks (Lei et al., 2021; Li et al., 2019). Nevertheless, there are little or no published works on the geochemical characteristics and hydrocarbon generation potential from the Paleocene source rocks in the Jiaojiang sag.

Based on the bulk geochemistry study on 57 samples and molecular biomarker study on 11 samples, this reseach focuses on the evaluation of organic matter richness, type and thermal maturity of the Paleocene source rocks in the Jiaojiang sag through investigating their geochemical characteristics such as Rock-Eval pyrolysis and H/C, O/C atomic ratios, etc. In addition, depositional environment and organic matter origin of the Paleocene source rocks are determined using biomarker indicators. Finally, hydrocarbon generation capacity and process of the Paleocene source rocks are determined by reconstructing buried, thermal, maturity and hydrocarbon generation histories with basin modeling, so as to provide insight into further oil and gas exploration in the Jiaojiang sag.

1. GEOLOGICAL SETTING

The ECSB is the largest offshore petroliferous basin in China, with a total area of 240 000 km² and a maximum deposition thickness of about 15 000 m. It is a typical back-arc rift basin and can be divided into three structural zones from west to east (Figure 1a), of which the western depression zone includes the Taibei and Yangtze depressions, the central uplift zone includes the Yushan, Haijiao and Hupijiao uplifts, and the eastern depression zone includes the Diaobei, Xihu and Fujiang depressions. The focus of this reseach, Jiaojiang sag, is NE extending in the west of Taibei Depression with length and width of about 72 km (from south to north) and 50 km (from east to west) and water depth of less than 100 m. Controlled by NE trending fault, the sag can be structurally subdivided into Jiaojiang eastern sub-sag and Jiaojiang western sub-sag (Figure 1b). The Jiaojiang eastern sub-sag covers 2 300 km² with a maximum deposition thickness of 7 500 m. It is graben and semi-graben in southern and dustpan-like fault (faulting in the east and overlap in the west) in the northern. The Jiaojiang western sub-sag covers 1 200 km² with a maximum deposition thickness of 6 100 m, which is dustpan-like fault with faulting in the east and overlap in the west (Figure 1c).

Figure  1.  Map (a) showing location and structural units of the East China Sea Basin, maps (b) and (c) showing tectonic setting of the Jiaojiang sag.

DownLoad: Full-Size Img PowerPoint

Jiaojiang sag is a Cenozoic sedimentary basin developed on the top of Mesozoic residual basins with Mesoproterozoic metamorphic rocks and Mesozoic igneous rocks as basement. It successively experienced four tectonic evolution phases, namely, syn-rifting, post-rifting, uplifting and regional subsidence (Jiang et al., 2015). Correspondingly, the deposition environment evolved from continental lake to marine-continental transitional water, and then to marine (Zhu et al., 2019). Intensive fault activities during syn-rifting phase from the Late Cretaceous to Paleocene cut basement and controlled location of deposition and subsidence center, depositing the Paleocene E₁y, E₁l and E₁m formations successively (Figure 2). Mudstone and coal-measure mudstone sediments in the E₁y, E₁l and E₁m formations are regarded as potential source rocks in the Jiaojiang sag: (1) the E₁y was primarily a set of lacustrine deposits with dark mudstone thickness of 200–800 m; (2) the E₁l was dominated by coastal marine deposits developed in transgressive environment with maximum dark mudstone thickness of 500 m; and (3) the E₁m was mainly deposited in regression environment with coal-measure mudstone thickness of 100–300 m. The Oujiang (E₂o) and Wenzhou (E₂w) formations of shallow marine facies were deposited during post-rifting phase from the Eocene to Oligocene, when weak activities of a few inherited faults had no obvious control on deposition. Uplift due to Yuquan orogenesis at the end of Eocene and Huagang orogenesis at the end of Oligocene resulted in the loss of the Eocene Pinghu (E₂p) Formation and Oligocene Huagang (E₃h) Formation, forming an obvious unconformity between the Eocene and Miocene. After that, Jiaojiang sag and associated open area in the east entered regional subsidence phase widely depositing the Miocene Longjing (N₁j), Yuquan (N₁y) and Liulang (N₁l) formations, Pliocene Santan (N₂s) Formation and Pleistocene Donghai (Qd) Formation of marine-continental transitional facies or shallow marine facies.

Figure  2.  Generalized stratigraphy of the Jiaojiang sag. Fm. Formation.

DownLoad: Full-Size Img PowerPoint

2. SAMPLES AND ANALYTICAL METHODS

2.1 Geochemical Analysis Procedures

Fifty-seven samples from the E₁y, E₁l and E₁m formations were collected from wells JJ4-1 and JJ6-1 (well location is seen in Figure 1b). The buried depths of sampling interval in these two wells vary from 1 944 to 2 520 m and 2 644 to 3 334 m, respectively. Measured vitrinite reflectance values (%R_o) of samples in these two wells range from 0.44% to 0.58%, and 0.48% to 0.64%, respectively.

All samples were washed and then crushed into fine powders. After that these samples were pyrolyzed using a Rock-Eval II instrument equipped with total organic carbon measurement function. For this analysis, S₁, S₂, S₃, T_max as well as TOC values were determined.

