Source Rock Evaluation and Hydrocarbon Expulsion Characteristics of Effective Source Rocks in the Fushan Depression, Beibuwan Basin, China

Abstract

This study presents an integrated approach using organic geochemistry and incident-light organic petrographic microscopy techniques to characterize the kerogen type, hydrocarbon potential, thermal maturity, and effective depositional environment of the Eocene Liushagang Formation intervals in the western Huangtong Sag, eastern Bailian Sag, central Huachang Sub-uplift, and Southern Slope Zone area in the Fushan Depression, Beibuwan Basin. The results show that the hydrocarbon potential of these organic-rich lacustrine shale areas is mainly dependent on the depositional environment and the present-day burial depth of sediments. Oscillations and transitions between (i) rocks with dominant allochthonous organic matter (including primary/reworked vitrinite and inertinite macerals and terrestrial debris particles) representing a large influence of continental sediments (e.g., source supply direction) and (ii) rocks with dominant autochthonous organic matter (e.g., alginite) indicate a distal and stable lacustrine basin depositional environment. The source rock thickness ranges from 40.1 to 387.4 m. The average TOC of the Liushagang Formation in the Fushan Sag is between 0.98% and 2.00%, with the highest organic matter abundance being in the first and second sections of the Liushagang Formation, presenting as high-quality source rocks. The organic matter is predominantly Type II₁ and Type II₂. The highest vitrinite reflectance (1.14%) is in the Huangtong and Bailian Sags. The source rocks of the second section of the Liushagang Formation are primary hydrocarbon generators, contributing 55.11% of the total generation. Hydrocarbon sequestration peaks at %Ro 0.80%, with a maximum efficiency of 97.7%. The cumulative hydrocarbon generation of the Liushagang Formation is 134.10 × 10⁸ tons, with 50.52 × 10⁸ tons having been expelled and 83.58 × 10⁸ tons remaining. E₂L_2X and E₂L_2S have maximum hydrocarbon displacement intensities of 184.22 × 10⁴ t/km² and 45.39 × 10⁴ t/km², respectively, with cumulative displacements of 52.99 × 10⁸ tons and 15.58 × 10⁸ tons. The oil and gas accumulation system is highly prospective, showing significant exploration potential.

1. Introduction

The Beibuwan Basin, an oil- and gas-rich sedimentary basin on the northern margin of the South China Sea, has a proven geological reserve of crude oil of approximately 4.05 × 10⁸ t and a proven geological reserve of natural gas of approximately 50 × 10⁸ m³ [1,2,3]. Although its petroleum exploration started in 1965, early research focused on the structural analysis of the Beibuwan Basin [4,5,6,7]. In 1977, the first well in the Beibuwan Basin was drilled, showing that the Palaeogene Liushagang Formation (E₂L) had good source rocks, and the industrial oil flow was tested in the third member of the Liushagang Formation (E₂L₃) [8]. After that, the source rock evaluation in the Beibuwan Basin started to gain increasing attention, and it is believed that the source rocks of the Paleogene Liushagang Formation function as the main source rocks in the Beibuwan Basin [9,10,11,12], However, since there are multiple sets of source rocks, like seven sub-members of the Liushagang Formation that have developed in the depression [13,14,15], the quality and hydrocarbon seepage characteristics are still unclear. This has limited the understanding of the resource potential and the selection of favorable resource fields.

The major hydrocarbon-producing source rocks in the Beibuwan Basin include the Weixinan Depression, Wushi Depression, Maichen Depression, Fushan Depression, and Haizhong Depression [16,17,18,19], among which the Fushan Depression, located on the southern margin of the Beibuwan Basin, has turned out to be a new oil-producing block, demonstrating proven geological reserves of oil of about 3139.41 × 10⁴ t and proven geological reserves of natural gas of 123.87 × 10⁸ m³ by the end of 2021 [13,20,21]. Therefore, this small, but abundant, oil- and gas-rich sag has received a lot of attention from the industry [22], and the hydrocarbon generation and expulsion characteristics of the source rocks of the Liushagang Formation (E₂L₃, E₂L₂, and E₂L₁) in the Fushan Depression should be evaluated systematically and analyzed thoroughly.

As the amount of hydrocarbon generation and expulsion from source rocks determines the calculation of the amount of hydrocarbon that can be extracted [23,24,25,26,27], it is of significance to understand the evaluation scheme of the source rocks and the potential of the oil and gas resources [28,29,30,31,32,33]. Therefore, the current widely applied calculation methods for evaluating hydrocarbon expulsion are the diffusion hydrocarbon expulsion mode, fluid seepage method, simulation experiment method, and hydrocarbon expulsion threshold method [34,35,36,37], among which there is not much agreement [26,38,39,40]. This could be explained by the limitations of these methods. For example, only light hydrocarbons are involved in diffusion throughout the hydrocarbon migration process. Therefore, the diffusion hydrocarbon expulsion model is only suitable for guiding natural gas-related migration simulation [41]. The critical saturation of hydrocarbon seepage from the source rock is the same as the factors influencing the oil seepage concentration. Hence, a variation profile can be established according to the principle of two-phase seepage to perform analogy analysis [42]. The hydrocarbon expulsion from the thermal simulation method can provide parameters for calculating the resource quantity. But, the obtained hydrocarbon production rate is not accurate enough [43]. Therefore, this work selected the source rocks of the Liushagang Formation in the Fushan Depression for a comprehensive evaluation of source rocks, simulating the hydrocarbon generation and expulsion characteristics of the source rocks and calculating the amount of oil and gas from different accumulation systems. This study will deepen the understanding of the source rocks in the Fushan Depression, thus providing a reference for the further study of oil and gas migration and accumulation in the Fushan Depression and a scientific basis for the further exploration and development of the study area.

2. Geological Setting

2.1. Tectonic Setting

The Fushan Depression has a total area of 2920 km² and is located on the southern margin of the Beibuwan Basin on the continental shelf of the South China Sea and the slope of the Haizhong uplift. The shape of the nearly triangular depression in the Fushan Depression is closely related to the left-lateral strike-slip of the NW-trending Red River Fault and the clockwise rotation of the Palaeogene Hainan Island [11,44,45,46]. On the western side, it is adjacent to the Lingao uplift, and on the eastern side, the Yunlong uplift is visible [45,47]. The Shenhu movement at the end of the Late Cretaceous formed a series of NE-trending faults and rifts [48,49]. It was controlled by the Cenozoic regional tectonic deformation of the northern continental margin of the South China Sea, and a series of NE-trending faults and small fault structures developed in the Beibuwan Basin during this period [50,51]. The Lingao Fault and Yunlong Fault control the area, forming a superimposed depression, which includes several small adjustment faults, including the Meihua Fault and the Lianhua Fault. This geological arrangement results in a pattern of alternating [52,53]. In the northern sector of the Meihua Fault, the following significant deep sag regions are discernible: the Huangtong Sag to the west and the Bailian Sag to the east. Sandwiched between these sags lies the central interval structural transformation zone, known as the Huachang Sub-uplift. Collectively, these features delineate the northern deep concave area. Conversely, the area situated to the south of the Meihua Fault is characterized by the Southern Slope Belt [54,55] (Figure 1).

2.2. Sedimentology

The Palaeogene Changliu Formation, Liushagang Formation, and Weizhou Formation, along with Neogene and Quaternary sediments, have been sequentially deposited upon the Cretaceous basement in the Fushan Depression. Notably, the Liushagang Formation, characterized by its lacustrine deltaic deposits, serves as the principal source of oil and a key reservoir within the Fushan Depression [58,59]. The Eocne Liushagang Formation was categorized into three members in this area, namely, E₂L₁, E₂L₂, and E₂L₃. It is divided into E₂L_3X, E₂L_3Z, E₂L_3S, E₂L_2X, E₂L_2S, E₂L_1X, and E₂L_1S sequentially, and hydrocarbon source rocks are developed in these layers [60], among which E₂L₁ and the E₂L₃ are the main oil- and gas-producing layers [13] (Figure 2). The boreholes examined in this study are predominantly located in the onshore regions of structural highs, encompassing the sedimentary core of the western Huangtong Sag, the central Huachang Sub-uplift, and the elevated Southern Slope Belt [61,62]. The objective is to ascertain the distribution and stratigraphic variations in the source rocks within the onshore areas. Controlled by the Meihua fault, E₂L₃ in the eastern Huangtong Sag is denuded. The northern sea area is the subsidence center of the Bailian Sag in the northeast. The distribution range of the non-well area is wide, and the basic data on lithology and layer thickness are lacking [3].

