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Article

Geological Conditions and Sedimentary Models of Oligocene and Eocene Effective Source Rocks in the Northern Yinggehai Basin

by
Jianxiang Pei
1,2,
Gaowei Hu
2,
Zhipeng Huo
3,4,*,
Zhihong Chen
2,
Yabing Chen
2,
Xiaofei Fu
5,
Weihong Wang
5,
Haiyu Liu
2,
Yanan Wang
3,6,
Jingshuang Luo
5 and
Guofei Chen
5
1
Institute of Advanced Studies, China University of Geosciences, Wuhan 430074, China
2
Research Institute of Exploration and Development, Hainan Branch of CNOOC (China) Co., Ltd., Haikou 570311, China
3
School of Resources and Materials, Northeastern University at Qinhuangdao, Qinhuangdao 066099, China
4
National Engineering Research Center of Offshore Oil and Gas Exploration, Beijing 100028, China
5
College of Geosciences, Northeast Petroleum University, Daqing 163319, China
6
School of Energy Resources, China University of Geosciences, Beijing 100083, China
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2025, 13(1), 100; https://doi.org/10.3390/jmse13010100
Submission received: 25 August 2024 / Revised: 15 November 2024 / Accepted: 22 November 2024 / Published: 7 January 2025
(This article belongs to the Special Issue High-Efficient Exploration and Development of Oil & Gas from Ocean—2nd Edition)
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Abstract

:
The development of the effective source rocks of the Eocene and Oligocene directly determines the oil and gas exploration potential in the northern Yinggehai Basin in China. Based on the analogy with the Hanoi Depression in Vietnam and the Yacheng District in the Qiongdongnan Basin and the comprehensive analysis of self-geological conditions, the development conditions of Eocene and Oligocene source rocks in the northern Yinggehai Basin are examined, focusing on tectonic evolution, sedimentary facies, and the paleoenvironment. Finally, the sedimentary models for the effective source rocks are established. The tectonic activity controlled the formation of the sedimentary deep depression and the migration of the sedimentary trough center, which migrated from east to west and then south from the Eocene to the Oligocene, leading to the sedimentary migration of good muddy source rocks. There are multiple sedimentary facies in favor of source rocks, including lacustrine facies, shallow marine facies, and delta plain swamps. The paleoenvironment indicates that the paleoclimate transitioned from warm and humid to cold and arid, the redox conditions evolved from semi-reducing to oxic, and paleoproductivity increased from the early to late Oligocene. Therefore, the early Oligocene was more conducive to the enrichment of organic matter. It is speculated that the warm and humid paleoclimate, reducing environment, and high paleoproductivity of the Eocene promoted the sedimentation and preservation of more organic matter. The above studies show that the northern Yinggehai Basin, especially the sedimentary period of the Eocene and Oligocene, has favorable geological conditions for the development of effective source rocks. The sedimentary models for Eocene lacustrine mudstones and Oligocene marine mudstones and marine–continental transitional coal-measure source rocks were established. These studies make up for the serious deficiency of previous research and mean that there is great exploration potential for oil and gas in the northern Yinggehai Basin in China.