Eleven mudstone samples with high TOC content were chosen for further gas chromatography (GC), gas chromatography/mass spectrometry (GC/MS), and elemental composition and stable carbon isotope of kerogen measurements. Chosen samples were extracted for 72 h using chloroform solution to obtain the rock extractions. Firstly, the rock extractions were separated into saturate, aromatic and resin fractions using column chromatography method. Saturate hydrocarbon was conducted for GC and GC/MS measurement using an Agilent 7890 instrument (a PONA fused silica column, 60 m × 0.25 mm i.d., film thickness 0.25 μm) and an Agilent 7890A-GC/5975C-MS instrument coupled with a HP-5MS fused silica column (30 m × 0.25 mm i.d., film thickness 0.25 μm), respectively. Refer to Dong et al. (2015) for the detailed operation process. Secondly, the extracted samples were demineralized with acid (e.g., HCl and HF) to separate kerogen. Elemental composition analysis for kerogen was performed on a VARIOEL III instrument. Stable carbon isotope of kerogen was determined using a DELTA-PLUS-XL apparatus, and then the result was reported relative to PDB standard (Coplen, 2011).

2.2 Basin Modeling Procedure

Well JJ7-1 (a pseudo well) in the center of Jiaojiang sag is selected to accurately investigate hydrocarbon generation potential of source rocks through reconstructing burial, thermal, maturity and hydrocarbon generation histories with BasinMod 1D (Version 7.61) software from Platte River company of the United States. Stratigraphic horizon, deposition and erosion age involved in the model construction are shown in Table 1. The key geological parameters required for the basin modeling are lithology, source rock properties, boundary conditions and eroded thickness (Hakimi and Abdullah, 2015). For low-explored area with minor measured geological data, reasonably selecting geological parameters is the key to give reliable modeling results. Lithology and geochemical parameters (e.g., TOC, HI and kerogen type) of Well JJ7-1 are analogized using data from surrounding wells. The boundary conditions are specifically paleo-heat flow, paleo-water depth and paleo-sedimentary water interface temperature, while paleo-heat flow is the most important parameter affecting modeled thermal, maturity and hydrocarbon generation histories (Yu et al., 2020; Gottardi et al., 2019). Its reconstruction process can be summarized as: estimating heat flow value based on tectonic evolution, modifying value until calculated vitrinite reflectance datas (BASIN %R_o) well matching with measured datas (Hakimi et al., 2018; Makeen et al., 2016). However, it is a challenging task for determining the heat flow value of Well JJ7-1 due to lacking of measured vitrinite reflectance data in the center of Jiaojiang sag. Heat flow in the sag center is typically lower than that in the surrounding uplift (Wang et al., 2000; Kutas, 1984), resulting in an over-estimation of source rock thermal maturity by using measured vitrinite reflectance data of uplift to calibrate the heat flow of Well JJ7-1. Hence, predicted vitrinite reflectance data in the center of Jiaojiang sag published by Tong et al. (2012) can be used as calibration data, and then the heat flow value of Well JJ7-1 is calculated to be 48.3 mW/m². The paleo-water depth was derived from the Cenozoic sea level variation of the ECSB published by Wu et al. (1998), which varied between 0 and 120 m. The paleo-sedimentary water interface temperature was acquired according to the global average temperature at sea level (Wygrala, 1989), which was in a range of 18–25 ℃. Jiaojiang sag also experienced multiple intensive uplifts during geological history, e.g., Oujiang orogenesis at the end of Paleocene, Yuquan orogenesis at the end of Eocene, Huagang orogenesis at the end of Oligocene and Longjing orogenesis at the end of Miocene (Jiang et al., 2015), while two primary erosion events, Oujiang and Yuquan orogenesis, were considered in this model. The estimated eroded thickness for the Oujiang and Yuquan orogensis in the Jiaojiang sag are 800–1 000 and 500–600 m, respectively (Li et al., 2015). In addition, mechanical compaction model was used to simulate burial history, transient heat flow model was used to model thermal history, Easy% R_o model proposed by Sweeney and Burnham (1990) and chemical kinetic model proposed by Behar et al. (1997) were utilized to determine thermal history and hydrocarbon generation history, respectively.

Table  1.  The key geological parameters used to reconstruct hydrocarbons generation history for Well JJ7-1