3. Methods and Workflows

3.1. Samples

This study meticulously analyzed 119 sediment samples from 65 wells located in the Fushan Depression. The spatial distribution of samples from the Liushagang Formation within this depression exhibits a pronounced and discernible pattern. The wells were categorized into two distinct regions as follows: the Huantong Sag in the west and the Bailian Sag in the east of the Fushan Depression (Figure 3). Core observations, total organic carbon (TOC) analysis, rock pyrolysis, bitumen extraction (extracted bitumen A), and vitrinite reflectance measurements were conducted on a total of 119 samples. These analyses were performed to evaluate the quality of the primary source rock intervals (E₂L_3X, E₂L_3Z, E₂L_3S, E₂L_2X, E₂L_2S, E₂L_1X, and E₂L_1S) of the Liushagang Formation. Simultaneously, the threshold for hydrocarbon displacement was ascertained to differentiate between effective and ineffective source rocks.

3.2. Laboratory Methods

The abundance, type, and thermal maturity of organic matter in source rocks are important factors in assessing the hydrocarbon generation capacity of source rocks in sedimentary basins. Leco CS230 and Rock-Eval 2 pyrolysis analyses, as per the methodology [63], were conducted to ascertain the total organic carbon (TOC), temperature of maximum hydrocarbon generation (Tmax), thermally extractable HCs (S₁), and remaining HC generating potential (S₂) content. In the context of the HAWK pyrolysis analysis, the samples underwent initial pyrolysis at 300 °C for 3 min, followed by a ramping rate of 25 °C per minute up to a final temperature of 650 °C. The liberated hydrocarbons were quantified as S₁ and S₂ employing a Flame Ionization Detector (FID). The data about TOC, S_1, and S₂ exhibited an accuracy and precision exceeding 95%, and the Tmax determination was accurate to within ±2 °C. These assessments were validated through the analysis of standard reference materials, which encompass Chinese standard reference materials [64,65].

Maceral identification such as the sapropel group, quantitative counting, and vitrinite reflectance (Ro, %) measurements were also conducted in this work. The samples were sectioned either perpendicularly or parallel to the bedding planes and subsequently processed in accordance with the guidelines outlined in [66]. Reflectance assessments were conducted using a Leica 4500 P microscope (Wetzlar, Germany), developed by Leica Company in Germany, equipped with a CRAIC microscope photometer or a Leica DM4 M microscope, both of which were integrated with a Hilgers Technisches Büro Fossil system. These systems were furnished with both white and ultraviolet light sources, adhering to the standards set forth in [67,68]. Random reflectance (Rr) measurements were executed utilizing an oil immersion objective lens with a 50× magnification. The infrared spectral parameters of kerogen, Tmax from pyrolysis, and vitrinite reflectance (Ro, %) were all used in this work to identify the degree of thermal evolution of organic matter. In addition, vitrinite reflectance was used as a determination index to establish the fitting relationship with burial depth to determine the maturity characteristics of organic matter in the target horizon of the study area.

3.3. Hydrocarbon Generation and Expulsion Conceptual Model

This study employed PetroMod. V9.0 software, developed by the IES Company in Germany, to simulate the burial, hydrocarbon generation, and thermal histories of key individual wells within the Fushan Depression. Specifically, Yong 7 well in the western region and Lian 23 well in the eastern region were selected for detailed single-well simulation analysis (Figure 3).

The hydrocarbon generation and expulsion conceptual model is a systematic method for analyzing the effectiveness of source rocks. The hydrocarbon generation potential index (S₁ + S₂)/TOC represents the hydrocarbon generation potential per unit of organic matter. It increases with Ro during the evolutionary process until it reaches the current maximum hydrocarbon generation potential index (HCIo). Following that, the cumulative hydrocarbon generation equals the residual hydrocarbon saturation of the source rock, which is the limit at which the hydrocarbon generation potential index begins to decrease and the source rock begins to expel hydrocarbons [36,69].

The key procedures for establishing the source rock hydrocarbon generation and expulsion model [70] using the hydrocarbon generation potential are as below:

(1)

Establishment of the hydrocarbon generation potential profile: The pyrolysis database of source rocks in the study area was set up. The profile of the hydrocarbon generation potential index with vitrinite reflectance or burial depth of source rocks in the study area was made determined on the data.

(2)

Calculation of the hydrocarbon expulsion rate: After determining the hydrocarbon generation and expulsion threshold of the study area according to the establishment of the step (1) handle, ${\mathit{HCI}}_o$ and ${\mathit{HCI}}_P$ were brought into Equation (2), which was used to calculate μ, which was then brought into Equation (3), and the ${\mathit{HCI}}_o^{\prime }$ corresponding to any point on the evolution profile of hydrocarbon generation potential index were calculated in batches. The hydrocarbon expulsion rates $q_e\left(Z\right)$ corresponding to different depths Z were determined.

$$q_e\left(Z\right)={\mathit{HCI}}_{\mathit{o}}^{\prime }\left(\mathit{Z}\right)-{\mathit{HCI}}_{\mathit{P}}\left(\mathit{Z}\right)$$

(1)

$$\mu =(1-0.083\times {\mathit{HCI}}_{\mathit{P}})/(1-0.083\times {\mathit{HCI}}_{\mathit{o}})$$

(2)

$${\mathit{HCI}}_{\mathit{o}}^{\prime }=\mu \times {\mathit{HCI}}_{\mathit{o}}$$

(3)

where Z represents different depths. ${\mathit{HCI}}_{\mathit{o}}$ is the hydrocarbon generation potential index corresponding to the hydrocarbon expulsion threshold of the maximum current hydrocarbon generation potential. ${\mathit{HCI}}_{\mathit{P}}$ is the actual hydrocarbon generation potential index. $\mu$ is the restitution coefficient of the hydrocarbon generation potential index. ${\mathit{HCI}}_{\mathit{o}}^{\prime }$ is the maximum original hydrocarbon generation potential. ${\mathit{q}}_{\mathit{e}}\left(\mathit{Z}\right)$ is the hydrocarbon expulsion rate corresponding to Z.

(3)

Calculation of hydrocarbon expulsion intensity: ${\mathit{q}}_{\mathit{e}}\left(\mathit{Z}\right)$ calculated in step (2) and H, Z, and the organic carbon mass fraction of the source rock corresponding to different burial depths Z were introduced into Equation (4).

$${\mathit{E}}_{\mathit{hc}}={\int }_{{\mathit{Z}}_0}^{\mathit{Z}}{\mathit{q}}_{\mathit{e}}(\mathit{Z})\times \mathit{H}\times \rho (\mathit{Z})\times \mathrm{TOC}\times \mathrm{d}\mathit{Z}$$

(4)

where $\mathit{H}$ and $\rho \left(\mathit{Z}\right)$ are the rock thickness and density, among which Z₀ represents the hydrocarbon expulsion threshold of the study horizon. The final integral operation is the hydrocarbon expulsion intensity E_hc with a unit of 10³ t/km².

(4)

Calculation of the hydrocarbon expulsion rate: The hydrocarbon expulsion rate was defined as the change in the hydrocarbon expulsion rate in the unit thermal evolution degree [71,72].

$${\mathit{V}}_{\mathit{e}}=\Delta {\mathit{q}}_{\mathit{e}}/\Delta R_o$$

(5)

where ${\mathit{V}}_{\mathit{e}}$ is the hydrocarbon expulsion rate, $\Delta {\mathit{q}}_{\mathit{e}}$ is the hydrocarbon expulsion rate difference of adjacent data points, and $\Delta R_o$ is the vitrinite reflectance difference and R_o.

(5)

Calculation of the hydrocarbon expulsion efficiency: The ratio of hydrocarbon expulsion to cumulative production at a certain point can be replaced by the proportion of the difference between${\mathit{HCI}}_{\mathit{o}}^{\prime }$ and ${\mathit{HCI}}_{\mathit{o}}$ in ${\mathit{HCI}}_{\mathit{o}}^{\prime }$. By bringing the relevant parameters obtained in Equation (2) into Equation (6), ${\mathit{R}}_{\mathit{eo}}$ was calculated [73].

$${\mathit{R}}_{\mathit{eo}}={\mathit{q}}_{\mathit{e}}\left(\mathit{Z}\right)/{\mathit{HCI}}_{\mathit{o}}^{\prime }\left(\mathit{Z}\right)\times 100\%$$

(6)

where ${\mathit{R}}_{\mathit{eo}}$ is the hydrocarbon removal efficiency at a given point.