1. Introduction

Effective source rocks not only produce and discharge hydrocarbons but also significantly contribute to the formation of commercial reservoirs [1]. The evaluation criteria for effective source rocks vary depending on the region. In this study, source rocks with a total organic carbon (TOC) greater than 0.5% are considered effective. Previous studies have identified two main sets of gas source rocks in the Yinggehai Basin: the Oligocene Yacheng Formation and the middle Oligocene Sanya Formation and Meishan Formation [2,3,4], as well as the possibility of another set of source rocks in the Eocene Lingtou Formation [5,6]. However, current analyses of source rocks in the Yinggehai Basin are limited to the marine source rocks of the Miocene Sanya and Meishan Formations, which are mainly distributed in the Yinggehai Depression with organic matter types dominated by humic types (III-II2) [4,7,8,9]. However, few domestic studies have explored the geological conditions, distribution, and paleoenvironments of potential Paleogene source rocks [5,10,11].
Because of the considerable thickness of the overlying Neogene and Quaternary strata in the Yinggehai Basin, the Paleogene strata are rarely exposed through drilling. Only a few exploratory wells have been drilled in the Lingao Bulge in the northwestern section and the Yingdong Slope in the northeastern section. However, Paleogene strata have been encountered in exploratory wells in the Vietnamese portion of the Yinggehai Basin, and the development conditions of Paleogene (mainly Oligocene and Eocene) source rocks have been thoroughly studied [12,13]. The Oligocene and Eocene source rocks in the Vietnam region have accumulated more than 3000 × 104 tons of recoverable reserves of hydrocarbons, primarily derived from the Eocene and Oligocene source rocks [14]. The Oligocene coal-measure source rocks have also been encountered in the Yacheng District in the western portion of the Qiongdongdong Basin [15]. This provides a useful reference for studying the Oligocene and Eocene source rocks in the northern Yinggehai Basin and offers hope for exploration. In summary, the northern Yinggehai Basin has a low level of exploration which has been stopped for 20 years, and the study of the geological conditions of oil/gas accumulations is still in its early stages [5]. The oil/gas source conditions are unclear, particularly whether the deep Eocene and Oligocene have favorable geological conditions for developing effective source rock. This uncertainty affects the resource potential evaluation in the northern Yinggehai Basin and has become a prominent problem, restricting exploration.
Because the basin genesis and geological conditions of the Hanoi Depression and Yacheng District of the Qiongdongdong Basin are similar to those of the northern Yinggehai Basin, this study discussed the development conditions of the Oligocene and Eocene source rock in the northern Yinggehai Basin from the perspectives of tectonics, sedimentary facies, and paleo-sedimentary environments by drawing analogies with these conditions and established source rock sedimentary models. These studies will not only guild the oil and gas potential evaluation and exploration in the northern Yinggehai Basin but will also make up for the serious deficiency of previous research and play a role in encouraging other marine and petroleum geologists to study the deep Eocene and Oligocene source rocks in the northern Yinggehai Basin or similar basins.

2. Geological Background

The Yinggehai Basin holds significant importance as one of the prominent Cenozoic sedimentary basins located on the northern continental shelf of the South China Sea between China’s Hainan Province and Vietnam. The basin spans an expansive area of more than 11 × 104 km2 and features a rhombic structure with a NNW strike and a length-to-width ratio of approximately 2.5:1 (Figure 1) [16]. The basin is interconnected with the Beibuwan Basin to the north, the Kunsong Uplift in the northwest, and the Qiongdongdong Basin in the southeast in a nearly vertical direction across the Yingdong I Fault Belt, which is a strike-slip tensile basin primarily composed of Cenozoic sediments. The maximum thickness of these sediments exceeds 17 km, making them rich sources of hydrocarbons, particularly oil and gas, in the northwestern part of the South China Sea [16,17]. The basin can be classified into three primary tectonic units: the central depression, the Yingdong slope, and the Yingxi slope. The central depression can be further divided into three secondary tectonic units, namely, the Hanoi Depression, Lingao Uplift and the Yinggehai Depression with a NW–SE orientation [5].
The Cenozoic tectonic evolution of the basin can be divided into four distinct stages. The early rifting stage characterized by the deposition of the Eocene Lingtou Formation (E2l), and the late rifting stage characterized by the deposition of the Oligocene Yacheng Formation (E3y) and the Lingshui Formation (E3l). E3y and E3l may be divided into three sections, namely, the first member (Eyc1), the second member (Eyc2), and the third member (Eyc3) for E3y, and the first member (Els1), the second member (Els2), and the third member (Els3) for E3l. During the thermal subsidence zone, the Miocene Sanya Formation (N1s), Meishan Formation (N1m), and Huangliu Formation (N1h) were deposited, whereas the rapid subsidence zone gave rise to the Pliocene Yinggehai Formation (N2y) and the Quaternary Ledong Formation (Ql) (Figure 2). Throughout the Eocene, lacustrine deposition dominated, whereas during the Oligocene, a prevalence of marine—continental intersection and coastal marine deposition occurred. The Miocene and upper strata are characterized by a predominance of marine deposits [5].
In fact, the oil and gas exploration has been stagnant in the past two decades in the northern Yinggehai Basin. There are only about ten exploratory wells (LG20-X1, LG20-X2, HK17-X1, HK29-X1, HK29-X2, HK30-X1, LG35-X1, and LG36-X1). Only two wells (LG20-X1 and LG20-X2) drilled into the Oligocene Lingshui Formation, but they did not penetrate into the deeper Oligocene Yacheng Formation and the Eocene. At present, there are not significant oil and gas discoveries in the northern Yinggehai Basin.

