Controls and Geological Significance of Macerals in Hybrid Shales: A Case Study on the Gaoyou Sag, Subei Basin, East China

Abstract

In the Gaoyou Sag located within the Subei Basin, the hybrid shales from the second member of the Funing Formation (E₁f₂) have been identified as a prolific source of shale oil production, despite their characteristically low organic matter content (TOC < 1.5%). This observation suggests that specific macerals within these hybrid shales demonstrate a pronounced hydrocarbon generation potential, thus unveiling a new frontier for shale oil exploration endeavors. In this study, 16 samples were rigorously extracted from both mudstone and hybrid shale strata within the E₁f₂. An exhaustive suite of organic and inorganic geochemical analyses was conducted on these specimens. The analyses elucidated several key findings: (1) The maceral composition within the hybrid shales is predominantly comprised of alginite, solid bitumen, and inertinite. Remarkably, the variability of alginite content within the hybrid shales is more pronounced than that observed in high-TOC mudstones. High-TOC mudstones are characterized by a preponderance of lamalginite and a paucity of telalginite, leading to a diminished aggregate hydrocarbon potential. (2) Biomarker ratios (e.g., sterane/hopane, C₂₃ tricyclic terpane/αβ C₃₀ hopane, C₂₄ tetracyclic terpane/C₂₆ tricyclic terpane, etc.) suggest a primary derivation from lower aquatic organisms, with a secondary contribution from terrigenous organic matter within the hybrid shales. (3) The accretion of macerals is governed by an intricate set of factors, including the input of terrigenous detritus, paleosalinity, and paleoproductivity. (4) Alginite is identified as the principal constituent responsible for hydrocarbon genesis in hybrid shales. The proliferation of alginite facilitates the concurrent enrichment of shale oil and organic matter within the hybrid shales of the Subei Basin, illustrating a cooperative mechanism underlying the accumulation of shale oil and organic matter. This indicates that even hybrid shales with scant organic content exhibit considerable potential for shale oil exploration.

1. Introduction

The burgeoning demand for oil and gas, set against the backdrop of a pronounced supply demand disequilibrium, accentuates the critical necessity for the exploration of innovative resources. Within the domain of continental shale oil exploration in China, three principal source-reservoir paradigms have been identified, each characterized by distinct tectonic and sedimentary regimes, namely, the interlayered shale, hybrid shale, and clayey shale types.¹⁻⁴ Among these typologies, hybrid shale has ascended as a pivotal element within the ambit of both domestic and international shale oil exploration and development endeavors. As a quintessential exemplar of the source-reservoir archetype, the paramount importance of hybrid shale is manifest across various geological formations, notably including the Shahejie Formation (members 3 and 4) within the Dongying-Zhanhua Sag from the Bohai Bay Basin, the Lucaogou Formation in the Jimsar Sag, and the Kongdian Formation (member 2) in the Cangdong Sag in China as well as the Eagle Ford Formation. These stratigraphic units have heralded initial successes in hydrocarbon yields.⁵⁻⁹

Lacustrine hybrid shale, distinguished by its marked heterogeneity, a broad spectrum of mineralogical compositions, and a significant content of carbonate rock minerals, is often considered a critical reservoir type. Previous research efforts have primarily focused on the analysis of mineral composition, the elucidation of structural features, and the dynamics of porosity within these formations.¹⁰ Intriguingly, the organic matter content in hybrid shale does not meet the conventional benchmarks traditionally postulated for the enrichment of shale oil resources, which stipulate that the TOC should exceed 2.0%, chloroform bitumen “A” should be greater than 0.4%, and S₁ should surpass 2.0 mg/g. Moreover, the thresholds for hydrocarbon generation are characterized by a vitrinite reflectance (R_o) ranging between 0.7% and 1.1%.^8,10−13 During mid to low maturation phases, these shales display propitious oil-bearing properties yet exhibit diminished fluidity, frequently categorizing them as “ineffective” within the context of shale oil resources.¹¹⁻¹³

The recent success in extracting shale oil from the hybrid shales of the Funing Formation within the Gaoyou Sag of the Subei Basin, culminating in a production exceeding 10,000 tons from over five new wells, has ignited a burgeoning interest in hybrid shale as a potent source rock.² This milestone raises pivotal inquiries regarding whether the extraction from low-TOC hybrid shale can be ascribed to a distinctive composition of hydrocarbon-generating elements, thus necessitating an in-depth exploration of the dynamics of organic matter enrichment. Numerous studies indicate that different types of macerals exhibit varying hydrocarbon potentials.¹⁴⁻¹⁶ Alginite exhibits very high potential for oil and gas generation, whereas vitrinite and inertinite typically lack oil generation potential.¹⁷⁻²⁰ Therefore, determining the maceral composition of the hybrid shale in the Funing Formation of Gaoyou Sag is of great significance for the exploration and development of shale oil. Consequently, this investigation, with a focus on the Subei Basin, embarks on an exhaustive analysis incorporating total organic carbon (TOC), Rock-Eval pyrolysis, organic petrography, comprehensive major and trace element assessments, and gas chromatography–mass spectrometry (GC-MS) of saturated hydrocarbons. The aim is to delineate the maceral composition within hybrid shale, identify the primary factors driving enrichment, and discern their implications for the hydrocarbon potential and oil-bearing capacity. This endeavor seeks to provide a robust theoretical foundation for the prospective exploration of hybrid shales with low organic abundance.

2. Geological Setting

The Subei Basin, spanning an extensive area of 3.8 × 10⁴ km², stands as a distinguished Cretaceous-Neogene faulted basin and epitomizes the most expansive Mesozoic-Cenozoic basin located in the southeastern region of China.²¹ This basin, located within the terrestrial bounds of the Subei South-Yellow Sea Basin, is defined by the Subei Uplift to the north and the Lusu Ancient Land to the south. It is bordered on the west by the Tanlu Fault and on the east by the expanse of the South Yellow Sea Basin^22,23 (Figure 1A). Structurally, the Subei Basin is segmented into three principal sections: the Yanfu Depression, the Jianhu Uplift, and the Dongtai Depression.²³

Figure 1.

A) Geologic element division map for the Gaoyou Sag, Subei Basin. Adapted with permission from Gao et al.²¹ and Quaye et al.²⁴ Copyright 2018 and 2022 Elsevier. B) Lithological column of E₁f₂ in Gaoyou Sag.

Nestled within the Dongtai Depression, the Gaoyou Sag spans an area of 2,670 km², making it the most substantial subsidence center in the Subei Basin. Its development is credited to the rapid and continuous subsidence prompted by the Late Cretaceous Yizheng Movement and the Paleocene Wubao Movement. The stratigraphic sequence in this region exhibits Mesozoic-Cenozoic deposits with a cumulative thickness exceeding 7,000 m. The structural composition of the basin is uniquely defined, featuring a fault zone to the south, a central deep depression, and a northern slope, all aligned along a north–south axis. The fault system is primarily oriented along the northeast (NE), north–northeast (NNE), and east-southeast (ESE) directions, creating a configuration akin to a dustpan, characterized by a pronounced southern incline and a gentler northern slope. This structural arrangement is particularly advantageous for the development of secondary oil and gas reservoirs.²¹ Stratigraphically, Gaoyou Sag encompasses a sequence of formations extending from the Upper Cretaceous to the Quaternary period. This sequence includes the Cretaceous Taizhou Formation (K₂t), the Paleogene Funing Formation (E₁f), the Paleogene Dainan Formation (E₁d), the Paleogene Sanduo Formation (E₁s), the Neogene Yancheng Formation (N₂y), and the Quaternary Dongtai Formation.^23,26 The Paleogene Funing Formation, which represents the foremost exploration target, is distinguished by its considerable resource potential and favorable conditions for the accumulation of shale oil. This formation is stratified into four members in ascending order from bottom to top (E₁f₁–E₁f₄).²³