(6)

Calculation of the amount of hydrocarbon expulsion: According to Equation (3), E_hc and the other parameters required for the calculation, such as $\mathit{S}\left(\mathit{n}\right)$, were put into Equation (7) for integral operation and to determine the amount of ${\mathit{Q}}_{\mathit{e}}$.

$${\mathit{Q}}_{\mathit{e}}={\int }_1^{\mathit{n}}{{10}^{-3}\times \mathit{E}}_{\mathit{hc}}\times \mathit{S}(\mathit{n})\times \mathrm{d}\mathit{n}$$

(7)

where $\mathit{S}\left(\mathit{n}\right)$ is the unit area of the source rock and ${\mathit{Q}}_{\mathit{e}}$ is the hydrocarbon expulsion in each sedimentary period of the study area that can be obtained.

4. Geochemical Characteristics of Eocene Source Rocks

4.1. Organic Matter Abundance

Based on total organic carbon (TOC) measurements, and hydrocarbon source rock extracts, and applying the established criteria for lacustrine mudstone abundance in China, the regions of elevated TOC within E₂L₃ are predominantly localized in the Bailian Sag and the Southern Slope Belt. E₂L_3X is classified within the medium source rock category, exhibiting a comparatively lower concentration of organic matter in comparison with the other constituent sub-segments. E₂L_3Z and E₂L_3S belong to the range of good source rocks, and the abundance of organic matter is higher. The high TOC value of E₂L₂ is mainly in the Huangtong Sag and Bailian Sag, and the organic matter abundance is better than that of other layers. The organic matter abundance of E₂L_2X and E₂L_2S is higher, and the hydrocarbon generation potential is up to 19.79 mg/g. The high-value area is located in the western Huangtong Sag, with an average value of 3.64 mg/g, which has great hydrocarbon generation potential and is a good source of rock with high quality. The TOC value of E₂L₁ is highest in the two hydrocarbon generation centers, Huangtong Sag and Bailian Sag, and decreases towards the edge of the Huachang Sub-uplift and Southern Slope Belts. The organic matter abundance of E₂L_1X and E₂L_1S is high, which belongs to the range of good source rocks (

Table 1. The organic matter abundance of the source rocks of the Liushagang Formation and the Fushan Depression.

Characteristics		TOC/wt %		Extracted Bitumen “A”/%		S1+S2/(mg HC/g Rocks)
		Distribution	Samples	Distribution	Samples	Distribution	Samples
E2L1S	Huangtong Sag	0.52~1.9 0.91	24	0.014~0.117 0.068	12	0.33~4.52 2.11	11
	Source rock evaluation	Medium		Medium		Medium
	Bailian Sag	0.64~1.36 1.03	9	0.031~0.19 0.088	4	0.98~3.92 2.55	4
	Source rock evaluation	Good		Medium		Medium
E2L1X	Huangtong Sag	0.67~3.61 1.40	36	0.025~0.32 0.091	33	1.38~8.43 3.07	30
	Source rock evaluation	Good		Medium		Medium
	Bailian Sag	0.48~1.75 1.30	9	0.042~0.107 0.078	3	1.46~3.49 2.75	3
	Source rock evaluation	Good		Medium		Medium
E2L2S	Huangtong Sag	0.77~3.04 1.46	42	0.013~0.363 0.102	30	1.4~7.17 3.78	40
	Source rock evaluation	Good		Good		Medium
	Bailian Sag	0.49~1.86 1.49	10	0.0242~0.1335 0.0739	5	0.5~3.04 1.62	5
	Source rock evaluation	Good		Medium		Poor
E2L2X	Huangtong Sag	-	-	-	-	-	-
	Source rock evaluation	-		-		-
	Bailian Sag	0.16~2.45 1.61	44	0.008~0.245 0.114	31	0.08~7.67 3.17	35
	Source rock evaluation	Good		Good		Medium
E2L3S	Huangtong Sag	-	-	-	-	-	-
	Source rock evaluation	-		-		-
	Bailian Sag	0.44~1.89 1.40	13	0.073~0.172 0.121	5	1.88~4.62 2.67	5
	Source rock evaluation	Good		Good		Medium
E2L3X	Huangtong Sag	1.14	1	0.0607	1	2.78	1
	Source rock evaluation	Good		Medium		Medium
	Bailian Sag	-	-	-	-	-	-
	Source rock evaluation	-		-		-

, and Figure 4, Figure 5 and Figure 6).

4.2. Kerogen Type

The hydrogen index (HI) and Tmax were used to identify the organic matter type. The results show that E₂L_3X is mainly II₂ and III in the Southern Slope Belt, while the kerogen type of E₂L_3Z changed from II₁ in the Meihua Fault to II₂ and III in the Southern Slope Belt. During the deposition of E₂L_3S, the kerogen of the Huachang Sub-uplift was type II₂ and type II₁, the kerogen of the Huangtong Sag was type II₂, and the kerogen of the Southern Slope Belt was type II₂ and type III. The kerogen types of the Huachang Sub-uplift in the E₂L_2X depositional period were type II₁ and type II₂. The kerogen type of the Huangtong Sag is mainly type II₂ but some are type II₁ and type III. The kerogen type of E₂L_2S transits from type II₁ to type III from west to east, and all three types of kerogens are found in the Southern Slope Belt. The organic matter type of E₂L_1X transits from type II₁ to type II₂ from west to east, and the Southern Slope Belt is dominated by type II₂ kerogen, followed by type III. The northern part of E₂L_1S is type II₂ kerogen, and type III kerogen occurs in the Southern Slope Belt (Figure 7).

The distribution range of E₂L₃ types II₂ and III organic matter is the widest, and the type of E₂L_3S organic matter is better with mainly type II₁. The E₂L₂ organic matter type is better in the Huangtong Sag and Huachang Sub-uplift, mainly type II₁. The organic matter type of E₂L₁ is better in the Huangtong Sag and Huachang Sub-uplift, mainly type II₂, and type II₁ exist. The type II₂ organic matter of E₂L₁ has the widest distribution area.

The hydrocarbon generation threshold for the Huangtong Sag, Bailian Sag, and Huachang Sub-uplift is delineated by the depth at which the vitrinite reflectance of source rocks reaches a value of 0.5%, which corresponds to a threshold depth of approximately 2800 meters. Consequently, the majority of the E₂L₁ source rocks have reached the threshold for hydrocarbon generation. The source rocks have also reached the peak of oil generation, and the depth above 4200 m has entered the high maturity stage. The E₂L₂ source rock is in the low maturity stage of development in the Huachang Sub-uplift. The E₂L_2X and E₂L₃ source rocks have reached the peak of oil generation and have entered the high maturity stage at a depth of 4200 m. The burial depth of the Southern Slope Belt is shallow, so the maturity of the source rock is also low. The variation trend in the vitrinite reflectance of the Southern Slope Belt source rocks with depth, shows that the threshold depth corresponding to the hydrocarbon generation threshold (Ro = 0.5%) is 2550 m and 1900 m, respectively (

Table 2. Rock-Eval pyrolysis data used in this study.