3. Samples and Methods

3.1. Samples

In this study, 29 rock debris samples were collected from four wells in the study area. These samples were obtained from wells LG20-X2 (6 samples from the Lingshui Formation, northern Yinggehai Basin), YC13-X1 (2 samples from the Yacheng Formation, Yacheng District), YC13-X2 (17 samples from the Yacheng Formation, Yacheng District), and YC13-4X (4 samples from the Yacheng Formation, Yacheng District), and the total organic carbon (TOC), major element, and trace element contents were analyzed. Furthermore, the CNOOC Hainan Branch provided 28 samples from the Yacheng District in the Qiongdongdong Basin for geological analysis.

3.2. Geological Analogy

The exploration level of the northern Yinggehai Basin was relatively low, and the drilled wells did not encounter the Oligocene Yacheng Formation or the Eocene [18]. Thus, evaluating the geological conditions and models for the effective source rocks of the Oligocene and Eocene is challenging owing to the limited available data. This includes the assessment of both geological development conditions and the hydrocarbon source rock sedimentary model. However, the tectonic and sedimentary conditions of the study area are similar to those of the Yacheng District in the Qiongdongnan Basin and Hanoi Depression in Vietnam [12,19]. This study employed a geological analogy to examine the tectonic evolution, sedimentary types, and paleoenvironmental conditions of each layer of the Oligocene and Eocene in the northern Yinggehai Basin. Although the analogy method cannot fully and accurately reproduce the geological conditions and characteristics of hydrocarbon source rock development in the northern Yinggehai Basin, an analogous study is a feasible and effective method given the lack of deep drilling data.

3.3. Structural Evolution Profile Drawing Methods

In this work, by employing the stratigraphic back-stripping method and adhering to the principle of a “balanced profile”, the current evolution profile of the Hanio Depression and the study area is gradually regressed to its original and undeformed state before the deposition of each stratum [10]. This approach allows the reconstruction of the structural development history profile of the area. The tectonic evolution profile encompasses three primary processes: profile selection, application of actual geological data, and profile equilibrium [20]. A profile line perpendicular to the direction of tectonic movement is typically chosen to portray the evolution of underground geological structures reasonably and effectively. In this study area, the prevailing tectonic stress direction is primarily east–west extension and displacement, resulting in the majority of fractures and tectonic developing in a nearly south–north direction. Consequently, several main survey line profiles in the east–west direction were selected to create the evolution profile, and the tectonic period and tectonic style of the study area were analyzed in conjunction with regional geologic features to establish the regional geotectonic framework. In this study, by employing the stratigraphic back-stripping method and adhering to the principle of a balanced profile, the current evolution profile of the Hanio Depression and the study area was gradually regressed to its original and undeformed state before the deposition of each stratum [10]. This approach allows the reconstruction of the structural development history profile of the area.