This research concentrates on the second member of the Funing Formation (E₁f₂), renowned for its lacustrine sedimentary layers, predominantly consisting of gray-black shale. Additionally, this member is characterized by the localized presence of the oil shale and stratified carbonate rock interbeds. Remarkably, it is abundant in fossils, such as the Funing Chinese Venus and the Jiangsu Crown Charophyte, offering a crucial understanding of the environmental conditions favorable for the genesis and aggregation of shale oil and gas. The E₁f₂ is acknowledged as the primary hydrocarbon source rock series within the Subei Basin.^21,22,24

3. Samples and Analytical Methods

3.1. Samples

In this study, a comprehensive collection of 16 core samples was scrupulously obtained from the mudstone and hybrid shale formations within the E₁f₂ section of Well Y1, situated in the Gaoyou Sag, Subei Basin. These specimens were meticulously extracted from stratigraphic depths extending from 3473.40 to 3711.50 m, as illustrated in Figure 1B. Each specimen was subjected to a comprehensive and intricate analytical regimen designed to illuminate its geochemical and petrological properties. This included the determination of TOC, Rock-Eval pyrolysis, and organic petrography. Additionally, an extensive evaluation of both major and trace elements was undertaken, augmented by the utilization of GC-MS for the analysis of saturated hydrocarbons. This multifaceted analytical approach was designed to yield a comprehensive understanding of the geochemical properties and hydrocarbon potential of the sampled formations.

3.2. Analytical Methods

3.2.1. Total Organic Carbon Content and Rock-Eval Pyrolysis

In the initial stage of sample preparation, 100 g of shale undergoes ultrasonic treatment in deionized water to facilitate the removal of surface clay. This is succeeded by a procedure entailing drying and then pulverizing the material to achieve a fine granularity of 200 mesh. A 100 mg portion of the resulting powder is then transferred into a crucible, and 5% hydrochloric acid is incrementally added dropwise until the cessation of foaming occurs. The sample is subjected to an acid bath for a duration of 4 h and subsequently rinsed multiple times with deionized water until it attains a neutral pH, thereby ensuring the elimination of inorganic carbon. The desiccated sample is then subjected to TOC analysis by using the LECO–CS230 analyzer. Furthermore, an additional 100 mg of sample is processed using Rock-Eval 6 pyrolysis to evaluate its potential for hydrocarbon generation.

3.2.2. Organic Petrography

In the preparatory phase for sample analysis, sample preparation followed standard organic petrography procedures²⁵ The macerals of organic matter are identified and documented employing a Leica DM4500P microscope in conjunction with a Leica DFC 310 FX digital camera. Vitrinite reflectance (R_o) assessments are conducted using a Zeiss Photoscope III reflection microscope, which is interfaced with a TIDAS PMT IV photometric system. This measurement utilizes a 50× oil immersion objective lens at a wavelength of 546 nm. A sapphire standard, possessing a known reflectance of 0.589%, serves as the calibration reference. The procedure involves the measurement of 50 points to ascertain an average reflectance value.

3.2.3. Major and Trace Element Measurements

In this investigation, an X-ray fluorescence spectrometer (XRF) was utilized for the quantitative analysis of major elements. The process entailed several steps: (1) The powdered sample was dried in an oven at a stable temperature of 105 °C for 2 h to ensure complete dryness. Subsequently, a 0.5 g portion of this dried powder was accurately measured and placed into a diminutive crucible. (2) This portion was then subjected to calcination in a muffle furnace maintained at a constant temperature of 1000 °C for 1 h. The mass was measured postcalcination to ascertain the loss on ignition. (3) A concoction comprising the sample, a cosolvent, and an oxidizing agent was prepared in a platinum crucible and incinerated at 1100 °C, thereafter being allowed to cool to form a vitreous sheet for the analysis of major elements. Major element concentrations were determined by using a Thermo Scientific Fisher ARL 9900 X-ray fluorescence (XRF) spectrometer. Analytical precision was usually better than ±3%.

An Agilent 7700e Inductively Coupled Plasma Mass Spectrometer (ICP-MS) was employed for the analysis of trace elements, with the sample preparation process detailed as follows: (1) Sample powders with a granularity of 200 mesh were desiccated in an oven at 105 °C for 12 h. (2) A 50 mg sample was placed into a Teflon sample dissolving vessel. Subsequently, 1 mL of HNO₃ and 1 mL of HF were sequentially added to the vessel, which was then securely sealed. (3) The sample-containing vessel was dried in an oven at 190 °C for 24 h. (4) The vessel was positioned on an electric heating plate set at 140 °C to evaporate the mixture to near dryness. Thereafter, 1 mL of HNO₃ was reintroduced to the mixture and evaporated to near dryness once more, with the vessel remaining uncovered during this process. (5) An additional 1 mL of HNO₃, 1 mL of MQ water, and 1 mL of an internal standard were added to the sample. The vessel was then returned to the oven and dried at 190 °C for 12 h. (6) The resultant solution was transferred into a polyethylene bottle and diluted with 2% nitric acid to a final volume of 100 g for analysis via ICP-MS. The accuracy of the elemental analysis was better than ±3%.

3.2.4. Gas Chromatography–Mass Spectrometry (GC-MS) Analysis of Saturated Hydrocarbons

The shale specimen is finely ground to a granularity of 100 mesh. Subsequently, a 20 g portion of this powdered sample undergoes Soxhlet extraction in a water bath for 72 h to derive a chloroform-soluble bitumen “A” organic extract. Following air drying, this extract is amalgamated with n-hexane and subjected to ultrasonication for 5 min, then left undisturbed for 12 h to facilitate the precipitation of asphaltenes. The resultant filtrate is further processed using column chromatography with a silica gel/alumina matrix at a ratio of 3:2, employing a sequence of elution steps with n-hexane, a dichloromethane/n-hexane mixture (1:2), and a dichloromethane/methanol mixture (93:7). This procedure effectively segregates the saturated hydrocarbons, aromatic hydrocarbons, and nonhydrocarbon fractions. The isolated saturated hydrocarbon fraction is subsequently analyzed with an HP Agilent 6890/5975 GC-MS system. Detailed descriptions of this methodology and the specific instrument settings have been elaborated in prior publications.^27,28

4. Results

4.1. TOC and Rock-Eval Pyrolysis

The analytical results from the 16 samples indicate that the TOC content spans from 0.68% to 2.47% with an average of 1.24%. The mudstone samples demonstrate elevated TOC content, ranging between 2.39% and 2.47% and averaging 2.43%. Conversely, the hybrid shale specimens present a wider range of TOC values, from 0.68% to 2.46%, with an average of 1.24% (Table 1 and Figure 2).