Fm.	Top (m)	Bottom (m)	Thickness (m)	Deposition age (Ma)			Erosion age (Ma)		Erosion thickness (m)	Lithology (%)				Source rock properties
				From	To		From	To		Sand.	Silt.	Mud.		TOC (%)	HI (mg/g)	Kerogen type
Qd	83	488	405	2.6	0					18.9	23.5	57.6		–	–	–
N2	488	857	369	5.3	2.6					38.4	17.9	43.7		–	–	–
N1	857	1 302	445	23.3	5.3					56.8	14.4	28.8		0.4	46	Type Ⅲ
E2p	–	–	–	41.2	36.0		36.0	23.3	600	57.2	16.6	26.2		0.2	23	Type Ⅲ
E2w	1 302	1 863	561	47.8	41.2					35.9	45.4	18.7		0.3	35	Type Ⅲ
E2o	1 863	2 498	635	54.5	47.8					34.2	20.7	45.1		0.4	42	Type Ⅲ
E1m	2 498	3 994	1 496	58.5	55.0		55.0	54.5	800	17.6	35.6	46.8		0.9	77	Type Ⅲ
E1l	3 994	5 322	1 328	61.0	58.5					3.8	11.5	84.7		2.5	324	Type Ⅱ
E1y	5 322	6 342	1 020	66.5	61.0					23.0	9.2	67.8		2.3	312	Type Ⅱ
Geological time scales are from Cohen et al. (2013); the top and bottom depths of strata units are determined from seismic data in combination with time-depth relationship data. Sand. Sandstone; Silt. siltstone; Mud. mudstone.

 | Show Table

DownLoad: CSV

3. RESULTS AND DISCUSSION

3.1 Bulk Compositions and Properties

3.1.1   Richness, type and thermal maturity of organic matter

Organic matter in source rocks is material supplier for hydrocarbon generation, which determines oil and gas resource potential (Peters, 2005). TOC in the E₁y mudstone ranges from 1.86 wt.% to 2.65 wt.% with average value of 2.31 wt.%, while S₁ + S₂ varies between 3.20 and 9.46 mg/g with average value of 7.46 mg/g. TOC in the E₁l mudstone ranges from 2.13 wt.% to 3.75 wt.% with average value of 2.82 wt.%, while S₁ + S₂ is in a range of 5.04–15.29 mg/g with average value of 9.76 mg/g. E₁m mudstone is low in TOC with a range of 0.34 wt.%–2.49 wt.% and average value of 0.97 wt.%, and S₁ + S₂ varies between 0.30 and 4.21 mg/g with average value of 1.09 mg/g. However, E₁m carbonaceous mudstone and coal measures are abundant in organic matter with TOC of 8.66 wt.%–63.24 wt.% and S₁ + S₂ of 8.17–90.43 mg/g. Given multiple source rock types in the Jiaojiang sag, hydrocarbon generation capacity of the E₁y and E₁l is determined based on organic matter abundance evaluation criterion of lacustrine source rock, while that of the E₁m is evaluated according to organic matter abundance evaluation criterion of coal-measure source rock (Chen et al., 1997). Results show that E₁y and E₁l mudstones are high-quality source rocks with high hydrocarbon generation capacity (Figure 3a), while most E₁m mudstone is poor source rock with low hydrocarbon generation capacity (Figure 3b). It should be pointed out that, although higher than that of mudstone, the hydrocarbon generation potential of carbonaceous mudstone and coal in the E₁m Formation is much lower than that of Jurassic coal (S₁ + S₂ values up to 238 mg/g) in Northwest China and Carboniferous–Permian coal (S₁ + S₂ values up to 260 mg/g) in North China (Zhao et al., 2018; Chen et al., 1997). Furthermore, strong hydrocarbon adsorption caused by high organic matter content in coal measures hinders oil expulsion from themselves. Therefore, E₁m carbonaceous mudstone and coal in the Jiaojiang sag cannot act as oil source, but can be good gas source at high maturity stage.

Figure  3.  Variation of TOC values with S₁ + S₂ values (a), (b) and HI values with T_max values (c) for the E₁y, E₁l and E₁m samples in the Jiaojiang sag. Classifications of organic matter type are from van Krevelen (1993).

DownLoad: Full-Size Img PowerPoint

Kerogen type is an important parameter to measure source rock quality, since hydrocarbon generation potential and products vary greatly with kerogen type. According to hydrogen indexs (HIs), Peters and Moldowan (1991) classified kerogen at R_o of 0.6% into three types, i.e., Type I or oil-prone with hydrogen indexs > 300 mg/g, Type II or oil/gas-prone with hydrogen indexs of 200–300 mg/g, Type III or gas-prone with hydrogen indexs of 50–200 mg/g. HIs of the E₁y, E₁l and E₁m samples range from 159–344, 184–394 and 31–209 mg/g (Figure 3c), respectively, indicating that organic matter in the E₁y and E₁l are typical Type II kerogen and the E₁m is dominated by Type III kerogen with certain proportion of Type II-III kerogen. In other words, the E₁y and E₁l can generate both oil and gas, while the E₁m has certain gas generation capacity at high maturation stage.

Thermal maturity can be utilized to express transformation degree of organic matter into hydrocarbon (Bechtel et al., 2012). Measured R_o values are in a range of 0.43%–0.64% (averaging 0.53%), with corresponding T_max values of 415–443 ℃ (averaging 433 ℃), indicating that E₁y, E₁l and E₁m source rocks in the Jiaojiang sag are generally immature to low mature. Obviously, E₁y, E₁l and E₁m source rocks have not yet considerably transformed into liquid hydrocarbon. Importantly, shallow buried depth and no significant variation in thermal evolution greatly limit their hydrocarbon generation capacity, since all samples are located in the uplift or the sag edge. Source rocks in the center of Jiaojiang sag with larger buried depth are speculated to be at higher thermal maturity stage.

