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The coal-measure source rock in the Chinese sea area plays a significant role as a hydrocarbon source rock, with its genetic environment, development and distribution, and hydrocarbon generation potential serving as essential factors for the exploration of coal-type oil and gas fields. This study focuses on the coal-measure source rock of the Paleogene Enping Formation in the Zhu I Depression, located in the northern South China Sea. The main geological insights obtained are as follows. The coal measures of the Enping Formation are developed in a warm and wet tropical-subtropical climate. The development environment of the coal-measure source rock in the Enping Formation includes the braided river delta upper plain peat swamp, characterized by dry forest swamp coal facies with relatively thick coal seams and a small number of layers. The braided river delta lower plain swamp-interdistributary bay of braided river delta front represents a forest edge-wetland herbaceous swamp coal facies with numerous layers of thin coal seams and poor stability. The shore swamp corresponds to an open water swamp coal facies with multiple layers of thin coal seams and poor stability. The organic matter abundance in the braided river delta upper plain is the highest, followed by the braided river delta lower plain-braided river delta front, and the shore-shallow lake. The organic matter type is predominantly type II1. Thermal evolution analysis suggests that the organic matter has progressed into a substantial oil generation stage. The hydrocarbon generation potential of the coal-measure source rock in the Enping Formation is the highest in the braided river delta upper plain, followed by the braided river delta lower plain-braided river delta front and the shore-shallow lake. Overall, this study proposes three organic facies in the coal-measure source rock of the Enping Formation: upper-plain swamp-dry forest swamp facies, lower plain-interdistributary bay-forest-herbaceous swamp facies, and lake swamp-herbaceous swamp facies. [ABSTRACT FROM AUTHOR]

Tuffaceous sandstone reservoirs of Paleogene Wenchang Formation in Huizhou and Lufeng area of Zhu I Depression, Pearl River Mouth Basin were selected, and core-scale fluid-rock interaction simulation experiments were designed and carried out to explore the dissolution and modification effect of acidic fluid on tuffaceous components in clastic reservoirs during burial process and its controlling factors. The dissolution characteristics and physical property response characteristics of sandstone reservoirs under different fluid flow rates and tuffaceous contents before and after the experiments were compared and analyzed through microscope observation, fluid composition analysis, physical property characterization and other methods. The results showed that tuffaceous dissolution was common in acidic fluid environment, but the intensity of tuffaceous dissolution and the response of reservoir physical properties were different under different experimental conditions. Among them, the openness and closure of diagenetic system determined the intensity of tuffaceous dissolution. Under the same rock and acidic fluid conditions, the amount of tuffaceous dissolution in the high flow rate open system was higher than that in the closed system, and the dissolution products in the closed system tended to precipitate, which was not conducive to the preservation of dissolution pores. In addition, the content of tuffaceous components significantly affected dissolution effect, the tuffaceous-rich sandstone would not be conducive to dissolution porosity, while the tuffaceous-containing sandstone and tuffaceous-poor sandstone could increase dissolution porosity, and tuffaceous-containing sandstone has higher dissolution efficiency. Overall, in the reservoirs with relatively moderate to low contents of tuffaceous components, the open diagenetic system in the shallow-middle burial stage is most conducive to the development of dissolution pores of tuffaceous components. After the late stage of acidic fluid dissolution and modification, it is easier to form secondary dissolution-type high-quality reservoirs. This study is of great significance for the prediction of dissolution-type reservoir sweet spots in different areas.

The element geochemical characteristics and diagenetic alteration products of tuffaceous components in sandstone reservoirs of Paleogene Wenchang Formation in typical subsags of the Huizhou-Lufeng area of the Zhu I Depression, Pearl River Mouth Basin, were identified through microscopic analysis and quantitative analysis of main and trace elements. The impacts of dissolution of different tuffaceous components on physical properties of reservoirs were discussed through quantitative characterization of reservoir physical properties. The results show that there are mainly four types of tuffaceous components in the study area, which are acidic, intermediate, basic and alkaline tuffaceous components. The acidic tuffaceous components evolved in a process of strong alteration and weak dissolution of alteration products, with a large amount of kaolinite precipitated during alteration to disenable the improvement of porosity and permeability. The intermediate and alkaline tuffaceous components evolved in a process of strong dissolution of tuffaceous components and strong alteration of residual tuffaceous components; the dissolution of tuffaceous components created intergranular pores, but the alteration products such as autogenic quartz, apatite and illite deteriorated the pore structure; ultimately, the dissolution of tuffaceous components resulted in the increase of porosity but no increase of permeability of the reservoir. The basic tuffaceous components dominantly evolved in a process of dissolution of tuffaceous components to strong dissolution of alteration products; both tuffaceous components between particles and laumontite generated from alteration can be strongly dissolved to create pores; thus, the dissolution of tuffaceous components can significantly increase the physical properties of the reservoir.