Table 1. TOC and Rock-Eval Pyrolysis Data for Hybrid Shale and Mudstone from the Funing Formation in the Gaoyou Sag, Subei Basin.

no.	depth (m)	lithology	TOC (%)	S1 (mg/g)	S2 (mg/g)	S1+S2 (mg/g)	free oil(mg/g)	total oil(mg/g)	Tmax (°C)	HI(mg/g)
S-1	3473.40	mudstone	2.39	1.85	7.47	9.32	4.31	9.33	445	312.55
S-2	3482.22		2.47	4.41	9.20	13.61	10.25	15.17	445	372.47
S-3	3589.75	hybrid shale	0.68	0.35	1.17	1.52	0.82	1.56	439	171.55
S-4	3670.90		1.15	1.07	2.04	3.11	2.49	3.52	443	177.39
S-5	3674.78		1.01	0.66	2.56	3.22	1.54	3.24	438	253.47
S-6	3676.46		1.53	0.84	3.69	4.53	1.96	4.48	449	241.18
S-7	3680.47		1.39	0.39	3.97	4.36	0.91	3.93	444	285.61
S-8	3680.95		1.43	0.75	4.17	4.92	1.75	4.72	435	291.61
S-9	3683.65		2.46	0.74	9.23	9.97	1.72	8.85	437	375.20
S-10	3686.72		1.12	1.01	3.44	4.45	2.35	4.57	437	307.14
S-11	3688.46		0.90	5.37	1.01	6.38	12.51	10.12	429	112.22
S-12	3696.68		1.69	1.52	5.73	7.25	3.54	7.33	437	339.05
S-13	3697.40		1.06	0.46	2.08	2.54	1.07	2.50	441	196.23
S-14	3698.37		0.90	0.40	2.06	2.46	0.93	2.38	430	228.89
S-15	3701.35		1.10	0.43	2.37	2.80	1.00	2.69	442	215.45
S-16	3703.01		0.99	0.32	2.11	2.43	0.75	2.28	429	213.13

Figure 2.

Vertical variations in TOC, S₁, S₂, HI, Total oil, Free oil, and R_o data for both the hybrid shale (3–16) and mudstone (1–2) from the Funing Formation in the Gaoyou Sag, Subei Basin.

Rock-Eval pyrolysis results demonstrate that the free hydrocarbon content (S₁) across these samples fluctuates from 0.32 to 5.37 mg of HC/g of rock, averaging 1.29 mg of HC/g of rock. Specifically, S₁ in the mudstone samples ranges from 1.85 to 4.41 mg of HC/g of rock (average 3.13 mg of HC/g of rock), while hybrid shale samples varied from 0.26 to 5.37 mg of HC/g of rock (average 1.02 mg of HC/g of rock). The pyrolysis hydrocarbon content (S₂) spans from 1.01 to 9.23 mg HC/g rock (average 3.89 mg HC/g rock). Specifically, mudstone samples have an average of 8.34 mg HC/g rock, whereas hybrid shale samples hold an average of 3.26 mg HC/g rock. The Hydrogen Index (HI) values range from 112 to 375 mg HC/g rock, averaging 256 mg HC/g rock (Table 1 and Figure 2).

4.2. Organic Petrological Characteristics of Organic Matter

The organic constituents identified within the analyzed samples predominantly comprised alginite, solid bitumen, vitrinite, and inertinite (Figure 3). Alginite emerges as the chief maceral, particularly within the context of mudstone, where it constitutes approximately 80–83 vol %. The proportion of alginite within hybrid shales exhibits notable variability, extending from 15 to 88 vol %, with a mean value of 56.8 vol % (Table 2). Conversely, the prevalence of vitrinite, solid bitumen, and inertinite within mudstone samples is comparatively modest, averaging 9.5, 5.0, and 4.0 vol %, respectively. In contrast, hybrid shale samples manifest a relatively elevated concentration of vitrinite and solid bitumen, averaging 15.3 and 16.6 vol %, respectively. Among the analyzed macerals, inertinite exhibits the lowest average concentration, at 11.3 vol % (Table 2).

Figure 3.

Macerals composition diagram of hybrid shale and mudstone from the Funing Formation in the Gaoyou Sag, Subei Basin.

Table 2. Macerals Composition of Hybrid Shale and Mudstone Within the Funing Formation in the Gaoyou Sag, Subei Basin.

	mudstone			hybrid shale
	min.	max.	avg.	min.	max.	avg.
macerals	%(v/v) for macerals
alginite	80	83	81.5	15	88	56.8
vitrinite	8	11	9.5	4	35	15.3
solid bitumen	4	6	5.0	2	35	16.6
inertinite	3	5	4.0	3	55	11.3

The organic matter in the samples predominantly comprised alginite, solid bitumen, vitrinite, and inertinite (Figure 3). Alginite is the predominant maceral, particularly in mudstone, where it accounted for 80–83 vol %. The alginite content in hybrid shales exhibits notable variability, extending from 15 to 88 vol %, with an average of 56.8 vol % (Table 2). Vitrinite, solid bitumen, and inertinite in mudstone is comparatively modest, averaging of 9.5, 5.0, and 4.0 vol % respectively. The content of vitrinite and solid bitumen in hybrid shale is relatively high, with an average of 15.3 vol % and 16.6 vol %. Among the analyzed macerals, inertinite exhibits the lowest average content, with an average of 11.3 vol % (Table 2).

Under oil-immersion reflected light, alginite manifests an amber coloration, while it exhibits a bright yellow fluorescence under ultraviolet illumination (Figure 4). In the E₁f₂ mudstone samples, Tasmanites and Botryococcus algae are prolific, intricately interspersed throughout the shale matrix, demonstrating pronounced fluorescence (Figure 4b,d). The alginite in these mudstones is primarily dominated by telalginite, with a subordinate presence of lamalginite. In contrast, hybrid shale alginite is characterized by lower fluorescence intensity, adheres in parallel alignment to bedding planes, and is distributed in a striated pattern (Figure 4f,g). Within these hybrid shales, lamalginite is notably present, forming distinct algal layers along the stratifications (Figure 4j), whereas telalginite, which is indicative of superior cellular structure preservation, is found in lesser quantities (Figure 4h).

Figure 4.

Microscopic photos of macerals in the mudstone and hybrid shale from E₁f₂ in Gaoyou Sag. Scale bar = 50 μm. (a) Sample S1, Microscopic photo of Tasmanite algae (oil immersed reflected light); (b) Sample S1, Microscopic photo of Tasmanite algae (fluorescent); (c) Sample S1, Microscopic photo of Botryococcus algae (oil immersed reflected light); (d) Sample S1, Microscopic photo of Botryococcus algae (fluorescent); (e) Sample S1, Microscopic photo of telalginite and lamalginite (oil immersed reflected light); (f), Sample S1, Microscopic photo of telalginite and lamalginite (fluorescent); (g) Sample S5, Microscopic photo of telalginite and lamalginite (oil immersed reflected light); (h), Sample S5, Microscopic photo of telalginite and lamalginite (fluorescent); (i) Sample S9, Microscopic photo of laminar algal (oil immersed reflected light); (j), Sample S9, Microscopic photo of laminar algal (fluorescent); (k) Sample S9, Microscopic photo of solid bitumen (oil immersed reflected light); (l), Sample S9, Microscopic photo of solid bitumen (fluorescent); (m) Sample S4, Microscopic photo of bituminite, solid bitumen and mineral bituminous groundmass (oil immersed reflected light); (n), Sample S4, Microscopic photo of bituminite, solid bitumen and mineral bituminous groundmass (fluorescent); (o) Sample S6, Microscopic photo of solid bitumen and mineral bituminous groundmass (fluorescent); (p) Sample S16, Microscopic photo of fusinite (oil immersed reflected light).Tas is Tasmanite algae; TA is telalginite; LA is lamalginite; Bot is Botryococcus algae; MBG is mineral - bituminous groundmass; B is bituminite; SB is solid bitumen.