3.4. TOC Analysis

The total organic carbon (TOC) content was measured via an ELTRR CS-800 Sulfur-Carbon Analyzer (Equipment source: Verder, Shanghai, China) in accordance with the standards outlined in GB/T 19145-2003 [21]. To commence the TOC measurement process, dilute hydrochloric acid was first employed to remove carbonate minerals from the sample under controlled conditions at 68 °C in a water bath. The TOC present in the sample was subsequently completely combusted via a CS-230 analyzer (Equipment source: LECO, St. Joseph, MO, USA) operated under high-temperature conditions. Finally, the TOC content was determined on the basis of the quantity of CO2 produced, as monitored via an infrared detector.

3.5. Analysis of Major and Trace Element Contents

After the sample was thoroughly dried at 105 °C, it was weighed accurately and placed in a platinum crucible. Next, a mixture of lithium tetraborate, lithium metaborate, and lithium nitrate, which served as the melting agent, was added to ensure homogeneity between the sample and the melting agent. The sample was then melted via a high-precision melting machine at 1050 °C, and the resulting melt was poured into a platinum mold and cooled to form a frit. The quality of the frit was then assessed to determine whether it met the necessary standards (if it did not, it was reweighed and remelted). Once the frit was deemed acceptable, the flake was weighed and melted again, and the major element content was determined via a PANalytical PW2424 X-ray fluorescence spectrometer (Malvern Panalytical, Malvern City, UK).
To determine the presence of trace elements, perchloric acid, nitric acid, and hydrofluoric acid were added to the samples, which were then evaporated to near dryness by heating. The samples were then dissolved and fixed with dilute hydrochloric acid, and the resulting solution was analyzed via plasma emission spectroscopy and plasma mass spectrometry with an Agilent 7900 (Equipment source: Agilent, Santa Clara, CA, USA) instrument.