3.1.2   Elemental composition of kerogen

Elemental composition of kerogen, e.g., carbon (C), hydrogen (H), oxygen (O) and nitrogen (N) can be applied to decipher original organic matter origin. Hunt (1991) and Powell (1988) regarded H/C atomic ratios of 0.8–0.9 as the floor level for liquid hydrocarbon potential. As shown in Table 2, H/C atomic ratios of the E₁y, E₁l and E₁m samples vary in a range of 0.93–1.04, 1.11–1.14 and 0.69–0.76, respectively. Thus, organic matter in the E₁y and E₁l formations seem to be more oil prone compared with the E₁m Formation. O/C atomic ratios of the E₁y, E₁l and E₁m samples are in a range of 0.08–0.10, 0.08–0.10 and 0.11–0.28, respectively. Based on the van Krevelen diagram, the E₁y and E₁l samples fall within Type II kerogen region, while the E₁m samples fall within Type III kerogen region (Figure 4).

Table  2.  Elemental composition and stable carbon isotope of kerogen as well as vitrinite reflectance for the E₁y, E₁l and E₁m samples in the Jiaojiang sag

Well	Depth (m)	Fm.	Ro (%)	Elemental composition of kerogen							δ13C value of the kerogens (‰, PDB)
				N (%)	C (%)	H (%)	O (%)	N/C	H/C	O/C
JJ4-1	2 412–2 433	E1y	0.47	2.28	66.11	5.74	7.49	0.03	1.04	0.08	-27.2
JJ4-1	2 505–2 508	E1y	0.58	2.05	65.63	5.06	8.59	0.03	0.93	0.10	-26.3
JJ4-1	2 550–2 565	E1y	0.55	2.32	68.19	5.43	8.94	0.03	0.96	0.10	-26.2
JJ4-1	2 286–2 289	E1l	0.44	1.54	45.93	4.23	5.86	0.03	1.11	0.10	-25.8
JJ4-1	2 319–2 328	E1l	0.43	2.09	63.38	6.03	6.81	0.03	1.14	0.08	-26.2
JJ4-1	2 361–2 364	E1l	0.52	2.30	68.30	6.35	8.17	0.03	1.12	0.09	-26.4
JJ6-1	2 760–2 780	E1m	0.48	1.35	66.73	4.04	17.32	0.02	0.73	0.19	-25.6
JJ6-1	2 860–2 875	E1m	0.52	1.53	65.10	3.72	24.63	0.02	0.69	0.28	-25.0
JJ6-1	3 085–3 100	E1m	0.59	1.72	70.62	4.47	13.03	0.02	0.76	0.14	-25.7
JJ6-1	3 154	E1m	0.62	1.80	72.49	4.18	10.29	0.02	0.69	0.11	-25.0
JJ6-1	3 184–3 199	E1m	0.64	1.37	69.50	4.22	15.01	0.02	0.73	0.16	-25.3

 | Show Table

DownLoad: CSV

Figure  4.  Variation of H/C atomic ratios with O/C atomic ratios of kerogen for the E₁y, E₁l and E₁m samples in the Jiaojiang sag.

DownLoad: Full-Size Img PowerPoint

3.1.3   Stable carbon isotope composition of kerogen

δ¹³C values of kerogen in the E₁y, E₁l and E₁m samples range from -26.2‰ to -27.2‰, -25.8‰ to -26.4‰ and -25.0‰ to -25.7‰, averaging of -26.6‰, -26.1‰ and -25.3‰ (Table 2), respectively. Schoell (1984) reported that carbon isotopic composition of kerogen is mainly dominated by biological source, where light δ¹³C value represents aquatic organism source, and heavy δ¹³C value means terricolous organism source. In general, Type I kerogen has δ¹³C value fewer than -27.5‰, Type II kerogen between -27.5‰ and -26.0‰, and Type III kerogen has δ¹³C value more than -26.0‰. According to this criterion, E₁y and E₁l mudstones should contain Type II kerogen, and E₁m mudstones with somewhat heavy isotopes have Type III kerogen. Obviously, this conclusion is in agreement with interpreted kerogen type from HI values as well as cross-plot of H/C and O/C atomic ratios.

3.2 Biomarkers and Paleo-Environment

3.2.1   n-alkanes and isoprenoids

The identified n-alkanes in most mudstone samples of Jiaojiang sag is dominated by C₁₃–C₃₆ (Figure 5). The distribution pattern of n-alkanes is a good indicator of organic matter source (Sachsenhofer et al., 2017). For immature to low mature samples, low carbon number components (< nC₂₀) are dominant in n-alkanes from algae and microorganisms, and high-carbon number components (> nC₂₅) are dominant in n-alkanes from terrestrial higher plants (Volkman et al., 1990). The carbon number of n-alkanes in the E₁y and E₁l samples is distributed in unimodal pattern with the max peak at C₂₃ and/or C₂₇, and the abundance of n-alkanes with low carbon number and high carbon number is similar (represented by moderate C₂₁^-/C₂₂⁺ ratio, Table 3), which indicates that organic matter was sourced from both aquatic organisms and terrestrial higher plants. The carbon number of n-alkanes in the E₁m sample is distributed in asymmetric bimodal pattern with the max peak at C₂₇, where the abundance of n-alkanes with high carbon number significantly increases (represented by low C₂₁^-/C₂₂⁺ ratio, Table 3), indicating considerable terrestrial higher plants input. Furthermore, all analyzed samples have high CPI and OEP values, ranging of 1.39–1.90 and 1.18–1.84 (Table 3), respectively, which further confirms immature to low mature organic matter.