The secondary organic material within the hybrid shales is predominantly typified by solid bitumen. It presents itself as slender, irregularly contoured lenses, positioned among shale particulates and in congruence with the stratification planes (Figure 4m,l). Under oil-immersion reflected light, the solid bitumen appears gray to black, displaying minimal fluorescence (Figure 4m), although certain samples exhibit a brownish-red fluorescence (Figure 4l). Bituminite, present in trace amounts, exclusively in certain hybrid shale samples, appears amorphous and forms organic stripes parallel to the shale layers under reflected light (Figure 4m), with negligible fluorescence (Figure 4n). It is often associated with a complex matrix of minor organic matter and inorganic minerals, termed mineral bituminous groundmass, which is challenging to discern under reflected light but shows extensive yellow-green fluorescence (Figure 4n).

Vitrinite is dispersed among shale particles in fragmented granular and striated forms, appearing grayish-white under an oil-immersion reflection without any fluorescence response (Figure 4o). Reflectance measurements for vitrinite (R_o) in the E₁f₂ shale samples range from 0.81% to 0.89% (Figure 2). Inertinite, dispersed in a granular form within the shale matrix, is bright white under reflected light (Figure 4p) and primarily consists of inert debris with occasional occurrences of semifusinite and fusinite (Figure 4p).

4.3. Molecular Characteristics of Saturated Hydrocarbons

Biomarker compound parameters, detailed in Table 3, exhibit distinct patterns in mudstone and hybrid shale. The average ratios of Pristane/Phytane (Pr/Ph), Pristane/nC₁₇ (Pr/nC₁₇), and Phytane/nC₁₈ (Ph/nC₁₈) in mudstone are 0.62, 0.30, and 0.35, respectively. In contrast, hybrid shale shows average Pr/Ph, Pr/nC₁₇, and Ph/nC₁₈ values of 0.28, 0.43, and 1.06. The average ratios of gammacerane to αβC₃₀ Hopane (G/C₃₀H), 4-methylsterane index (4-MSI), and C₂₄ tetracyclic terpane to C₂₆ tricyclic terpane (C₂₄/C₂₆) in mudstone are 0.06, 1.67, and 4.11, whereas in hybrid shale they are 0.99, 0.14, and 1.37. Furthermore, the mean ratios of C₁₉/C₂₃ tricyclic terpane (C₁₉/C₂₃) and C₂₀/C₂₃ tricyclic terpane (C₂₀/C₂₃) in mudstone and hybrid shale samples manifest as 0.27, 0.48, and 0.14, and 0.37, respectively. The sterane/hopane (S/H) ratios and C₂₃ tricyclic terpane to αβC₃₀ Hopane (C₂₃/C₃₀H) values are 0.11 and 0.22 for mudstone and 1.02 and 0.40 for hybrid shale, respectively. In the sterane mass-to-charge ratio (m/z) of 217 fragmentograms, the distributions of C₂₇ααα(20R), C₂₈ααα(20R), and C₂₉ααα(20R) steranes exhibit distinctive asymmetrical V-shaped distribution patterns (Figure 5).

Table 3. Biomaker Parameters of the Hybrid Shale and Mudstone from the Funing Formation in the Gaoyou Sag, Subei Basin^a.

no.	depth (m)	Pr/nC17	Ph/nC18	Pr/Ph	G/C30H	C24/C26	ETR	C23/C30H	C19/C23	C20/C23	S/H	4-MSI	C27ααα(20R) (%)	C28ααα(20R) (%)	C29ααα(20R) (%)	nC21+22/nC28+29
S-1	3473.40	0.27	0.31	0.72	0.04	3.94	0.29	0.02	0.27	0.50	0.08	0.47	21.04	32.96	46.00	1.35
S-2	3482.22	0.32	0.39	0.53	0.08	4.27	0.26	0.03	0.27	0.47	0.14	2.87	21.86	22.97	55.17	1.48
S-3	3589.75	0.15	0.38	0.35	0.36	1.94	0.60	0.13	0.21	0.56	0.45	0.13	19.45	30.64	49.91	1.17
S-4	3670.90	0.31	0.98	0.31	1.05	0.86	0.72	0.32	0.12	0.34	0.79	0.16	24.35	33.11	42.54	2.14
S-5	3674.78	0.34	0.76	0.27	1.23	0.95	0.67	0.40	0.11	0.31	0.76	0.08	26.43	27.86	45.71	1.78
S-6	3676.46	0.39	1.11	0.33	1.28	0.63	0.80	0.41	0.09	0.29	1.08	0.28	24.70	35.05	40.25	1.99
S-7	3680.47	0.42	0.96	0.23	1.06	1.12	0.68	0.41	0.10	0.31	0.85	0.07	18.62	28.53	52.85	0.96
S-8	3680.95	0.41	0.95	0.41	1.23	1.11	0.71	0.37	0.15	0.39	1.03	0.23	18.46	31.41	50.14	1.28
S-9	3683.65	0.46	1.03	0.19	1.03	0.92	0.71	0.54	0.07	0.33	0.88	0.27	21.48	33.76	44.76	1.71
S-10	3686.72	0.60	1.59	0.21	0.79	1.55	0.67	0.30	0.13	0.30	1.07	0.10	21.48	30.20	48.32	0.95
S-11	3688.46	0.74	1.77	0.26	0.67	2.20	0.57	0.29	0.20	0.38	1.38	0.10	24.10	26.89	49.01	0.93
S-12	3696.68	0.34	0.77	0.18	1.11	1.78	0.59	0.70	0.15	0.46	1.22	0.06	21.12	29.05	49.83	2.01
S-13	3697.40	0.41	0.94	0.41	1.06	2.05	0.63	0.63	0.20	0.43	1.32	0.06	20.57	29.42	50.01	1.26
S-14	3698.37	0.50	1.31	0.18	0.81	1.76	0.57	0.30	0.10	0.34	1.17	/	21.96	26.48	51.56	0.86
S-15	3701.35	0.45	1.24	0.35	1.17	0.91	0.73	0.36	0.15	0.34	1.27	0.16	22.91	30.7	46.38	1.11
S-16	3703.01	0.27	0.68	0.31	0.37	3.41	0.24	0.13	0.38	0.46	1.37	0.03	22.89	29.47	47.64	1.03

^aPr/nC₁₇ = Pristane/C₁₇n-alkane; Ph/nC₁₈ = Phytane/C₁₈n-alkane; Pr/Ph = Pristane/Phytane; G/C₃₀H = Gammacerane/αβC₃₀ Hopane; C₂₄/C₂₆ = C₂₄ tetracyclic terpane/C₂₆ tricyclic terpane; ETR = (C₂₈ + C₂₉)TT/(C₂₈TT + C₂₉TT + Ts); C₂₃/C₃₀H = C₂₃ tricyclic terpane/αβC₃₀ Hopane; C₁₉/C₂₃ = C₁₉/C₂₃ tricyclic terpane; C₂₀/C₂₃ = C₂₀/C₂₃ tricyclic terpane; S/H = sterane/hopane; 4MSI = 4-methylsterane index (∑4-methylsterane/∑C₂₉ steranes); C₂₇ααα(20R) = C₂₇ααα(20R) sterane/(C₂₇ααα(20R) + C₂₈ααα(20R) + C₂₉ααα(20R) sterane); C₂₈ααα(20R)= C₂₈ααα(20R) sterane/(C₂₇ααα(20R) + C₂₈ααα(20R) + C₂₉ααα(20R) sterane);C₂₉ααα(20R) = C₂₉ααα(20R)sterane/(C₂₇ααα(20R) + C₂₈ααα(20R) + C₂₉ααα(20R) sterane); (nC₂₁ + nC₂₂)/(nC₂₈ + nC₂₉) = (C₂₁n-alkane + C₂₂n-alkane)/(C₂₈n-alkane + C₂₉n-alkane); “/” represents no data.

Figure 5.