Area	Well	Depth	Formation	Sapropelic	Liptinite	Vitrinite	Inertinite	Type Coefficient	Organic Matter Type	Ro (%)	N	Std. Dev.
Huangtong Sag	C6X	2408.42	E2L1S	61.3	-	38.7	-	32.3	Ⅱ2	-	-	-
Huangtong Sag	C6X	2683.85	E2L1S	63.7	-	36.3	-	36.5	Ⅱ2	-	-	-
Huangtong Sag	C6X	2685.97	E2L1S	74.3	-	25.7	-	55	Ⅱ1	-	-	-
Huachang Sub-uplift	H1	2250	E2L1S	20.3	1	76.3	2.3	-	Ⅱ1	0.44	35	0.0002
Huachang Sub-uplift	H1	2300	E2L1S	22.3	1.3	73.3	3	-	Ⅱ1	0.41	31	0.0001
Huachang Sub-uplift	H1	2400	E2L1S	23.3	2.3	70.7	3.7	-	Ⅱ1	0.44	26	0.0039
Huachang Sub-uplift	H131x	2983.62	E2L1S	-	75	19	6	17.3	Ⅱ2	0.84	30	0.08
Huachang Sub-uplift	H132a	3447.35	E2L1S	-	73	19	8	14.3	Ⅱ2	0.78	27	0.06
Huachang Sub-uplift	H132a	3448.62	E2L1S	-	72	23	5	13.8	Ⅱ2	0.73	30	0.06
Huachang Sub-uplift	H2-2	2141.45	E2L1S	74.7	5.7	19.7	-	62.8	Ⅱ1	-	-	-
Huachang Sub-uplift	H2-2	2145.41	E2L1S	73.7	4	22.3	-	59	Ⅱ1	-	-	-
Huachang Sub-uplift	H2-2	2149.4	E2L1S	53.7	5.7	40.7	-	26	Ⅱ2	-	-	-
Huachang Sub-uplift	H2-2	2150.1	E2L1S	48	6.7	45	0.3	17.3	Ⅱ2	-	-	-
Huachang Sub-uplift	H2-2	2159.5	E2L1S	53.3	1	45.7	-	19.5	Ⅱ2	-	-	-
Huachang Sub-uplift	H7	2401.62	E2L1S	-	73	18	9	14	Ⅱ2	0.57	30	0.03
Huangtong Sag	J1x	3214.39	E2L1S	-	77	19	4	20.3	Ⅱ2	0.74	30	0.06
Huangtong Sag	J2x	3560.3	E2L1S	-	72	24	4	14	Ⅱ2	0.72	24	0.06
Huangtong Sag	J2X	3561.97	E2L1S	-	75	22	3	18	Ⅱ2	0.71	30	0.05
Huangtong Sag	J2X	3563.5	E2L1S	-	69	27	4	10.3	Ⅱ2	0.73	23	0.05
Huangtong Sag	Y10X	3276.1	E2L1S	65.7	-	34.3	-	40	Ⅱ2	-	-	-
Huangtong Sag	Y11X	3289.29	E2L1S	54.3	-	45.3	0.3	20	Ⅱ2	-	-	-
Huangtong Sag	Y11X	3548.9	E2L1S	69.3	-	30.7	-	46.3	Ⅱ1	-	-	-
Huangtong Sag	Y11X	3550.95	E2L1S	67.7	0.3	32.0	-	43.9	Ⅱ1	-	-	-
Huangtong Sag	Y11X	3552.53	E2L1S	76.0	-	24.0	-	58	Ⅱ1	-	-	-
Huangtong Sag	Y11X	3555.93	E2L1S	71.3	-	28.7	-	49.8	Ⅱ1	-	-	-
Huangtong Sag	Y11X	3557.93	E2L1S	67.0	-	33.0	-	42.3	Ⅱ1	-	-	-
Huangtong Sag	Y11X	3567.72	E2L1S	66.3	-	33.7	-	41	Ⅱ1	-	-	-
Huangtong Sag	Y11X	3568.92	E2L1S	69.0	-	31.0	-	45.8	Ⅱ1	-	-	-
Huangtong Sag	Y11X	3641.8	E2L1S	66.0	-	34.0	-	40.5	Ⅱ1	-	-	-
Huangtong Sag	Y11X	3643.65	E2L1S	65.7	-	34.3	-	40	Ⅱ2	-	-	-
Huangtong Sag	Y11X	3647.45	E2L1S	68.7	-	31.3	-	45.2	Ⅱ1	-	-	-
Huangtong Sag	Y11X	3653.12	E2L1S	68.7	-	31.3	-	45.2	Ⅱ1	-	-	-
Huangtong Sag	Y13x	3586.7	E2L1S	-	69	24	7	9.5	Ⅱ2	0.8	30	0.06
Huangtong Sag	C6-2X	2939.5	E2L1X	78.0	-	22.0	-	61.5	Ⅱ1	-	-	-
Huangtong Sag	C6-2X	2943.5	E2L1X	65.0	-	35.0	-	38.8	Ⅱ2	-	-	-
Huachang Sub-uplift	H1	2500	E2L1X	40.7	2.7	54	2	-	III	0.534	0	0
Huachang Sub-uplift	H1	2600	E2L1X	33.7	3	61	2	-	III	0.45	23	0.0034
Huachang Sub-uplift	H123-8X	3526.64	E2L1X	44.0	-	56.0	-	2	Ⅱ2	-	-	-
Huangtong Sag	Y10X	3499.67	E2L1X	80.7	-	19.3	-	66.2	Ⅱ1	-	-	-
Huangtong Sag	Y10X	3540.29	E2L1X	72.0	0.3	27.7	-	51.4	Ⅱ1	-	-	-
Huangtong Sag	Y11X	3670.48	E2L1X	71.7	0.3	28.0	-	50.9	Ⅱ1	-	-	-
Huangtong Sag	Y11X	3683.95	E2L1X	69.3	-	30.7	-	46.3	Ⅱ1	1.05	22	0.0275
Huangtong Sag	Y11X	3691.65	E2L1X	69.7	0.3	30.0	-	47.4	Ⅱ1	-	-	-
Huangtong Sag	Y11X	3694.9	E2L1X	72.0	-	28.0	-	51	Ⅱ1	-	-	-
Huangtong Sag	Y11X	3708.33	E2L1X	71.0	-	29.0	-	49.3	Ⅱ1	-	-	-
Huangtong Sag	Y11X	3715.74	E2L1X	77.7	-	22.3	-	61	Ⅱ1	-	-	-
Huangtong Sag	Y11X	3721.71	E2L1X	70.3	-	29.7	-	48	Ⅱ1	-	-	-
Huangtong Sag	Y11X	3725.29	E2L1X	77.3	-	22.7	-	60.3	Ⅱ1	-	-	-
Huangtong Sag	Y11X	3732.24	E2L1X	73.7	0.3	26.0	-	54.4	Ⅱ1	-	-	-
Huangtong Sag	C12X	3473.47	E2L2S	72	-	28	-	51	Ⅱ1	-	-	-
Huangtong Sag	C12X	3475.87	E2L2S	69.7	-	30.3	-	47	Ⅱ1	-	-	-
Huangtong Sag	C12X	3511.8	E2L2S	68.7	-	31.3	-	45.2	Ⅱ1	-	-	-
Huangtong Sag	C12X	3513.55	E2L2S	74	-	26	-	54.5	Ⅱ1	-	-	-
Huangtong Sag	C20X	1399.19	E2L2S	64.3	0.3	35.3	-	62.5	Ⅱ2	-	-	-
Huangtong Sag	C20X	1399.19	E2L2S	70.3	-	29.7	-	38	Ⅱ1	-	-	-
Huangtong Sag	C20X	1399.19	E2L2S	72.3	0.3	27.3	-	48	Ⅱ1	-	-	-
Huangtong Sag	C20X	1399.19	E2L2S	75.3	0.7	24	-	52	Ⅱ1	-	-	-
Huangtong Sag	C20X	1399.19	E2L2S	78.3	0.3	21.3	-	57.6	Ⅱ1	-	-	-
Huangtong Sag	C6X	3160.25	E2L2S	76.3	-	23.7	-	58.5	Ⅱ1	-	-	-
Huachang Sub-uplift	H1	2700	E2L2S	77.3	3.3	18.7	0.7	64.2	Ⅱ1	0.44	35	0.001
Huachang Sub-uplift	H1	2800	E2L2S	57	9.7	32.7	0.7	36.6	Ⅱ2	0.45	8	0.0061
Huachang Sub-uplift	H1	2900	E2L2S	86.7	0.7	12.7	-	77.5	Ⅱ1	0.49	34	0.0001
Huangtong Sag	J1x	3606.05	E2L2S	-	72	23	5	13.8	Ⅱ2	0.6	28	0.05
Huangtong Sag	J1x	3620.45	E2L2S	-	80	15	5	23.8	Ⅱ2	0.67	30	0.06
Huangtong Sag	Y7	3744.12	E2L2S	-	69	26	5	10	Ⅱ2	0.87	30	0.07
Huangtong Sag	Y7	3889.94	E2L2S	-	69	25	6	9.8	Ⅱ2	1.01	24	0.06
Huachang Sub-uplift	H1	3000	E2L2X	61	1.7	36.7	0.7	33.6	Ⅱ2	0.51	39	0.0017
Huachang Sub-uplift	H1	3100	E2L2X	66	1.7	31	1	42.8	Ⅱ2	0.52	34	0.0007
Huachang Sub-uplift	HD1-1	3332.25	E2L2X	-	69	23	8	9.3	Ⅱ2	1.03	25	0.06
Huachang Sub-uplift	HD1-1	3341.8	E2L2X	-	77	16	7	19.5	Ⅱ2	0.96	30	0.06
Huachang Sub-uplift	H1	3151	E2L3S	76	4	19	1	62.8	Ⅱ1	0.67	26	0.1288
Huachang Sub-uplift	H1	3165	E2L3S	59.7	2.3	36.3	1.7	31.9	Ⅱ2	0.64	19	0.133
Huachang Sub-uplift	H1	3195	E2L3S	54.7	1.3	42	1.7	-	Ⅱ2	0.54	30	0.001
Huachang Sub-uplift	H1	3300	E2L3S	37.3	0.7	60	1.7	-	III	0.57	30	0.0001
Huachang Sub-uplift	H7	3410.81	E2L3S	-	70	22	8	10.5	Ⅱ2	0.62	27	0.05
Huachang Sub-uplift	H7	3448.67	E2L3S	-	70	20	10	10	Ⅱ2	0.86	30	0.06
Huachang Sub-uplift	HD6-1X	3673.13	E2L3S	81.3	1.0	17.3	0.3	68.5	Ⅱ1	-	-	-
Huachang Sub-uplift	HD6-1X	3678.08	E2L3S	78.7	0.7	20.3	0.3	63.5	Ⅱ1	-	-	-
Huachang Sub-uplift	H1	3400	E2L3Z	81.7	0.3	17.3	0.7	-	Ⅱ1	0.54	30	0.0032
Huachang Sub-uplift	H1	3500	E2L3Z	24	1.3	69.3	5	-	III	0.56	30	0.0014
Southern Slope Belt	M17x	3801.56	E2L3Z	-	72	20	8	13	Ⅱ2	0.99	30	0.06
Southern Slope Belt	M17x	3804.1	E2L3Z	-	74	19	7	15.8	Ⅱ2	0.97	26	0.06
Huachang Sub-uplift	H1	3600	E2L3X	58.3	1	37.7	3	-	Ⅱ2	-	-	-
Huachang Sub-uplift	H1	3700	E2L3X	70	1.3	27.7	1	-	Ⅱ1	0.9	0	0
Huachang Sub-uplift	H1	3795	E2L3X	61.3	2	35.7	1	-	Ⅱ2	0.55	37	0.0012