Figure  5.  M/z 191 and m/z 217 mass chromatograms of saturate hydrocarbon for the E₁y, E₁l and E₁m samples in the Jiaojiang sag.

DownLoad: Full-Size Img PowerPoint

Table  3.  Biomarker compound parameters for the E₁y, E₁l and E₁m samples in the Jiaojiang sag

Well	Depth (m)	Fm.	n-alkanes and isoprenoids							Terpanes (m/z = 191)								Steranes (m/z = 217)
										Tricyclic terpanes			Hopane terpanes					Regular steranes (%)			C2920S /(20R+20S)	C29ββ /(αα+ββ)		Diasteranes/steranes
			Pr/Ph	Pr/nC17	Ph/nC18	MP	CPI	OEP	C21-/C22+	TiC19/TiC23	TiC20/TiC23	TeC24/TiC26	C29/C30	Gam/C30	O/C30	Ts/Tm		C27	C28	C29
JJ4-1	2 412–2 433	E1y	2.01	1.97	1.16	C27	1.48	1.30	0.62	0.42	0.97	1.03	0.52	0.12	0.01	0.53		19	24	57	0.13	0.17		0.19
JJ4-1	2 505–2 508	E1y	2.32	1.67	0.69	C23	1.39	1.18	0.56	0.52	0.98	1.29	0.53	0.12	0.02	0.45		22	23	55	0.17	0.18		0.22
JJ4-1	2 550–2 565	E1y	1.39	2.01	1.29	C23	1.45	1.19	0.68	0.48	1.03	0.98	0.45	0.11	0.01	0.48		24	25	51	0.12	0.19		0.27
JJ4-1	2 286–2 289	E1l	1.51	3.12	2.40	C27	1.90	1.80	0.59	0.27	0.83	0.84	0.35	0.09	0.01	0.66		27	24	50	0.06	0.18		0.23
JJ4-1	2 319–2 328	E1l	1.86	3.10	1.79	C27	1.77	1.84	0.49	0.44	1.01	1.00	0.36	0.07	0.01	0.62		26	22	51	0.11	0.18		0.26
JJ4-1	2 361–2 364	E1l	1.65	2.54	1.55	C27	1.65	1.60	0.51	0.43	1.10	0.86	0.44	0.09	/	0.57		22	22	56	0.13	0.18		0.27
JJ6-1	2 760–2 780	E1m	9.53	7.89	0.87	C27	1.78	1.61	0.33	7.39	4.06	3.85	0.63	0.04	0.04	0.05		10	15	75	0.11	0.27		0.33
JJ6-1	2 860–2 875	E1m	4.71	4.37	0.98	C27	1.76	1.64	0.36	7.35	6.42	4.84	0.66	0.04	0.02	0.07		7	16	77	0.15	0.29		0.49
JJ6-1	3 085–3 100	E1m	5.92	7.52	1.31	C27	1.47	1.35	0.33	27.63	6.38	/	0.70	0.02	0.05	0.07		5	14	81	0.26	0.29		0.53
JJ6-1	3154	E1m	5.23	7.93	1.37	C27	1.57	1.43	0.36	6.10	2.89	1.96	0.59	0.02	0.30	0.06		14	9	77	0.31	0.30		0.66
JJ6-1	3 184–3 199	E1m	8.65	8.47	0.94	C27	1.64	1.49	0.26	10.57	4.63	7.31	0.68	0.02	0.07	0.07		7	16	77	0.20	0.29		0.46
MP. Max PeakCPI. Carbon Preference Index (2[C23 + C25 + C27 + C29]/[C22 + 2{C24 + C26 + C28} + C30]); OEP. Odd-Even Predominance ([C25 + C27 + C29]/2[C26 + C28]); C29/C30. C29-norhopane/C30-hopane; Gam/C30. Gammacerane/C30-hopane; O/C30. Oleanane/C30-hopane; Diasteranes/steranes. C29Diasteranes/C29 regular steranes