Typical biomarker distributions of shale samples from the E₁f₂ in Gaoyou Sag, Subei Basin, showing the order of terpanes (m/z 191) and steranes (m/z 217) distribution. C_21–C26 = C₂₁ – C₂₆ tricyclic terpanes; C₂₄ Tet T = C₂₄ tetracyclic terpane; C₃₀H = C₃₀ αβ hopane; C_27–29 = C₂₇ – C₂₉ααα(20R) sterane.

4.4. Elemental Geochemistry

The compositional analysis of major and trace elements, as detailed in Table 4, reveals significant variations between mudstone and hybrid shale specimens. The average concentrations of aluminum oxide (Al₂O₃), potassium oxide (K₂O), and magnesium oxide (MgO) in mudstone are registered at 10.91%, 1.74%, and 1.65%, respectively. Conversely, in hybrid shale, these values are markedly higher, recorded at 14.73%, 3.50%, and 3.43%, respectively.

Table 4. Major and Trace Elements of the Hybrid Shale and Mudstone from the Funing Formation in the Gaoyou Sag, Subei Basin.

no.	Na2O (%)	MgO (%)	Al2O3 (%)	K2O (%)	CaO (%)	Fe2O3 (%)	Ni (ppm)	Cu (ppm)	Zn (ppm)	Sr (ppm)	Ba (ppm)	V (ppm)	Cr (ppm)	Mn (ppm)
S-1	1.26	1.88	12.00	1.84	14.50	4.58	41.80	46.3	34.2	618	620	70.2	43.1	1288
S-2	1.46	1.42	9.81	1.63	7.32	4.74	26.10	39.7	58.1	311	788	66.6	51.0	804
S-3	2.98	2.10	13.40	3.04	6.44	3.66	26.20	80.0	150.0	554	1042	65.6	58.6	393
S-4	3.70	2.61	17.10	3.63	2.15	5.31	42.10	54.0	77.6	269	324	103.0	76.0	485
S-5	2.57	3.98	16.50	4.34	2.67	5.43	37.80	51.8	83.7	301	362	94.6	74.4	560
S-6	3.46	3.16	16.20	3.46	5.58	5.64	39.90	45.5	76.5	520	306	115.0	72.8	684
S-7	2.01	3.61	16.70	4.72	1.88	5.42	42.30	49.2	77.9	228	355	105.0	85.0	521
S-8	1.78	4.56	14.60	4.08	4.41	5.44	32.90	40.6	70.2	402	334	87.9	72.9	487
S-9	2.80	3.71	14.70	3.52	5.83	4.90	40.10	44.9	64.8	485	332	95.0	73.7	500
S-10	3.48	2.51	12.90	2.65	5.80	3.97	36.40	89.2	113.0	500	392	68.9	57.6	459
S-11	2.98	2.57	14.20	3.28	5.12	3.99	36.70	55.0	72.3	407	490	93.2	71.4	503
S-12	2.37	3.23	17.60	4.62	1.12	5.35	49.70	37.4	81.7	213	416	100.0	92.8	473
S-13	2.70	3.35	16.90	4.07	1.59	5.01	52.40	47.6	82.7	220	368	106.0	92.0	571
S-14	2.57	5.17	13.40	2.71	6.81	4.99	45.00	69.7	82.3	580	315	81.3	74.7	697
S-15	2.02	2.96	8.66	1.41	22.70	4.16	28.20	44.2	51.6	1209	172	73.7	47.8	620
S-16	1.74	4.48	13.30	3.40	6.94	4.64	35.90	54.9	56.6	521	409	88.7	70.6	518

In terms of trace elements, hybrid shale samples exhibit elevated average concentrations of nickel (Ni), copper (Cu), zinc (Zn), strontium (Sr), vanadium (V), and chromium (Cr), measured at 38.97 54.57, 79.07, 529.00, 89.40, and 72.88 ppm, respectively. This contrasts with the mudstone samples, which demonstrate lower average concentrations of these elements, specifically 33.95 ppm Ni, 43.00 ppm Cu, 46.15 ppm Zn, 464.50 ppm Sr, 68.40 ppm V, and 47.05 ppm Cr. Additionally, the analysis reveals disparities in the average concentrations of barium (Ba) and manganese (Mn) between the mudstone and hybrid shale, with mudstone exhibiting higher values of 704.00 ppm Ba and 1046.00 ppm Mn, whereas hybrid shale displays lower concentrations of 424.33 ppm Ba and 553.64 ppm Mn.

5. Discussion

5.1. Microscopic Characteristics and Compositions of Macerals

Within the E₁f₂ hybrid shale, lamalginite is identified as the primary constituent of alginite (Figure 4f,g,j), serving as a crucial source for oil generation. Telalginite, also encountered within the E₁f₂ hybrid shale (Figure 4f,h), is characterized by its distinct cellular structures when observed under fluorescence microscopy, yet the precise biological provenance of this maceral remains indeterminate (Figure 4h). Lamalginite, primarily originating from planktonic algae,²⁹ undergoes sapropelification post-mortem, followed by diagenesis, ultimately resulting in the development of a maceral with considerable capacity for oil generation, although it is generally inferior in comparison to telalginite.²⁹⁻³¹ Certain samples exhibit layered algal accumulations of lamalginite, exceeding 30 μm in thickness and 200 μm in length (Figure 4i,j). This structural characteristic is emblematic of substantial oil-generating capabilities, comparable to telalginite.²⁹ The chemical composition of telalginite is predominantly aliphatic, which plays a pivotal role in its pronounced potential for hydrocarbon generation.^29,31−33

Solid bitumen, a secondary organic constituent prevalent in hybrid shale, emerges from the transformation of oil-generating organic matter, indicative of petroleum migration, accumulation, and postdepositional alteration.^31,34−36 Within the E₁f₂ source rocks, solid bitumen manifests as slender, irregularly shaped lenses aligned parallel to the shale bedding, exhibiting negligible fluorescence (Figure 4l,n). Bituminite, an amorphous microscopic constituent, arises under anoxic conditions through the transformation of organic materials such as algae, zooplankton, and bacteria, signifies microbial degradation.^30,31,37 It frequently co-occurs with mineral bituminous groundmass (Figure 4l,n), which is composed of a mixture of sapropelic organic matter alongside clay or other inorganic constituents, such as carbonates and silica, commonly associated with microbial alterations.^30,31,37,38 Wang posited that mineral bituminous groundmass content is independent of the organic carbon content in source rocks but relates to microbial contributions.³⁷ Tang proposed that highly reductive environments, alongside epochs conducive to microbial proliferation, specifically bacteria, facilitate the genesis of mineral bituminous groundmass.³⁸ The presence of bituminite and mineral bituminous groundmass implies significant biological alteration of the organic matter in E₁f₂ hybrid shale during diagenesis.

Compared with mudstone, hybrid shale demonstrates significant vertical heterogeneity in alginite content, with a higher content of solid bitumen, bituminite, inertinite, and vitrinite (Figure 3). Tasmanites and Botryococcus algae present in mudstone and categorized as freshwater green algae, exhibit intense fluorescence and possess substantial hydrocarbon generating capacity.^{31−33,39,40} Within hybrid shale, the dominance of lamalginite and the emergence of telalginite indicate a moderate potential for hydrocarbon generation. The simultaneous presence of bituminite and lamalginite, in conjunction with a well-developed mineral bituminous ground mass and a hydrogen index not surpassing 250 mg HC/g TOC in samples rich in bituminite, points to significant biodegradation throughout the sedimentation process of this hybrid shale sequence.