and

Table 3. Rock-Eval pyrolysis data used in the sample of the Liushagang Formation in the Fushan Depression.

Area	Well	Depth (m)	Formation	Mudstone Color	TOC (wt %)	Extracted bitumen “A” (%)	Tmax (℃)	S1(mg HC/g Rock)	S2(mg HC/g Rock)	S1 + S2 (mg HC/g Rocks)	Ro (%)
Bailian Sag	L1	2749	E2L2X	grey black	1.85	0.20	475	0.72	1.99	2.71	0.51
Bailian Sag	L102X	3234	E2L1X	brown grey	0.78	0.04	441	0.06	1.40	1.46	0.68
Bailian Sag	L2	3133	E2L2X	grey black	0.94	0.07	450	0.13	1.63	1.76	0.66
Bailian Sag	L22X	2079	E2L3S	dark grey	1.10	0.07	439	0.27	1.61	1.88	0.51
Bailian Sag	L23-1	3570	E2L1X	grey black	1.42	0.08	436	0.37	3.12	3.49	0.63
Bailian Sag	L23-1	4062	E2L2X	grey black	2.02	0.15	439	0.70	2.70	3.4	0.99
Bailian Sag	L23-1	3975	E2L2S	grey black	1.65	0.13	437	0.60	2.44	3.04	0.92
Bailian Sag	L23-1	3860	E2L2S	grey black	1.24	0.02	423	0.23	0.60	0.83	0.87
Bailian Sag	L23x	3923.8	E2L2X	grey black	1.84	0.15	442	0.42	3.19	3.61	1.11
Bailian Sag	L23x	4047.42	E2L2X	brown grey	0.99	0.09	443	0.13	1.86	1.99	0.65
Bailian Sag	L23x	3928	E2L2X	grey black	1.29	0.07	458	0.25	0.84	1.09	1.49
Bailian Sag	L23x	3930.05	E2L2X	grey black	1.08	0.09	460	0.21	0.7	0.91	1.46
Bailian Sag	L23x	4033.06	E2L2X	brown grey	0.92	0.09	469	0.14	0.55	0.69	1.45
Bailian Sag	L23x	4035.26	E2L2X	brown grey	0.88	0.07	476	0.09	0.34	0.43	1.69
Bailian Sag	L27X	3531.8	E2L2X	dark grey	2.39	0.25	447	1.08	3.96	5.04	0.82
Bailian Sag	L27X	3524.36	E2L2X	dark grey	1.97	0.19	443	0.74	2.82	3.56	0.64
Bailian Sag	L27X	2936	E2L2S	dark grey	0.93	0.19	434	0.21	3.14	3.35	0.58
Bailian Sag	L27X	3594.35	E2L2X	dark grey	2.06	0.16	448	0.63	2.64	3.27	1.11
Bailian Sag	L27X	3601.07	E2L2X	dark grey	1.36	0.10	449	0.25	1.65	1.90	0.84
Bailian Sag	L27X	3604.55	E2L2X	dark grey	1.42	0.10	451	0.26	1.57	1.83	1.03
Bailian Sag	L3-2X	3429	E2L2X	brown grey	1.65	0.18	442	0.41	2.63	3.04	0.83
Huangtong Sag	C12X	3473.17	E2L2S	grey black	1.53	0.18	443	0.47	5.18	5.65	0.75
Huangtong Sag	C12X	3511.35	E2L2S	grey black	0.85	0.15	443	0.26	3.35	3.61	0.76
Huangtong Sag	C12X	3511.35	E2L2S	grey black	0.94	0.36	444	0.40	3.57	3.97	0.76
Huangtong Sag	C12X	3473.17	E2L2S	grey black	1.28	0.20	444	0.40	4.13	4.53	0.75
Huangtong Sag	C2	2493	E2L2S	grey	1.90	0.08	436	0.22	4.30	4.52	0.49
Huangtong Sag	C20X	1393.59	E2L2S	brown grey	1.33	0.06	430	0.08	4.13	4.21	0.39
Huangtong Sag	C20X	1393.59	E2L2S	brown grey	1.21	0.06	431	0.05	3.29	3.34	0.39
Huangtong Sag	C20X	1393.59	E2L2S	brown grey	1.07	0.04	430	0.06	3.65	3.71	0.39
Huangtong Sag	C20X	1393.59	E2L2S	brown grey	1.01	0.04	431	0.05	3.2	3.25	0.39
Huangtong Sag	C5X	3660	E2L2S	grey black	1.81	0.03	425	0.47	4.65	5.12	0.54
Huangtong Sag	C5X	3857	E2L2S	grey black	2.09	0.11	433	0.38	3.41	3.79	0.66
Huangtong Sag	C5X	3778	E2L2S	grey black	1.55	0.12	431	0.38	3.28	3.66	0.57
Huangtong Sag	C5X	3700	E2L2S	grey black	1.52	0.13	431	0.48	2.68	3.16	0.90
Huangtong Sag	C5X	3812	E2L2S	grey black	1.56	0.16	425	0.51	2.59	3.1	0.51
Huangtong Sag	Fc1	3129.97	E2L1X	grey black	3.61	0.14	433	0.46	6.29	6.75	0.64
Huangtong Sag	Fc1	3120.07	E2L1X	grey black	1.99	0.09	435	0.27	3.12	3.39	0.60
Huangtong Sag	Fc1	3371.37	E2L2S	grey black	1.51	0.10	437	0.30	2.82	3.12	0.55
Huachang Sub-uplift	H104X	2750	E2L1S	brown grey	1.13	0.05	437	0.05	1.59	1.64	0.52
Huachang Sub-uplift	H109-3X	2501	E2L1S	brown grey	1.01	0.11	434	0.15	1.65	1.80	0.50
Huachang Sub-uplift	H113X	3125	E2L1S	brown grey	2.04	0.26	435	0.30	3.27	3.57	0.54
Huachang Sub-uplift	H115X	2698	E2L1X	dark grey	1.04	0.54	400	2.96	3.29	6.25	0.57
Huachang Sub-uplift	H2-15X	2913	E2L2S	brown grey	1.86	0.20	440	0.91	4.92	5.83	0.67
Huachang Sub-uplift	H2-2	2145.8	E2L1S	brown grey	0.74	0.03	431	0.15	1.42	1.57	0.51
Huachang Sub-uplift	H2-2	2155.6	E2L1S	grey black	0.85	0.04	438	0.09	1.06	1.15	0.53
Huachang Sub-uplift	H2-2	2145.8	E2L1S	grey black	0.80	0.06	436	0.05	0.95	1.00	0.51
Huachang Sub-uplift	H2-2	2136.95	E2L1S	grey black	0.84	0.05	434	0.06	1.33	1.39	0.50
Huachang Sub-uplift	H2-2	2136.95	E2L1S	grey black	0.90	0.04	434	0.05	1.06	1.11	0.50
Huachang Sub-uplift	H3-13X	3471	E2L3Z	Black grey	1.89	0.15	447	0.48	3.22	3.7	0.83
Huachang Sub-uplift	H7-6X	3979	E2L3S	grey black	1.74	0.18	396	0.75	5.55	6.3	1.04
Huachang Sub-uplift	H7-6X	3199	E2L2S	brown grey	1.74	0.19	445	0.73	4.06	4.79	0.70
Huachang Sub-uplift	H7-6X	3097	E2L1X	brown grey	1.48	0.17	446	0.57	3.55	4.12	0.62
Huachang Sub-uplift	H7-7X	3899	E2L3S	grey black	2.30	0.34	395	1.24	4.65	5.89	0.83
Huachang Sub-uplift	H7-7X	3052	E2L1X	brown grey	1.57	0.17	439	0.43	2.91	3.34	0.66
Huachang Sub-uplift	H7-7X	2477	E2L1S	dark grey	0.62	0.06	430	0.07	0.45	0.52	0.56
Huachang Sub-uplift	HD4-3X	3370	E2L2S	Black grey	1.17	0.04	433	0.13	0.49	0.62	0.76
Huachang Sub-uplift	HD6-1X	3821	E2L3S	grey black	1.00	0.10	444	0.31	1.74	2.05	0.91
Bailian Sag	JF4X	1991	E2L2X	brown grey	1.14	0.19	436	0.50	5.76	6.26	0.41
Bailian Sag	JF6	2535	E2L1X	grey black	0.45	0.04	439	0.10	1.00	1.10	0.48