4. Results and Discussion

4.1. Tectonic Conditions

The Paleogene Basin underwent initial rifting, during which time the Yinggehai Basin exhibited a characteristic graben–horst structure, with its eastern boundary fault (Yingdong I Fault Belt) experiencing heightened activity. The central part of the Hanoi Depression is double-faulted, whereas the eastern subdepressions are east-faulted and west-superfaulted (Figure 3). However, strong tectonic inversion occurred in the late Oligocene in the Hanoi Depression, and obvious truncation was observed at the T60 interface, seeing seismic line YGH01 (Figure 1 and Figure 3a). To compare the similarities and differences in the evolution of the Hanoi Depression and the northern Yinggehai Basin, the 2D seismic line 913393 (Figure 1) in the northern Yinggehai Basin was selected to construct the evolutionary history section (Figure 3b), which is NE-oriented and can clearly reflect the structural characteristics of the basin during the rifting period and fracture activities at the basin boundary. In the early stage of rift subsidence (Eocene), the northern Yinggehai Basin was characterized by east breaking and west superposition. In the late stage of rift subsidence (Oligocene), the sliding activity of the main trunk fracture decreased, and the Lingao Inversion Structural Belt developed in the central part of the basin. Thus, during the Eocene, the basin experienced a rift-based style with east breaking and west superpositioning, and several half-grabens developed in the Hanoi Depression and Yingdong slope. The northern Yinggehai Basin is connected to the Hanoi Depression and belongs to the same secondary tectonic unit. During the Oligocene, both faults had two dominant strike directions: NW–SE-oriented and E–W-oriented fractures. Most boundary fault zones are NW–SE-oriented and mainly consist of long-term active faults. The E–W-oriented faults, as secondary faults in the basin, are characterized by small fracture distances and small extension lengths and are very well developed in the upper plate of the eastern boundary faults. Both the northern Yinggehai Basin and Hanoi Depression experienced multi-deformation processes of cracking and trapping, followed by slip during the rifting period, and were controlled by northeast-oriented faults, which resulted in the same stage of overall tectonic evolution and the development of similar source rocks. In summary, the analogy of the tectonic evolution between the northern Yinggehai Basin and the Hanoi Depression shows that the northern Yinggehai Basin has favorable tectonic conditions for developing effective source rocks from the Oligocene and Eocene because it has been proved that the Hanoi Depression develops good source rocks [12,13,22].
In view of the wide scope of the study area and the lack of directly available drilling data but the rich seismic line resources, this study proposes a well–seismic joint analysis method. This method integrates the analysis of geological background and existing data and uses impression technology to restore ancient geomorphic features. The tectonic inversion periods, fold development characteristics, and degrees of tectonic evolution differ between the northern Yinggehai Basin and Hanoi Depression. The anticlines formed in the Hanoi Depression region during the late Oligocene–Miocene and Pliocene epochs, reflecting a strong inversion process and a tendency toward weakening in a southerly direction. Conversely, anticlines in the northern Yinggehai Basin emerged in two periods: the end of the Eocene and the late Oligocene–Miocene. The tectonic inversion period in the northern Yinggehai Basin is earlier than the Hanoi Depression, which means that the source rocks of the Eocene and Oligocene are possibly slightly worse than those of the Hanoi Depression.
The Hanoi Depression and the northern Yinggehai Basin are both located in the NW-oriented fracture zone, and the controlling effect of the eastern boundary fault zone is stronger; therefore, the deep-lying zones are concentrated in the east. Because the fracture zones controlling the Hanoi Depression and northern Yinggehai Basin are left-ordered rather than through faults, the depression zones are arranged in a bead-like fashion along the eastern boundary fracture zone because of differences in fracture activity. Although there is no complete stratigraphic interpretation of the Cenozoic basement owing to the depth of burial, the distribution of the Eocene sub-sag can be roughly inferred from the study of fracture activity, tectonic evolution, and tectonic deformation. During the depositional period of the Lingtou Formation (Eocene), the Yingdong I Fault Belt was more active, which led to strong subsidence control and the formation of the eastern deep depression zone (Figure 4a). In the late stage of rift sinking in the northern Yinggehai Basin, during the development of E3y and E3l, the activity of the Yingxi Fault Belt increased, the sliding activity of the basin’s backbone fracture generally decreased, and the Lingao Inversion Structural Belt developed in the central part of the basin. During the development of the Yacheng and Lingshui Formations, the strike-slip movement of the Honghe fracture zone strongly modified the pre-existing tectonics, controlling the migration of the sub-sag, and the center of the sub-sag migrated from east to southwest. During the depositional period of the Yacheng Formation, the controlling effect of the Yingdong I Fault Belt on subsidence decreased, a nose-like structure developed close to the eastern boundary fracture, the water body became shallow, and the center of subsidence migrated to the southwest (Figure 4b). In contrast, the eastern boundary fracture did not control subsidence during the Lingshui Formation, and the subsidence center migrated to the center and the south (Figure 4c). In summary, there is a difference in the activities of the Yingdong I Fault Belt and the Yingxi Fault Belt, which led to the migration of the center of the sedimentary depression, meaning the migration of good muddy source rocks.