5.2. Main Controlling Factors for Enrichment of Macerals

5.2.1. Origin and Productivity

The distribution pattern of n-alkanes is recognized as a crucial indicator for identifying the origin of organic matters.⁴¹ It is widely acknowledged that n-alkanes with lower carbon numbers are predominantly sourced from aquatic microorganisms, including plankton and algae, whereas n-alkanes with higher carbon numbers are typically attributed to land plants. Within the analyzed hybrid shale, the n-alkanes ratio (nC₂₁ + nC₂₂)/(nC₂₈ + nC₂₉) spans from 0.86 to 2.14, averaging 1.40, and the preponderance of samples manifest values above 1.0 (Table 2, Figure 6). This ratio strongly suggests that the organic matter in the hybrid shale is primarily sourced from lower aquatic organisms.

Figure 6.

Pristane/nC₁₇ vs Phytane/nC₁₈ reflecting the sources of organic matter and depositional environment.

Additionally, isoprenoid parameters, notably Pr/nC₁₇ and Ph/nC₁₈, are frequently utilized to deduce sedimentary environments and the origins of organic matter.⁴¹⁻⁴⁵ The intersection diagram of Pr/nC₁₇ and Ph/nC₁₈ (Figure 6) indicates that the organic matter in the hybrid shale is largely derived from algal sources, deposited under anoxic water conditions. Given that C₂₇ sterols are primarily produced by plankton, C₂₈ sterols by phytoplankton, and C₂₉ sterols prevalent in land plants, the comparative abundance of C₂₇ααα(20R), C₂₈ααα(20R), and C₂₉ααα(20R) regular steranes serves as a crucial indicator for ascertaining the predominant source of organic matter.^21,46Figure 7 shows that all of the shale samples fall in the plankton/bacterial and plankton/land plant fields. Huang undertook a detailed investigation on the individual carbon isotopes of C₂₇ααα(20R), C₂₈ααα(20R), and C₂₉ααα(20R) steranes, noting the average carbon isotope ratios to be −28.62‰, 28.65‰, and 28.02‰ respectively, with a variance of less than 1‰.⁴⁷ Consequently, they inferred that C₂₉ααα(20R) steranes in the shale from the E₁f₂ formation of the Subei Basin, predominantly originate from algal contributions. From the analyses provided, it can be concluded that the organic matter within the hybrid shale of the Gaoyou Sag primarily stems from planktonic sources, with fewer contributions from land plants and bacterial organisms.

Figure 7.

Ternary diagram of regular steranes (C₂₇, C₂₈, and C₂₉) showing the relationship between sterane composition and organic matter inputs for the E₁f₂ hybrid shale in Gaoyou Sag, Subei Basin Adapted with permission from the work of Gao et al.²¹ Copyright 2018 Elsevier.

Tricyclic terpane parameters are utilized to deduce terrestrial organic matter contributions.^22,41,48 Increased ratios of C₁₉/C₂₃ and C₂₀/C₂₃ serve as hallmarks for the input of terrestrial organic matters.^13,48−51 Rich C₂₄ tetracyclic terpene are considered indicators of carbonate and evaporite source rocks,^52,53 but some case studies suggest that high C₂₄ tetracyclic terpene concentrations may also indicate input of terrestrial organic matters.^48,49 Within the E₁f₂ hybrid shale, an increase in the C₁₉/C₂₃ and C₂₀/C₂₃ratio correlates with a rise in the C₂₄/C₂₆ ratio (Figure 8), suggesting the latter ratio as a reliable indicator for terrestrial organic input. Contrasting these data from When juxtaposed with the Bohai Bay Basin, Pearl River Mouth Basin, and Jianghan Basin,^{45,48,51,54,55} it emerges that the C₁₉/C₂₃ and C₂₀/C₂₃ ratios within the E₁f₂ are generally lower, whereas the C₂₃/C₃₀H ratio is comparatively higher. This distribution suggests a relatively subdued contribution of terrigenous organic matter to the E₁f₂ hybrid shale.

Figure 8.

Composite organic geochemistry profile showing changes in major biomarker parameters reflecting depositional conditions and/or organic matter input of E₁f₂. Abbreviations of biomaker parameters are explained in the legend of Table 3.

The sterane/hopane (S/H) ratio serves as an indicator of the relative contributions of eukaryotic organisms (algae and higher plants) versus prokaryotic organisms (bacteria) to the organic matter.^{28,43,48−50,56} Within the study area, the S/H ratio ranges from 0.45 to 1.38, averaging 1.02 (Table 3, Figure 8), signifying a pronounced influence of eukaryotic organisms. Analyzing the relationship among C₁₉/C₂₃, C₂₀/C₂₃, C₂₄/C₂₆, C₂₃/C₃₀H, and S/H, it was observed that the correlation between S/H and the earlier noted terpane ratios is not pronounced. Higher S/H values generally align with lower C₁₉/C₂₃ and C₂₄/C₂₆ ratios and higher S/H values generally align with higher C₂₃/C₃₀H (Figure 8). This suggests that eukaryotic contributions within the study area are primarily derived from algae, with a comparatively minor involvement of land plants. This observation is in alignment with the previously mentioned lower terpane ratios, substantiating the dominance of algal over land plant contributions.

According to Ebukanson and Kinghorn^57,58 and Hao,⁴⁸ bituminite can originate from both algal and higher plant sources, with its formation also linked to bacterial activity. Solid bitumen, as explicated by Cardott et al., Mastalerz et al., and Misch et al., is a complex product of petroleum migration, aggregation, and postdepositional changes, derived from a diverse biological origin.³⁴⁻³⁶ Consequently, this study refrains from delving into the sources and enrichment mechanisms of bituminite and solid bitumen. Analyses exploring the correlations between alginite content and the combined vitrinite+inertinite concentrations versus S/H ratios reveal that S/H increases with increased alginite content (Figure 9c). Conversely, the correlation with vitrinite+inertinite content is less evident. Higher S/H ratios correspond to relatively lower vitrinite+inertinite contents (Figure 9d), indicating a predominant eukaryotic contribution from algae, with lower input from land plants.

Figure 9.

Parameter maps of various biomarker ratios reflecting the organic matter sources of hybrid shales in the Subei Basin (a-b, e-f) as well as correlation plots of various biomarker parameters and maceral groups (c-d). Abbreviations of biomarker compound parameters are shown in the legend of Table 3.

4-Methyl sterane is predominantly sourced from dinoflagellate sterols or is associated with methane-producing bacterial activity during diagenesis.^48,59−61 The 4-methyl sterane index (4-MSI) serves as an indicator for identifying the origin of organic matter, with dinoflagellates in lacustrine environments proposed as the principal contributors to 4-methyl sterane.^48,59−61 The correlation between S/H and 4-MSI in hybrid shales of the study area is not significant, yet an increase in alginite content correlates with higher 4-MSI levels (Figure 9e,f). This suggests a complex source for 4-methylsterane, with dinoflagellates contributing to alginite enrichment.

The primary productivity of lakes is closely associated with the proliferation and development of algae.^62,63 Trace metals, including copper (Cu), iron (Fe), and barium (Ba), play a vital role in the proliferation and development of algae. The concentrations of these elements serve as significant indicators of paleoproductivity in lacustrine environments.⁶⁴⁻⁶⁷ Organic matter frequently forms organometallic complexes with such elements, including Cu, which are then accumulated in sedimentary deposits. Moreover, the biosynthesis rate of Ba is directly correlated with the levels of biological productivity.⁶⁶⁻⁶⁸ Consequently, the ratios of Cu/Al and Ba/Al are widely utilized to approximate the widely utilized to approximate levels.⁶⁶⁻⁶⁸ Within the analyzed hybrid shale, the mean Cu/Al ratio stands at 7.23 × 10^–4, and the average Ba/Al ratio is registered at 52.04 × 10^–4. The relatively low ratios of Cu/Al to Ba/Al suggest a modest level of paleoproductivity during the sedimentation of the hybrid shale in the study area.