Huangtong Sag	Y1	3509	E2L1X	brown grey	1.47	0.06	449	0.17	2.27	2.44	0.82
Huangtong Sag	Y1	3200	E2L1S	grey black	0.84	0.06	445	0.08	1.89	1.97	0.63
Huangtong Sag	Y1	3801.4	E2L1S	grey black	0.97	0.08	469	0.13	1.45	1.58	1.03
Huangtong Sag	Y1	3040	E2L1S	grey black	0.52	0.06	441	0.11	1.31	1.42	0.70
Huangtong Sag	Y1	3650	E2L1S	brown grey	1.37	0.07	449	0.06	1.34	1.40	0.82
Huangtong Sag	Y1	2903	E2L1S	brown grey	0.94	0.05	445	0.04	1.26	1.30	0.66
Huangtong Sag	Y1	2990	E2L1S	grey black	0.53	0.01	445	0.04	0.29	0.33	0.77
Huangtong Sag	Y10X	3274.2	E2L1X	dark grey	1.09	0.09	458	0.25	2.73	2.98	0.84
Huangtong Sag	Y10X	3536.54	E2L1X	dark grey	1.27	0.11	451	0.17	2.6	2.77	0.99
Huangtong Sag	Y10X	3495.07	E2L1X	dark grey	1.28	0.10	463	0.27	2.29	2.56	0.95
Huangtong Sag	Y11X	3724.89	E2L1X	brown grey	1.05	0.03	469	0.17	1.56	1.73	1.07
Huangtong Sag	Y11X	3641.15	E2L1X	brown grey	0.90	0.07	465	0.28	1.38	1.66	1.05
Huangtong Sag	Y11X	3551.23	E2L1X	brown grey	1.86	0.06	460	0.19	5.01	5.20	0.95
Huangtong Sag	Y11X	3547.3	E2L1X	brown grey	1.16	0.11	456	0.18	2.35	2.53	0.87
Huangtong Sag	Y11X	3641.15	E2L1X	brown grey	1.86	0.07	469	0.33	2.98	3.31	1.05
Huangtong Sag	Y11X	3286.79	E2L1S	dark grey	1.01	0.09	450	0.32	2.89	3.21	0.80
Huangtong Sag	Y11X	3692.1	E2L1X	brown grey	1.27	0.03	471	0.12	1.56	1.68	1.05
Huangtong Sag	Y11X	3560.12	E2L1X	brown grey	1.25	0.07	466	0.23	2.40	2.63	1.04
Huangtong Sag	Y11X	3547.3	E2L1X	brown grey	1.08	0.13	464	0.41	2.31	2.72	0.87
Huangtong Sag	Y11X	3666.38	E2L1X	brown grey	1.19	0.13	466	0.21	1.90	2.11	1.06
Huangtong Sag	Y11X	3717.06	E2L1X	brown grey	1.00	0.06	471	0.20	1.31	1.51	1.09
Huangtong Sag	Y11X	3551.23	E2L1X	brown grey	1.98	0.32	459	0.44	4.42	4.86	0.95
Huangtong Sag	Y11X	3641.15	E2L1X	brown grey	1.03	0.20	467	0.23	1.65	1.88	1.05
Huangtong Sag	Y11X	3551.23	E2L1X	brown grey	0.78	0.26	456	0.78	2.53	3.31	0.95
Huangtong Sag	Y11X	3724.89	E2L1X	brown grey	0.90	0.03	468	0.17	1.21	1.38	1.07
Huangtong Sag	Y11X	3560.12	E2L1X	brown grey	1.14	0.07	463	0.25	2.28	2.53	1.04
Huangtong Sag	Y11X	3683.95	E2L1X	brown grey	1.10	0.09	471	0.21	1.79	2.00	1.05
Huangtong Sag	Y11X	3700.36	E2L1X	brown grey	1.25	0.05	465	0.15	1.51	1.66	1.08
Huangtong Sag	Y11X	3675.25	E2L1X	brown grey	1.14	0.06	472	0.15	1.73	1.88	1.08
Huangtong Sag	Y11X	3560.12	E2L1X	brown grey	1.34	0.10	463	0.34	2.61	2.95	1.04
Huangtong Sag	Y7	3779.5	E2L2S	dark grey	1.96	0.14	441	0.29	4.42	4.71	0.84
Huangtong Sag	Y7	3782.35	E2L2S	dark grey	1.78	0.13	442	0.34	3.73	4.07	0.82
Huangtong Sag	Y7	3777.4	E2L2S	dark grey	1.48	0.13	438	0.23	3.13	3.36	0.72
Huangtong Sag	Y7	3785.15	E2L2S	dark grey	1.35	0.12	441	0.26	3.06	3.32	0.79
Huangtong Sag	Y7	3742.1	E2L2S	grey black	1.41	0.08	440	0.16	3.06	3.22	0.69
Huangtong Sag	Y7	3892.3	E2L2S	dark grey	1.83	0.14	449	0.41	2.07	2.48	0.92

, Figure 8).

In summary, the Southern Slope Belt crossed the hydrocarbon generation threshold during the early phase of the E₂L₂ source rock but has not yet reached the zenith of oil generation. In general, the high-grade area of Ro in the Liushagang Formation is located in the Huangdong Sag and Bailian Sag, and the overall change is positively correlated with the increase in burial depth. From E₂L_2X to E₂L₃, the maturity in the east is significantly higher than in the other regions, which is related to the intrusion of igneous rocks in E₂L₂ and E₂L₃. The thermal effect makes it appear as an unusually high-maturity area, up to 2.69% ([7,75]) (Figure 9).

5. Geological Characteristics of Source Rocks

The average thickness of the E₂L₃ source rock formation is less than 100 m, demonstrating a progressive reduction in thickness from the vicinity of the Meihua Fault towards the Southern Slope Belt. The E₂L_3X source rock, proximal to the Meihua Fault control boundary, is estimated to attain a maximum thickness of approximately 295 m. Within the Huangtong Sag and Bailian Sag, the E₂L₂ source rocks exhibit an average thickness exceeding 400 m, with a lateral extent that transitions from the central regions of the sags towards their peripheries. Within the central region of the Huachang Sub-uplift, a significant elevation of 424 m is observed, comparable to the other two sags. The E₂L₁ source rock in the Huangtong Sag, Bailian Sag, and Huachang Sub-uplift exhibits a similar thickness, reaching a maximum of 531 m. The significant thickness of the source rock is predominantly observed in the Huangtong and Bailian Sags, exhibiting a gradual reduction towards their peripheries (Figure 4). The E₂L_2X dark mudstone is exceptionally well-developed. In comparison with the source rocks from other stratigraphic units, the E₂L_2S source rock exhibits a substantial thickness of 750 m. Furthermore, the E₂L_2S source rock, with a thickness exceeding 250 m, is characterized by a more extensive areal distribution.