 | Show Table

DownLoad: CSV

Pristane (Pr) as well as phytane (Ph), most common acyclic isoprenoids in source rocks, are primary parameters to indicate deposition environment (Chandra et al., 1994; Ten Haven et al., 1987). This is achieved by Pr/Ph ratio, e.g., low Pr/Ph ratio (< 1.0) indicates reductive environment, moderate Pr/Ph ratio (1.0–3.0) represents weak-reducing to weak-oxidizing environment, and high Pr/Ph ratio (> 3.0) suggests oxidizing environment (Didyk et al., 1978). The influence of maturity on Pr/Ph ratio can be excluded, since samples analyzed in this paper are mainly immature and low mature (Koopmans et al., 1999). In the Figure 5, considerable Pr and Ph generally appear in all samples, higher than adjacent normal alkanes, with both Pr/nC₁₇ and Ph/nC₁₈ ratios greater than 1.0 (Table 3). In addition, Pr is much higher in concentration compared to Ph in the E₁y, E₁l and E₁m samples, with Pr/Ph ratios in the range of 1.39–2.32, 1.51–1.86 and 4.71–9.53, respectively. The moderate Pr/Ph ratios in the E₁y and E₁l and high Pr/Ph ratios in the E₁m indicate that sub-reducing condition extensively appeared in the bottom water during the E₁y and E₁l periods, whereas oxidizing conditions appeared during the E₁m period. Furthermore, the variation of Pr/nC₁₇ versus Ph/nC₁₈ is also utilized to explain organic matter origin and deposition environment (Moldwan et al., 1985; Shanmugam, 1985). As shown in Figure 6a, most E₁y and E₁l samples are characterized by mixed organic matter input at weak-reducing to weak-oxidizing environment, while the E₁m samples are sourced from terrestrial organic matter at oxidizing environment.

3.2.2   Steroids

Besides C₂₇, C₂₈, C₂₉ regular steranes, 4-methylsterane can be detected from m/z 217 GC/MS, which provides evidence for identifying organic matter origin. Although regular steranes and 4-methylsteranes have various sources, C₂₇ regular steranes are mainly sourced from aquatic organisms, C₂₉ regular steranes are primarily originated from terrestrial higher plants, and 4-methylsteranes are derived from bacteria and/or algae (Farhaduzzaman et al., 2012; Peters et al., 2005; Volkman, 1986). The relative C₂₇, C₂₈, C₂₉ regular steranes contents of the E₁y and E₁l samples are mainly distributed in "V" shape (Figures 5a–5b), which are in a range of 19.3%–24.3%, 23.2%–24.6%, 51.2%–56.7% and 22.1%–26.8%, 21.6%–23.5%, 49.7%–56.4%, respectively, indicating that organic matter was primarily sourced from terrestrial higher plants with aquatic organisms of different proportions. The relative C₂₇ and C₂₈ regular steranes contents in the E₁m samples are in a range of 5.2%–14.3% and 8.5%–16.3%, while C₂₉ regular steranes has highest content ranging from 74.8% to 81.2%. Typical inverted "L" shape in the E₁m samples indicates considerable terrestrial organic matter input (Figure 5c). In addition, 4-methylsterane only occurs in the E₁y and E₁l samples, which further confirms the contributions of aquatic organic matter.

The isomerization of sterane have the potential to describe thermal maturity of organic matter. Previous studies show that increasing thermal maturity can convert unstable "R" shape of sterane into stable "S" shape and transform unstable αα configuration to stable ββ configuration (Hanson, 2000). Therefore, the C₂₉ sterane 20S/(S + R) and C₂₉ sterane ββ/(ββ + αα) will increase with increasing thermal maturity (Seifert and Moldowan, 1986), with suggested thermodynamic equilibrium mixture at 0.52–0.55 and 0.67–0.71, respectively. The steranes of all analyzed samples in the Jiaojiang sag is lowly isomerized with above two parameters lower than 0.31 and 0.30 (Figure 6b), respectively, suggesting that organic matter in the E₁y, E₁l and E₁m samples is present in immature to low-mature stage. This result is consistent with conclusions from measured R_o, T_max, CPI and OEP values.

Figure  6.  Correlation between various biomarker parameters for the E₁y, E₁l and E₁m samples in the Jiaojiang sag, displaying original organic matter input, thermal maturity and deposition environment.

DownLoad: Full-Size Img PowerPoint

3.2.3   Terpenoids

Terpanes, e.g., C₂₇-18a(H)-tris-norneohopane (Ts), C₂₇-17a(H)-tris-norhopane (Tm), C₂₉-norhopane, C₃₀-hopane, (C₃₁–C₃₅)-homohopane, gammacerane, minor (C₁₉–C₂₆)-tricyclic terpane and C₂₄-tetracyclic terpane, etc., can be detected from m/z 191 GC/MS. C₃₀-hopane peak is significantly greater than C₂₉-norhopane peak in the E₁y, E₁l and E₁m samples, with C₂₉/C₃₀-hopane ratios of 0.45–0.53, 0.35–0.44 and 0.59–0.68, respectively (Figure 6c). The relative content of homohopane is positive sequence, i.e., C₃₁-homohopane > C₃₂-homohopane > C₃₃-homohopane > C₃₄-homohopane > C₃₅-homohopane, where the content of C₃₄-homohopane and C₃₅-homohopane are extremely low, also indicating suboxic deposition environment (Peters and Moldowan, 1991).