Lake primary productivity is not only dependent on nutrient availability but also significantly influenced by water chemistry factors such as paleoclimate and salinity.^48,49 The distribution, concentration, and interrelationships of major and trace elements in sediments are frequently employed to infer paleoclimatic conditions.⁶⁹⁻⁷² The paleoclimate index “C” is a commonly used metric for characterizing paleoclimate conditions,^42,43,70,71 and its calculation formula provided as follows:

1

The “C” value, serving as an indicator for paleoclimate conditions, where higher values are indicative of more humid climates.^42−44,73 Within the hybrid shale of the study area, the “C” value fluctuates between 0.14 and 0.45, averaging 0.33. Predominantly, these values fall within the range of 0.20–0.40, signifying that the hybrid shale was primarily deposited under semiarid paleoclimatic conditions.

Nesbitt and Young introduced the Chemical Index of Alteration (CIA), a metric extensively acknowledged for evaluating the extent of chemical weathering in source regions and for reconstructing paleoclimatic conditions.⁷⁴ The formula for calculating the CIA is as follows:

2

It is noteworthy that the unit for oxide in the formula is the molar concentration. CaO* represents the proportion of CaO in the silicate components. When employing CIA to infer paleoclimatic changes, it is essential to consider the influence of CaO present in the carbonate components. To adjust for this, a correction formula is employed:⁷⁵

3

Given the uncertainties surrounding the source of CaO in the hybrid shale samples and the high CaO content, CaO was omitted from the CIA calculation to minimize potential errors. Research suggests that higher CIA values correlate with humid paleoclimatic conditions.^42,74 In the study area, the CIA values for hybrid shale span from 47.40 to 58.76, averaging 54.06, which implies deposition in an arid paleoclimatic environment.

Biomarker compound parameters and inorganic metal elements are widely utilized to investigate lake paleosalinity.^42,48,49 The Gammacerane Index (G/C₃₀H) is is widely used to deduce variations in the salinity and alkalinity of water columns during sedimentation periods.^13,48,49 Gammacerane, believed to derive from the reduction of tetrahymanol, is commonly linked with bacterial ciliates that thrive at redox interfaces within stratified water columns. High gammacerane abundance often indicates conditions of heightened reduction and elevated salinity.^{13,48,49,59,76,77} Thus, a significant gammacerane presence is indicative of stratification in high salinity water columns. Early research indicates that the extended tricyclic terpane ratio (ETR) serves as an age-related indicator to distinguish crude oils originating from Triassic, Lower Jurassic, and Middle–Upper Jurassic marine source rocks.⁷⁸ ETR, as elucidated by Hao et al.,^48,49 Tan et al.,⁴² and He et al.,¹³ is also as an indicator to reflect the salinity of water columns. A strong association between ETR and both Pr/Ph and G/C₃₀H ratios (Figure 10a,b) indicates that within the Subei Basin ETR closely relates to redox and/or saline conditions during or immediately following the deposition of hybrid shale. Elevated G/C₃₀H and ETR values in the study area’s hybrid shale (Figure 8, Table 3) suggest high salinity levels and stratification occurred during the sedimentation process. Moreover, the Sr/Ba ratio is frequently employed to determine the paleosalinity of lake water columns, with higher ratios suggesting increased salinity.⁴² Within the E₁f₂ shale samples, Sr/Ba values vary from 0.51 to 7.03, with an average of 1.47, indicating that the hybrid shale is mainly deposited in salt lakes.

Figure 10.

Variations of ETR with Pr/Ph and G/C₃₀H (a-b) as well as correlation plots of paleoproductivity with paleoclimate, paleosalinity, and algintie groups (c-f). Abbreviations of biomarker compound parameters are shown in legend of Table 3.

Correlative analyses were undertaken to explore the relationships between “CIA”, “C”, Sr/Ba, and Cu/Al. The findings indicate a significant negative correlation between “CIA” and “C” concerning Cu/Al, whereas Sr/Ba exhibits a positive correlation with Cu/Al (Figure 10c-e). This suggests that increased paleoproductivity is associated with relatively drier paleoclimatic conditions and higher water column salinity. Additionally, the correlation between the Cu/Al and alginite content was investigated, revealing a positive correlation (Figure 10f). This observation suggests that heightened paleoproductivity fosters alginite enrichment.

5.2.2. Preservation Conditions

The redox conditions of water columns significantly play a significant role in the preservation of organic matter, with hypoxic environments favoring maceral enrichment.^42,79,80 Elements such as vanadium (V) and nickel (Ni), are crucial in determining ancient redox conditions. The ratio V/(V + Ni) serves as a widely accepted metric for indicating the redox environment of sedimentary water columns.^70,81,82 A V/(V + Ni) ratio below 0.45 indicates an oxidizing environment, while ratios falling between 0.45 and 0.6 indicate a suboxic environment. Ratios ranging from 0.6 to 0.85 point toward hypoxic conditions, and values exceeding 0.85 suggest a euxinic (sulfidic) environment. In the study area, the V/(V + Ni) ratio in hybrid shale ranges from 0.64 to 0.74, averaging 0.70, indicating deposition under anoxic conditions. The biomarker compound Pr/Ph values and the crossplot of isoprenoid further support this deduction of anoxic environment (Figures 6 and 8). Correlation analyses between V/(V + Ni) and various macerals reveal a weak association with alginite and (vitrinite+inertinite) content (Figure 11a-b), suggesting that redox conditions are not the primary factor influencing maceral enrichment.

Figure 11.

Intersection diagram of redox parameters in water column with the content of macerals (a-b), input of terrestrial debris with the content of macerals (c-d), intersection diagram of paleosalinity parameters in water column with the content of alginite (g-h), and intersection diagram of paleoclimate parameters with the content of (vitrinite + inertinite) (e-f).

Terrigenous debris inputs bring essential nutrients to lakes, aiding hydrobiont growth. However, excessive terrigenous inputs have the potential to either dilute or impair the preservation of organic matter.^13,83,84 The ratio (Fe₂O₃ + Al₂O₃)/(CaO + MgO) is commonly utilized to assess the magnitude of terrigenous debris inputs.⁴ Analysis indicates a positive correlation between (Fe₂O₃ + Al₂O₃)/(CaO + MgO) and alginite content (Figure 11c). With an increase in this ratio, the content of vitrinite and inertinite initially increases, followed by a subsequent decline (Figure 11d). This observation suggests that optimal terrestrial debris inputs promote alginite growth, whereas excessive inputs disrupt the preservation of vitrinite and inertinite.

The CIA and paleoclimate index “C” are utilized to evaluate chemical weathering and paleoclimate conditions, respectively. Both CIA and “C” values exhibit a positive correlation with the content of (vitrinite+inertinite) (Figure 11e-f), indicating that humid paleoclimates and intensive chemical weathering promote inputs from land plants, thereby enriching vitrinite and inertinite.

ETR and G/C₃₀H serve as effective indicators for characterizing water salinity and stratification. The positive correlations observed between ETR, G/C₃₀H, and algal biomass content (Figure 11g-h) are consistent with the notion that saline lakes often demonstrate high productivity, owing to the adaptability of bacteria and algae to saline conditions.^13,48,85 In the Subei Basin, elevated concentrations of lake nutrients foster algal proliferation. As the salinity increases, pronounced stratification occurs within the water column, resulting in anoxic conditions at the bottom. This anoxic environment facilitates the preservation of settling alginite, thereby contributing to its enrichment in hybrid shale.