Due to the lack of drilling data in the Zhu 1 Depression of the Pear River Mouth Basin, the Enping 17 Sag is taken as an example in this paper to solve the problem of comprehensive identification of underground distribution of high-quality source rocks. Based on the analysis of the geological background, sedimentary characteristics and petrophysical characteristics of source rocks, seismic inversion is guided by the constraints of the petrophysical characteristics of the source rocks in different sedimentary environments. According to the statistics of the relationship between the lithofacies of drilled source rocks and the inversion resistance, a semi-quantitative prediction of the spatial distribution of favorable source rocks is made by geological modeling, and the rationality of prediction method and results is analyzed in combination with the recognition of the geochemical origin type and source of crude oil, to discuss the geological and geophysical identification method of the underground distribution of high-quality source rocks with lack of drilling data. The more reductive the depositional environment of source rocks in the Paleogene Wenchang Formation, the lower the impedance value of high-quality source rocks, and the greater the impedance difference between poor source rocks or non-source rocks. The third, fourth and fifth members of Wenchang Formation are the main source rocks of the Enping 17 Sag, which can be supported by the study of crude oil biomarkers. In the small sags of the Pearl River Mouth Basin with basically clear petroleum geological background conditions, establishing the quantitative and semi-quantitative relationship between the lithofacies of source rocks and the inversion impedance value is helpful to quickly clarify the development interval and scale of main source rocks. This method has a good prospect of popularization and application.

The Pearl River Mouth Basin (PRMB) is one of the important offshore targets for oil and gas exploration in China and has great potential of oil and gas resources. The Eocene Wenchang Formation is one of the potential source rocks in the PRMB, but its geochemical characteristics are insufficiently studied, particularly for the Wenchang Formation source rocks in deep-water area of the PRMB. Due to the deep burial condition, core samples can rarely be obtained, and most samples are drilling cuttings which are easily to be contaminated by drilling mud, which makes it difficult to directly access the geochemical characteristics of Wenchang Formation source rocks. This paper aims to compare the geochemistry of the Wenchang Formation source rocks in the deep-water and shallow-water areas of the PRMB with an improved catalytic hydropyrolysis method to release the bound hydrocarbons from kerogen macromolecules. Based on the biomarkers and carbon isotopic compositions of individual n-alkanes released, the geochemical characteristics of Wenchang Formation source rocks in deep-water (mainly from Zhu Ⅱ Depression) and shallow water areas (mainly from Zhu Ⅰ Depression) from PRMB are compared. Results indicate that the source rocks of Wenchang Formation in PRMB can be classified into three types including semi-deep to deep lacustrine (WC-Ⅰ), shallow lacustrine (WC-Ⅱ), and semi-deep to deep lacustrine with algal blooming (WC-Ⅲ). The WC-Ⅰ are characterized by low C30 4-methylsteranes (4-Me/C29 < 0.2) and bicadinanes contents, and relatively light carbon isotopic composition of individual n-alkanes (-33‰ to -31‰). The WC-Ⅱ are characterized by low 4-Me/C29 ratios (average=0.06), low to high T/C30H ratios (average=1.04), and relatively heavier carbon isotope composition of individual n-alkanes (-30‰ to -27‰). The WC-Ⅲ are characterized by high content of C30 4-methylsteranes (4-Me/C29=0.66) and heavy carbon isotope compositions of individual n-alkanes (-25‰ to -23‰). The Wenchang Formation source rocks in deep-water and shallow-water areas have semi-deep to deep lacustrine facies (WC-Ⅰ) and shallow lacustrine facies (WC-Ⅱ). In addition, the semi-deep to deep lacustrine source rocks with algal blooming (WC-Ⅲ) also developed in shallow-water areas, but no such type of source rocks were found in deep-water area. This study compared the geochemical characteristics of bound hydrocarbons in source rocks of Wenchang Formation in different regions of PRMB, and found that the semi-deep to deep lacustrine source rocks of Wenchang Formation in shallow-water area can be divided into two types. It provides a scientific basis for a better understanding of the geochemical features of source rocks in different areas of Wenchang Formation in PRMB, and is helpful for oil-source correlations in this area.