The stratigraphic thickness of the dark mudstone serves as a pivotal determinant in ascertaining the thickness of the effective source rocks [61]. The areal extent and stratigraphic thickness of E₂L₃, characterized by dark mudstone, are significantly less extensive than those of E₂L₁ and E₂L₂. The northern strata of the E₂L₃ formation have been eroded along the fault line. During the depositional phases of the E₂L_3X and E₂L_3Z members, a singular subsidence center was established in the northeastern part of the Bailian Sag, where the sedimentary thickness is predominantly concentrated. This could be attributed to the influence of the Meihua Fault, with source supply from the northwest and southwest directions [54]. Within E₂L_3S, the Huangtong and Bailian Sags are characterized as two principal subsidence centers. Initially, during the depositional phase of the E₂L_2X formation, the expanse of dark mudstone in the deep-depression zone, specifically in the northern sector adjacent to the Meihua Fault, exhibited a more extensive distribution. At present, the subsidence center remains constant. Because of the relatively deep and stable hydrosphere, there is extensive development of thick dark mudstone within the reducing environment. For example, the thickness of the dark mudstone in the H18X well is 360 m, which is 100% of the thickness of the E₂L_2X formation, and the whole section is dark mudstone. Affected by the southern provenance [61], the distribution range of E₂L_2S in the Huangtong Sag becomes smaller; however, the dark mudstone of fan delta plain facies with a thickness of 570 m is developed in the C4X well. The thickness of source rock in the Huachang Sub-uplift is high, and the thickness of the E₂L_2S source rock is larger than that of the other layers. Under the condition that the subsidence center of E₂L_1X is unchanged, the thickness of the source rock in the deep depression area is smaller than that in E₂L₂. The hydrocarbon charge to the Huachang Sub-uplift, located in the southern sector of the E₂L_1S, has experienced a reduction in source input, resulting in the re-establishment of two subsidence centers, while concurrently, the areal extent of the source rock has diminished. Overall, the source rocks of the Liushagang Formation are mainly distributed in Sags, and the thickness and area of E₂L₂ source rocks are the largest (Figure 10).

6. Organic Petrology of Source Rocks

From the scatter data in the triangle diagram, it can be seen that the content of the sapropelic group and liptinite macerals of the E₂L₂ source rock is higher compared with the adjacent E₂L₁ and E₂L₃ sections, ranging from 62.7% to 87.4%, with an average of 72.57%. The sapropelic group is dominated by sapropelic amorphous bodies, and the content can reach more than 60%. The average vitrinite content is 25.56%, and the average inertinite content is 1.86%. The hydrocarbon-generating capacity of the organic matter in the E₂L₂ source rocks is high. In E₂L_2S, the content of the sapropelic group in the Huangtong Sag is higher, and there is no inertinite. The content ranges from 0.1% to 6.0%, with an average of 1.32%.

From the scatter data in the diagram (Table 2), it can be seen that the content of the E₂L_2X and E₂L_2S sapropel group and liptinite group is higher than that of the adjacent E₂L₁ and layers (Table 2, Figure 11). By calculating the maceral type index, it is concluded that type II₁ (40< TI <80) and type II₂ (0 < TI < 40) are the main organic matter types of the Liushagang Formation source rocks, and type I (80 < TI) and type III (TI < 0) kerogen are rarely developed, among which type II₁ kerogen accounts for 65% of all the samples. It can be concluded that dark mudstone has high organic matter content and hydrocarbon generation potential, and belongs to oil source rocks, especially those of the Huangtong Sag and Bailian Sag.

Combined with the analysis of the two methods, the E₂L_2X, E₂L_2S, and E₂L_1X of the Huangtong Sag to the west are dominated by type II₁ kerogen, which develops mainly from deep lake mudstone, and the provenance is mainly from the south [76]. The type II₂ kerogen sample (well Y11X) of E₂L_1S in the Huangtong Sag has a sapropelic content of 54.3%, which is much lower than that of the other beds. During this period, the western source was supplied in large quantities [77]. The gravel at the base of E₂L_1S (2535.0 m) can be observed, and the sorting is 3.03. E₂L_1S has a high TOC (2.29%) at this time but has no hydrocarbon generation potential. The Bailian Sag in the eastern part of the survey area is dominated by type II₂ kerogen, but most of the wells in the Sag changed from type II₂ to type III at E₂L_2X. At this time, the eastern source supply was sufficient, the grain size increased to 22.4 mm, the deep lake mudstone was intercalated with turbidite sandstone, and the vitrinite content reached 36.7%. During the deposition of the Liushagang Formation in the Southern Slope Belt, there was a sufficient source from the south [78], mainly type II₂ and type III kerogen. No sapropel group was identified in the M17 well observation, with 72% of the liptinite group. Affected by the source of the western source in the E₂L_1X period [7], tear-like mud particles appeared, the whole was inverted grain order, and there were multiple positive cycles.

This shows that the source supply is sufficient, the particle size increases, and the screening deteriorates. Meanwhile, the sapropel group content decreases, and the total HI decreases. This shows that during the depositional process of the Liushagang Formation, the external transport was mainly oxidized or polymerized vitrinite and inertinite, which have no hydrocarbon-generating capacity. At this time, the layer has high TOC but no hydrocarbon generation capacity. As the source supply weakens, the algae and debris in the lake basin produce sapropel, which has a high H index and hydrocarbon generation potential (Table 3, Figure 11 and Figure 12).

7. Hydrocarbon Expulsion Characteristics of Source Rocks

7.1. Simulation of Hydrocarbon Generation and Expulsion History

The comprehensive analysis of the burial, thermal, and hydrocarbon generation histories for individual wells across the Fushan Depression reveals a consistent evolutionary pattern in hydrocarbon generation and expulsion within the source rock strata, albeit with minor temporal variations. The initial phase of hydrocarbon expulsion was observed in the deep depressions of the Huangtong Sag and Bailian Sag, succeeded by the Huachang Sub-uplift, and culminated in the Southern Slope Belt.

The Yong 7X well, located in the western sector of the study area, demonstrates evidence of burial following the sedimentation of the E₂L₂ formation. It experienced three transient uplift and denudation events at 42–41 Ma, 36–33 Ma, and 23.5–20 Ma, separately, before entering a phase of rapid burial. Hydrocarbon generation in the Yong 7 well began around 37 Ma, stabilizing at 25 Ma. E₂L_2S source rocks also initiated hydrocarbon generation around 37 Ma, with a significant increase in the generation rate compared with the lower sub-member of Liu II, reaching the expulsion threshold around 28 Ma and commencing hydrocarbon expulsion. Tectonic events, such as the Zhuqiong movement of the Hainan Uplift at 36 Ma and 23.5 Ma, temporarily weakened or halted hydrocarbon generation and expulsion. The process resumed after further burial, with the restart time in the Huangtong Sag at 33 Ma and 20 Ma, respectively.

The burial and hydrocarbon generation history of the Lian 23 well in the Bailian Sag mirrors that of the Huangtong Sag, characterized by burial, three short uplift and denudation phases, and then followed by rapid burial. The hydrocarbon generation in the Lian 23 well during the E₂L_2X sedimentation period commenced around 36.5 Ma. E₂L_2S source rocks began generating hydrocarbons at a similar time but at a significantly higher rate than E₂L_2X. E₂L₂ reached the hydrocarbon expulsion threshold at approximately 24 Ma and 12 Ma, initiating expulsion. Similar to the Huangtong Sag, tectonic movements at 36 Ma and 23.5 Ma disrupted hydrocarbon generation and expulsion, which resumed after further burial, with restart times in the Bailian Sag at 33 Ma and 20 Ma (Figure 13).

These observations elucidate the impact of tectonic activities on the hydrocarbon generation and expulsion processes in the Fushan Depression, providing a refined understanding of the temporal evolution of hydrocarbon systems in this region.

7.2. Hydrocarbon Generation and Expulsion Models of Source Rocks

In accordance with the hydrocarbon generation and expulsion paradigm for the Fushan Depression, the source rocks are anticipated to cross the threshold for hydrocarbon generation at a depth of 2200 m and the threshold for hydrocarbon expulsion at 2700 m. The rates of hydrocarbon generation and expulsion initially increase and then decrease. Early in hydrocarbon generation, the rate changes little and remains low until reaching a peak generation rate at 2370 m, with a maximum value of 510 mg/g. After passing the expulsion threshold, hydrocarbon is gradually expelled as thermal evolution progresses, with the expulsion rate peaking at 3220 m and a maximum value of 430 mg/g HC/g rock. Throughout the process, hydrocarbon expulsion efficiency increases, reaching a maximum of 88%.