In the hybrid shale of the study area, the predominant source of organic matter originates from lower aquatic organisms, with a lesser contribution from terrestrial organic matter and a significant contribution from dinoflagellates, which contributes to the enrichment of algae. Contrary to the initial assumptions, the redox conditions within the water column do not predominantly influence the enrichment of macerals. Instead, the introduction of terrigenous debris, in conjunction with paleosalinity and paleoproductivity, emerges as the principal driving force behind maceral accumulation. The inflow of terrigenous debris introduces essential nutrient elements into the lake, thereby increasing nutrient salt concentrations, promoting algal proliferation and consequently enhancing primary productivity. Simultaneously, an increase in salinity induces pronounced stratification within the water column, resulting in the formation of an anoxic environment at the lakebed, which is conducive to the preservation of alginite. Moreover, the deposition of vitrinite and inertinite macerals is regulated not only by the input of organic matter from land plants but also by the prevailing paleoclimate, the intensity of chemical weathering, and the influx of terrestrial detritus. A relatively humid paleoclimate combined with a robust chemical weathering index favors the enrichment of vitrinite and inertinite macerals. An optimal quantity of terrigenous debris enhances the accumulation of these macerals, whereas an excessive influx potentially hinders their enrichment.

5.3. Geological Significance

5.3.1. Effect of Maceral on Shale Oil Enrichment

In source rock assessment, the abundance of organic matter is commonly evaluated using geochemical parameters such as Total Organic Carbon (TOC) and pyrolysis hydrocarbon contents (S₁ and S₂).^73,86,87 The S₁ component is particularly indicative of extant shale oil content, while both S₁ and S₂ collectively serve as key indicators of the hydrocarbon generation capacity of source rocks.^11,43,73,86 In the case of the hybrid shale from the study area, the TOC concentrations exhibit a broad range, spanning from 0.68% to 2.46%, with a mean value of 1.24%. The aggregate S₁+S₂ contents vary between 1.52 and 9.97 mg of HC/g of rock. According to established criteria for evaluating terrestrial hydrocarbon source rocks, the hybrid shale under study classifies as a medium to high-quality hydrocarbon source rock. This assessment is consistent with the understanding that the organic matter content in hybrid shale, despite its variability, holds significant potential for hydrocarbon generation. The maceral composition within these shales plays a crucial role in this context, influencing both the quality and quantity of recoverable hydrocarbons.

In assessing the oil content of shale oil, key metrics include the Oil Saturation Index (OSI), actual movable oil, and total oil content.^11,88,89 Multitemperature stage pyrolysis experiments provide parameters for assessing real movable oil and total oil content. The derived formulas are Free oil = 2.33 × S₁ and Total oil = 1.73 × S₁ + 0.82 × S₂.⁷⁷ In the hybrid shale under investigation, the free oil content varies widely, ranging from 0.75 to 12.51 mg of HC/g of rock, with an average of 2.38 mg of HC/g of rock. Similarly, total oil content spans from 1.56 to 10.12 mg HC/g rock, averaging 4.55 mg HC/g rock (Figure 2, Table 1).

Analyses were conducted to establish correlations between the content of alginite, vitrinite+inertinite with Total Organic Carbon (TOC), S₁+S₂, total oil, and free oil. The results indicate that TOC increases with the rise in alginite content, while showing no significant correlation with the combined content of vitrinite+inertinite. Both S₁+S₂ and total oil display a positive correlation with alginite content, yet there is no distinct correlation with vitrinite+inertinite content. Free oil content also correlates positively with alginite content but inversely with the content of vitrinite+inertinite (Figure 12). The positive relationship between TOC, total oil, and free oil with the alginite content underscores that alginite enrichment promotes the accumulation of shale oil and organic matter in the hybrid shale of the Subei Basin. This suggests a synergistic mechanism wherein shale oil enrichment and organic matter accumulation mutually reinforce each other.

Figure 12.

Maceral contents vs total organic carbon (TOC) (a, b), hydrocarbon potential (c, d), and total oil (e, f) and free oil (g, h) of hybrid shales in E₁f₂ from the Gaoyou Sag, Subei Basin.

5.3.2. Enrichment Patterns of Typical Macerals and Shale Oil in the Hybrid Shale

The research findings indicate that the E₁f₂ hybrid shale predominantly originates from aquatic organisms, revealing a synergistic mechanism between organic matter and shale oil enrichment. Arid paleoclimatic conditions led to significant evaporation in lake waters, resulting in a reduction in lake area. Concurrently, the input of terrigenous debris introduced nutrient elements into the lake, leading to an escalation in nutrient salt concentrations and promoting eutrophication. This process enhanced algal growth and increased primary productivity. The elevation in salinity resulted in pronounced water column stratification, ultimately leading to the formation of an anoxic environment at the lakebed. Under such conditions, minimal degradation of settled alginite occurred, facilitating its preservation and enrichment (Figure 13).

Figure 13.

Typical maceral components and shale oil enrichment patterns of hybrid shale in E₁f₂ from the Gaoyou Sag, Subei Basin.

Alginite, as the primary source material for hydrocarbon generation in E₁f₂ hybrid shale, holds a pivotal position in the genesis of shale oil. The enrichment of alginite fosters favorable conditions for the generation of shale oil. Furthermore, the geological characteristics of the hybrid shale, serving as both a source and reservoir system, provide favorable environments for the accumulation and preservation of shale oil, thereby promoting its enrichment within the hybrid shale matrix.

6. Conclusion

Comprehensive organic–inorganic geochemical analyses were performed on the E₁f₂ section of the Subei Basin’s hybrid shale, encompassing TOC, Rock-Eval pyrolysis, organic petrography, major and trace element analysis, and GC-MS analysis of saturated hydrocarbons. These analyses facilitated a thorough investigation into the microcomposition, enrichment genesis, and geological significance of the mixed shale. The key findings are summarized as follows:

• (1)The predominant macerals in the hybrid shale are alginite, solid bitumen, and inertinite. Compared to mudstone with higher alginite content, the hybrid shale exhibits a wider range of alginite concentrations and relatively higher levels of solid bitumen and bituminite. The predominant form of alginite is lamalginite, supplemented by telalginite, which collectively contribute to a moderate overall hydrocarbon potential.
• (2)Biomarker parameters such as S/H, C₂₃/C₃₀H, and C₂₄/C₂₆ reveal that the organic matter in the hybrid shale primarily derives from lower aquatic organisms, with less contribute from terrigenous organic matter. Additionally, dinoflagellates have contributed to the enrichment of alginite.
• (3)Maceral enrichment is influenced by a combination of factors, including terrigenous detrital input, paleosalinity, and paleoproductivity, with redox conditions playing a less significant role. The introduction of terrigenous detritus enhances lake nutrient salt concentrations, stimulating algal proliferation and augmenting primary productivity. Elevated salinity levels and pronounced water column stratification promote reduction conditions favorable for alginite preservation. The enrichment of vitrinite and inertinite is governed by organic matter derived from land plants, as well as by paleoclimate and chemical weathering factors. In relatively humid paleoclimates with high chemical weathering indices, there is a preference for the enrichment of vitrinite and inertinite. Adequate terrigenous debris input facilitates their accumulation, while excessive input is detrimental.
• (4)Alginite is the key hydrocarbon-generating component in low-abundance hybrid shale. The enrichment of alginite promotes the accumulation of shale oil and organic matter within the hybrid shale of the Subei Basin, indicating a synergistic mechanism between shale oil and organic matter enrichment. This suggests the significant potential for shale oil exploration even in low-abundance mixed shale formations.

The authors declare no competing financial interest.