The study of changes in normal fault systems during different rift stages is important to understand the genesis and evolution of multi-phase rift basins, such as the Panyu 4 Sag in the Zhu Ⅰ Depression. Using 2D and 3D seismic data and analogue modelling, the Zhu Ⅰ Depression was characterized as a series of half-grabens bounded by NE-NEE-trending normal faults, it was found to have undergone two phases of the extension during the Paleogene. The Zhu Ⅰ Depression exhibited four fault sets with different strikes, including NNE, NE-NEE, EW, and NWW. The main controlling faults were NE-trending and EW-trending with high activity rates during Rift Phase 1 and Rift Phase 2, respectively. The average azimuths of the dominant strikes for type Ⅰa, type Ⅰb, and type Ⅱ faults were 75°, 85°, and 90°, which revealed that the minimum principal stress (σ3) directions during the rift phases 1 and 2 of the Zhu Ⅰ Depression were SSE (∼165°) and near-EW (∼180°), respectively. Two phases of structural-sedimentary evolution, with different directions and analogue modelling results, illustrated that the Panyu 4 Sag was formed as a superimposed basin under multi-phase anisotropic extension. The structural evolution of the Panyu 4 Sag since the Paleogene was mainly controlled by the combined effects of the Pacific, Eurasian, and Indian plates. Since the orientation of subduction of the Pacific plate changed from NNW to NWW, the stress field shifted from NW-SE-trending tension to S-N-trending tension, causing the superimposition of late near-E-W-oriented structural pattern on the early NE-oriented structural pattern.

Through the analysis of the faults and their internal structure in Zhu I Depression, it is found that the internal structure of the late fault is obviously segmented vertically. It develops unitary structure (simple fault plane) in shallow layers, binary structure (induced fracture zone in hanging wall and sliding fracture zone in footwall) in middle, layers and ternary structure (induced fracture zone in hanging wall and sliding fracture zone in middle, and induced fracture zone in footwall) in deep layers. Because the induced fracture zone is a high porosity and permeability zone, and the sliding fracture zone is a low porosity and ultra-low permeability zone, the late fault in middle layers has the character of "transporting while sealing". The late fault can transport hydrocarbon by its induced fracture zone in the side of the hanging wall and seal hydrocarbon by its sliding fracture zone in the side of the footwall. In deep layers, the late fault has the character of "dual-transportation", induced fracture zones in both sides of hanging wall and footwall can transport hydrocarbon. The early fault that only developed in the deep layers is presumed to be unitary structure, which plays a completely sealing role in the process of hydrocarbon migration and accumulation due to inactivity during the hydrocarbon filling period. Controlled by hydrocarbon source, early/late faults, sand bodies and traps, two reservoir-forming models of "inverted L" and "stereo-spiral" can be proposed in middle layers, while two reservoir-forming models of "cross fault" and "lateral fault sealing" are developed in the deep layers of Zhu I Depression. [ABSTRACT FROM AUTHOR]

The study of source rocks is a key component in the exploration and discovery of hydrocarbon plays in the offshore petroliferous basins of China. Geochemical analyses, drilling, seismic, and microfossil data were integrated to document the formation mechanism of lacustrine source rocks contained in the Wenchang Formation in the Zhu I Depression of the Pearl River Mouth Basin, China. Several factors that control the development of lacustrine source rocks were evaluated in this study, including tectonics, sedimentary conditions, palaeoclimate, the supply of organic matter, and redox conditions. The deposition and occurrence of source rocks is a combined function of these factors. Lacustrine source rocks are characterized by high organic matter contents (average TOC value = 1.33 wt.%) and are dominated by type I and type II1–2 kerogen, which primarily originate from planktonic algae. The rifting of basement rocks, syn‐sedimentary faults, and palaeoclimate played significant roles in controlling the geochemical properties of the source rocks (e.g., controlling the abundance and types of organic matter). The Eocene lacustrine source rocks were deposited when the palaeoclimate was characterized by humid to semi‐humid, subtropical to tropical weather that did not have a stable temperature or humidity. Controlled by large‐scale faults, "Half‐graben patterns" and "Graben patterns" are the two main palaeotopographic patterns where the thermal stratification of deep lake water resulted in flourishing planktonic algae in its surface zones. Dysoxic to anoxic conditions prevailed during the deposition of the lacustrine source rocks. An underfilled basin situation caused the occurrence of abundant organic matter during the period of maximum surface flooding. [ABSTRACT FROM AUTHOR]