The contents of Ts and Tm vary as a function of lithology, kerogen type and thermal maturity (Moldowan et al., 1985; Seifert and Moldowan, 1978). All analyzed samples are lower in Ts content compared with Tm content, with Ts/Tm ratios lower than 0.7, especially for the E₁m samples, where ratios are all lower than 0.1 (Figure 6c). This may be explained by the limited catalytic reaction of clay in coal-measure strata (Moldowan et al., 1986). Gammacerane, an important biomarker, originated from tetrahymanol reduction (Damsté et al., 1995). Previous studies suggested that high gammacerane concentration was associated with high water salinity and commonly occurred in deposits derived from stable marine and lacustrine environment with a stratified water column (Hakimi et al., 2016; Sepúlveda et al., 2009; Ten Haven et al., 1989). As illustrated in Figure 5, this compound is commonly observed in all analyzed samples. Gammacerane index, expressed as Gammacerane/C₃₀-hopane, in the E₁y, E₁l and E₁m samples are 0.11–0.12 (averaging 0.11), 0.07–0.09 (averaging 0.08) and 0.02–0.04 (averaging 0.03), respectively, implying that the stability of water stratification was weakened correspondingly. Furthermore, samples with high Gammacerane indexs commonly have low Pr/Ph ratios, while samples with lower Gammacerane indexs generally have high Pr/Ph ratios (Figure 6d), which indicates that water stratification is responsible for reducing environment.

Tricyclic terpane and tetracyclic terpane are another important biomarkers representing terrestrial organic matter input (Adegoke et al., 2015; Hao et al., 2011), e.g., higher C₁₉ tricyclic terpane/C₂₃ tricyclic terpane (Ti_C19/Ti_C23), C₂₀ tricyclic terpane/C₂₃ tricyclic terpane (Ti_C20/Ti_C23) and C₂₄ tetracyclic terpane/C₂₆ tricyclic terpane (Te_C24/Ti_C26) generally indicate high terrestrial organic matter input. As expected, the E₁y and E₁l samples are characterized by low Ti_C19/Ti_C23, Ti_C20/Ti_C23, Te_C24/Ti_C26, e.g., values in the E₁y are 0.42–0.52, 0.97–1.03, 0.98–1.29, respectively, and values in the E₁l are 0.27–0.44, 0.83–1.10, 0.84–1.01 (Figures 6e–6f), respectively. In contrast, the E1m samples are characterized by high Ti_C19/Ti_C23, Ti_C20/Ti_C23, Te_C24/Ti_C26, which are 6.10–27.63, 2.89–6.42, 1.96–7.31, respectively. This means that the E₁m Formation has more terrestrial organic matter input than the E₁y and E₁l formations, which agrees well with the conclusions from the distribution pattern of n-alkanes and the relative content of C₂₇–C₂₉ regular steranes.

3.3 Basin Modeling of the Source Rocks

3.3.1   Burial and subsidence histories modeling

The burial and subsidence histories of Well JJ7-1 are shown in Figure 7. Modeled burial and subsidence histories show that Jiaojiang sag experienced multiple alternated subsidence and uplift in geological history. An initial subsidence occurred during the Paleocene with a subsidence rate of 320 m/Ma and residual thickness of 3 840 m. This rapid subsidence rate was developed in response to the intensive tectonic subsidence during syn-rifting phase. Oujiang orogenesis in later stage uplifted and strongly eroded strata with eroded thickness of 800 m. Subsidence rate decreased to 90 m/Ma in the Eocene with residual thickness of 1 190 m. Uplift event occurred again due to Yuquan orogenesis at the end of Eocene and Huagang orogenesis at the end of Oligocene, eroding the Eocene Pinghu Formation with eroded thickness of 600 m. The subsidence was smooth in the Miocene with subsidence rate of 25 m/Ma and residual thickness of 450 m. The subsidence rate increased to 145 m/Ma from the Pliocene to Quaternary with residual thickness of 770 m. The current burial depth of each formation is the maximum one in geological history.

Figure  7.  Calibration of thermal and maturity models (a), and thermal evolution history (b) for 1-D model constructed in Well JJ7-1.

DownLoad: Full-Size Img PowerPoint

3.3.2   Heat flow and thermal history modeling

Abdalla et al. (1999) regarded that current heat flow can be estimated by thermal conductivity of rocks in combination with geothermal gradient that can be derived from corrected bottom-hole temperature. The calculated geothermal gradient value in the center of Jiaojiang sag is 2.6 ℃/100 m, which is lower than 3.0 ℃/100 m in the uplift (Yang et al., 2004). Therefore, the current heat flow value of Well JJ7-1 is calculated to be 48.3 mW/m² based on transient heat flow model in BasinMod 1D software. Furthermore, "Instantaneous Uniform Extension Model" proposed by McKenzie (1978) was used to assess paleo-heat flows in different geological periods based on the tectonic evolution in the Jiaojiang sag. It was corrected based on calculated vitrinite reflectance (solid line) and calibration data (circle symbols), which agrees well in Figure 7a. Also, the calculated formation temperature (dotted line) in Well JJ7-1 matches well with the calibration data (box symbols), indicating reasonable paleo-heat flow model in this research. The results show that the highest paleo-heat flow in the Jiaojiang sag occurred in the Paleocene with value of 74.2 mW/m². It began to decay since the Early Eocene, and decreased to 44.8 mW/m² at the end of Miocene, which increased slightly and generally maintained between 46.0 and 48.3 mW/m² from the Pliocene to Quaternary.