Hydrocarbon displacement models for the three sets of source rocks (E₂L₁, E₂L₂, E₂L₃) in the Fushan Depression were also established. The hydrocarbon generation thresholds are 2200 m, 1850 m, and 2200 m, respectively, while the hydrocarbon expulsion thresholds are 2650 m, 2720 m, and 2640 m, respectively. The hydrocarbon expulsion peaks for these source rocks correspond to the burial depths of 3170 m, 3380 m, and 3600 m, respectively, with expulsion efficiencies exceeding 84%, indicating sufficient hydrocarbon expulsion. Differences in hydrocarbon expulsion characteristics among the Liushagang Formation source rocks are attributed to variations in burial history, thermal history, and the type and abundance of organic matter in different sedimentary intervals (Figure 14).

7.3. Hydrocarbon Generation, Expulsion Intensity, and Volume of Source Rocks

This study shows that there are some differences in the hydrocarbon expulsion period of the source rocks in each section of the Fushan Depression. Among them, the source rocks of E₂L₂ started to expel hydrocarbon in the Oligocene (the Huangtong Sag is 28 Ma; the Bailian Sag is 29 Ma). The source rocks of E₂L₃ started to expel hydrocarbons 26 Ma ago. The hydrocarbon source rocks of E₂L₁ started to produce hydrocarbons 25 Ma ago and migrated into the reservoir of the first member of the Liushagang Formation. This shows that from the beginning of the Oligocene, the source rocks of each layer began to expel a large amount of hydrocarbon, and the source rock hydrocarbon expulsion center of E₂L₂ of the flow was the largest, reaching the peak period of the source rock hydrocarbon expulsion. The burial depth of the source rock at this time is consistent with the burial depth of the peak period of hydrocarbon expulsion of the TH.

7.4. Tight Oil Resource Potentials of Source Rocks

The centers of hydrocarbon generation and expulsion within the dark mudstone strata of the E₂L₁ and E₂L₂ formations are predominantly localized in the Huangtong Depression and the Bailian Depression. The maximum hydrocarbon generation intensity of E₂L₁ is 475 × 10⁴ t/km² and 625 × 10⁴ t/km², respectively, and the maximum hydrocarbon expulsion intensity is 185 × 104 t/km² and 230 × 10⁴ t/km², respectively. Although the distribution areas of hydrocarbon generation and expulsion in the two layers are roughly similar, better than E₂L₁ in terms of the intensity, and it is a hydrocarbon generation stove with larger resources.

The hydrocarbon generation and expulsion epicenters within the E₂L₃ source rocks are predominantly localized in the Bailian Sag, Huangtong Sag, and Huachang Sub-uplift areas. Compared with E₂L₁ and E₂L₂, the main source rock of E₂L₃ in the Huachang Sub-uplift is added. The maximum hydrocarbon generation intensity is 240 × 10⁴ t/km², and the maximum hydrocarbon expulsion intensity is 90 × 10⁴ t/km².

In general, compared with the source rocks of the other two horizons, the hydrocarbon expulsion range of E₂L₂ is larger, and the hydrocarbon generation and expulsion intensities of the Bailian Sag are significantly higher than those of the Huangtong Sag (Figure 15).

The total volume of hydrocarbon generation and expulsion from the source rocks within the Fushan Sag were determined through the application of mathematical integration techniques, taking into account the spatial extent of the source rock formations and their respective intensities of hydrocarbon generation and expulsion. The total cumulative hydrocarbon generation amount in the Fushan Depression is 134.10 × 10⁸t, the cumulative hydrocarbon expulsion amount is 50.52 × 10⁸ t, and the residual hydrocarbon amount is 83.58 × 10⁸ t (Figure 16). In comparison to the source rocks of the other two stratigraphic horizons, the hydrocarbon expulsion window of the E₂L₂ formation is markedly broader. Moreover, the intensity of hydrocarbon generation and expulsion within the Bailian Sag is considerably more pronounced than that observed in the Huangtong Sag.

Overall, the hydrocarbon generation of the dark mudstone in E₂L₂ is 73.90 × 10⁸ t, which is higher than that of other layers, accounting for 55.11% of the total hydrocarbon generation of the source rocks in the Liushagang Formation, followed by the source rocks in E₂L₁, accounting for 29.85%. The hydrocarbon generation in E₂L₃ is the least, accounting for 15.04%. The hydrocarbon seepage is similar. The source rock of the second member of the Liushagang Formation accounts for the highest proportion of 52.75%, and the hydrocarbon expulsion amount of the source rock of E₂L₁ and E₂L₃ account for only 32.03% and 15.22%, respectively. This shows that the main source rocks in the Fushan Depression are mainly dark mudstone in E₂L₂, while the contribution of source rocks in E₂L₁ and E₂L₃ is relatively small (

Table 4. Data table of hydrocarbon generation and expulsion amount of source rocks in each layer of the Fushan Depression.

Formation	Hydrocarbon-Generating Quantity 108 t	The Proportion of Hydrocarbon Generation%	Hydrocarbon Expulsion Quantity 108 t	Proportion of Hydrocarbon Expulsion Amount %
E2L1	40.03	29.85	16.18	32.03
E2L2	73.90	55.11	26.65	52.75
E2L3	20.17	15.04	7.69	15.22
Total	134.10	100.00	50.52	100.00

).

In light of the hydrocarbon expulsion calculations, and in conjunction with the geological framework of the sag, as well as integrating prior research findings, the accumulation coefficients for the source rocks across various strata within the Fushan Depression were ascertained as follows: 56% for E₂L₂, 25% for E₂L₁, and 10.5% for E₂L₃. The prospective resources of the source rocks in the Liushagang Formation of the Fushan Depression were calculated accordingly (

Table 5. Hydrocarbon source rock prospective resource data table for the Fushan Depression.

Formation	Hydrocarbon Expulsion Quantity 108 t	Migration and Accumulation Coefficient %	Predicted Resources 108 t	Proportion of Resources %
E2L1	16.18	25.00	4.05	20.46
E2L2	26.65	56.00	14.92	75.46
E2L3	7.69	10.00	0.81	4.08
Total	50.52	/	19.77	100.00

). The prospective resources of E₂L₂ in the Fushan Depression are 14.92 × 10⁸ tons, accounting for 75.46% of the total prospective resources. The dark mudstone resources of E₂L₁ and E₂L₃ account for 20.46% and 4.08%, respectively. Thus, the main source rocks in the Fushan Depression are the dark mudstone of E₂L₂. While E₂L₁ and E₂L₃ have some hydrocarbon potential and prospective resources, their contributions are relatively minor.

8. Conclusions

The maximum thickness of the source rock is observed in the subsidence centers of the Huangtong Sag (west) and the Bailian Sag (east), with a progressive thinning towards the periphery and the Huachang Sub-uplift. Dark mudstone predominantly occurs at depths below 200 m. The Liushagang Formation source rocks in the Fushan Depression show high organic matter abundance, particularly in the northern Huangtong Sag and the Bailian Sag. The average TOC is 1.80%, extractable bitumen ‘A’ is 0.1279%, and the hydrocarbon generation potential (S₁ + S₂) is 3.63 mg/g. The organic matter types are mainly types II₁ and II₂. The simulations of hydrocarbon generation and expulsion suggest that E₂L_2X in the deep syncline to the north of the Meihua Fault has initiated the expulsion of hydrocarbons, whereas other regions remain inactive. The Huangtong Sag marks the initial phase of hydrocarbon expulsion, succeeded by the Bailian Sag, with the Huachang Sub-uplift experiencing the latest expulsion. Source rocks reach the hydrocarbon expulsion threshold at 2700 m, with peak generation at 2370 m (510 mg/g) and peak expulsion at 3220 m (430 mg/g). The maximum expulsion efficiency is 88%. The expulsion thresholds for E₂L₁, E₂L₂, and E₂L₃ are 2650, 2720, and 2640 m, with peaks at 3170, 3380, and 3600 m, respectively, all above an efficiency of 84%. The hydrocarbon generation and expulsion centers of E₂L₁ and E₂L₂ are in the Guangdong and Bailian Sags, while the centers of E₂L₃ are in Huangtong, Bailian, and Huachang. The prospective resources of E₂L₂ in the Fushan Depression hold 14.92 × 10⁸ tons (75.46% of total resources), making it the main source rock, compared with 20.46% of E₂L₁ and 4.08% of E₂L₃.