The established paleo-heat flows were employed to reconstruct thermal evolution of source rocks in Well JJ7-1, which can dynamically show thermal maturity in different geological periods in the Jiaojiang sag (Figure 7b). Significant subsidence and high paleo-heat flow during the Paleocene to Eocene contributed to the increased thermal maturity of the Paleocene source rocks. Specifically, E₁y source rock entered early maturation stage (0.5%–0.7% Ro) and Middle maturation stage (0.7%–1.0% Ro) successively in the Middle Paleocene, and then entered late maturation stage (1.0%–1.3% Ro) in the Late Paleocene and high maturation stage (1.3%–2.0% Ro) in the Middle to Late Eocene, respectively, which did not change further. E₁l source rock was low mature and middle mature in the Late Paleocene and was late mature in the Middle Eocene, which have not entered high maturation stage up to now. E₁m source rock had been in the early maturation stage since the Middle Eocene. Obviously, the thermal maturities of all source rocks in the Jiaojiang sag were unchangeable at the end of Eocene, which can be attributed to tectonic uplift from the end of Eocene to the end of Oligocene and decreased geotemperature in the Neogene.

3.3.3   Hydrocarbon generation history modeling

Hydrocarbon generation history of the Paleocene source rocks in the Jiaojiang sag was reconstructed based on modeled burial and maturity histories to exhibit hydrocarbon generation process and intensity. As mentioned above, Jiaojiang sag had experienced multiple burial and uplift events. The first burial event lasted to the end of Paleocene, while the maximum buried depth of E₁y source rock was 4 000–4 950 m after adequate sedimentary compensation, with corresponding Ro value of 0.7%–1.3%. It gave rise to the first hydrocarbon generation, with generated oil and gas volume of 261 and 52 mg/g, respectively. However, the uplift due to Oujiang orogenesis at the end of Paleocene stopped hydrocarbon generation, which was followed by the second burial event in the Eocene. The maximum buried depth of E₁y source rock was 4 900–5 850 m, with corresponding Ro value of 1.3%–2.0%. The second hydrocarbon generation occurred as response to temperature exceeding the maximum value of the first burial event, however, only gas was generated with generated gas volume of 50 mg/g. E₁y source rock did not generate further hydrocarbon after that (Figure 8a). Similarly, E₁l source rock also experienced two hydrocarbon generation events, specifically, the first occurred in the Late Paleocene with generated oil and gas volume of 142 and 25 mg/g, respectively. The second occurred from the Middle to Late Eocene, with generated oil and gas volume of 130 and 31 mg/g, respectively (Figure 8b). E₁m source rock only experienced one hydrocarbon generation event in the Late Eocene. It has not yet begun to transform into hydrocarbon significantly owing to the low maturation stage, with generated oil and gas volume of 19 and 5 mg/g, respectively (Figure 8c). Therefore, the E₁m Formation should be regarded as an important reservoir (Cai et al., 2016), rather than source rock.

Figure  8.  Relationship between the oil-gas generation and geological time of E₁y, E₁l and E₁m source rocks in Well JJ7-1.

DownLoad: Full-Size Img PowerPoint

Summarily, E₁y and E₁l source rocks in the Jiaojiang sag have high hydrocarbon generation capacity, which brings confidence to oil-gas exploration activities. Tong et al. (2012) proposed that drilling failure of wells JJ2-1, JJ5-1 and JJ6-1 could be attributed to their locating at ends of the long axis of the sag and slope zone of fault terrace, where were not the favorable direction of hydrocarbon accumulation. The central uplift belt is regarded as favorable oil and gas accumulation region. However, the failure of wells JJ3-1 and JJ4-1 in the central uplift can be explained by the trap failure and poor reservoir quality (Tong et al., 2012). Therefore, the future oil-gas exploration in the Jiaojiang sag should focus on hydrocarbon generation center and select targets in the central uplift formed before the Miocene with high-quality traps.

4. CONCLUSIONS

The Paleocene E₁y, E₁l and E₁m formations are three potential source rocks in the Jiaojiang sag. Hydrocarbon generation capacity and process were studied based on geochemical analysis and basin modeling. Conclusions are as follows.

(1) E₁y and E₁l mudstones are high-quality source rocks with high TOC content and Type II kerogen, which are characterized by high thermal maturity. In contrast, E₁m mudstone is poor-quality source rock with low TOC content and Type III kerogen, which is characterized by low thermal maturity. Obviously, hydrocarbon generation capacity of E₁y and E₁l source rocks is higher than that of E₁m source rock.

(2) Differences in hydrocarbon generation capacity of these three source rocks are associated with organic matter type and preservation/degradation. As indicated by biomarkers, organic matter in the E₁y and E₁l was sourced from both aquatic organisms and terrestrial higher plants deposited in sub-reducing environment, whereas that in the E₁m was originated from terrestrial higher plants deposited in oxidizing environment.

(3) Basin modeling suggests that the hydrocarbon generation process of the Paleocene source rocks primarily occurred from the Late Paleocene to Eocene. The future oil-gas exploration in the Jiaojiang sag should focus on hydrocarbon generation center and select targets in the central uplift formed before the Miocene with high-quality traps.