4.2. Depositional Conditions

The northern Yinggehai Basin is characterized by three river systems, including the Red River in the northwest, which follows the direction of the basin, the Ma River in the west, and the Changhua River in the east [23]. The northern Yinggehai Basin and Hanoi Depression are both affected by the Red River source. The Yinggehai Basin is more distant, resulting in greater formation of muddy source rocks. The Ma River is a crucial source of material for the Oligocene and Eocene source rocks on the western slopes of Vietnam and China. In contrast, the Changhua River is a unique source in the eastern part of the northern Yinggehai Basin, providing a continuous supply of material, which is beneficial for the development of mudstone and coal-measure source rocks.
In this study, we relied primarily on the conversion of seismic facies to sedimentary facies to determine the sedimentary characteristics. This conversion allows for multi-resolution analysis because different sedimentary facies types can produce the same seismic facies response. To convert seismic facies to sedimentary facies accurately and reasonably, it is essential to consider the geological meaning of the seismic facies and obtain a thorough understanding of regional sedimentary sequences and features. Typically, taking the seismic line 5600 as an example (Figure 1), the seismic facies in delta plains are wedge-shaped or mat-shaped, parallel or subparallel; in delta fronts, they are wedge-shaped, with large stacked-tile front deposits or diagonal intersections; in seafloor fans, they are lenticular or wedge-shaped, with disorganized reflections; and in shallow seas, they are mat-shaped, with parallel, subparallel, or wave-like patterns (Figure 5).
Considering the sedimentary and seismic facies characteristics of the drilled wells in the Yacheng District of the Qiongdongnan Basin and the sedimentary characteristics of the Hanoi Depression, the sedimentary pattern of the northern Yinggehai Basin is determined by analogy. The northern Yinggehai Basin exhibits seismic facies characteristics similar to those of the coal strata at the edge of the Yacheng District, Hanoi Depression, and other basins, characterized by medium–high continuity, low frequency, and strong amplitude [5,14,24]. Despite the limitations of seismic resolution, the integrated response of multiple thin coal seams was discernible, resulting in the same facies axis displaying low-, medium-, and high-amplitude seismic facies characteristics. The presence of several low-frequency, strong-amplitude reflections and prominent continuous strong-amplitude seismic facies in the northern Yinggehai Basin suggests that lacustrine mudstone, marine mudstone, and coal-measure source rocks were deposited. The stronger the amplitude is, the thicker the coal seams are. The wedge-shaped, subparallel, long-axis, high-frequency, strong-amplitude seismic facies that developed in the northern Yinggehai Basin are indicative of coal strata. The seismic and drilled data both prove that high-organic lacustrine mudstones developed in the Eocene and Lower Oligocene of the Hanoi Depression [5,13,21], and high-quality lacustrine source rocks were also present in the second section of the Eocene Liushagang Formation in the Beibuwan Basin, which is located in the eastern part of the Yinggehai Basin, particularly in deep and semi-deep lakes with thicker layer, parallel, continuous medium and a low frequency and strong amplitude [25,26,27]. By analogy, it is inferred that deep lake and semi-deep lake facies are also present in the northern Yinggehai Basin, which is favorable for developing effective lacustrine source rocks (Figure 5).
The Yacheng Formation in the northern Yinggehai Basin exhibited specific depositional characteristics. First, three to four fan deltas spanning a length of 35–45 km and a width of 30–35 km were used. Second, two to four fans were developed in the fan delta with a length of 25 km and a width of 20 km. Finally, the coal-measure source rocks are located primarily in coastal plains and the marshy facies of diversion plains, covering an average area of 5.0 × 103 km2 (Figure 6). The Lingshui Formation also has certain sedimentary characteristics. First, three to four fan deltas, measuring 30–40 km in length and 25–35 km in width, have developed at the front edge of the braided river delta. Second, two to four fans were present in the fan delta, extending 25 km in length and 10–25 km in width. Finally, the coal-measure source rocks are mainly found in the coastal plains and the swamp facies of the diversion plains, covering an average area of 4.5 × 103 km2 per stratum. Tectonic evolution influences sedimentary development. The Eocene was a fault-bound lake basin in the northern Yinggehai Basin, predominantly characterized by lake facies sedimentation, and a small-scale fan-delta system emerged in the eastern steep-slope area. The Oligocene primarily represented a marine–continental transitional facies with an expanding sea level and extensive fan-delta and braided river delta formations in the eastern steep-slope zone and northwestern slope. These conditions enabled the deposition of coal-measure source rocks in the swampy Delta Huchou plains and the development of marine mudstone in the south-central region, which was dominated by shallow coastal marine deposition (Figure 4, Figure 5 and Figure 6). The distribution of sedimentary facies corresponds to paleogeomorphology.

4.3. Paleoenvironmental Conditions

4.3.1. Paleoclimate

The Sr/Cu, Mg/Ca, Al2O3/MgO, SiO2/Al2O3, and FeO/MnO ratios are commonly used to distinguish paleoclimatic conditions [28,29]. An arid and hot climate accelerates the evaporation of water, resulting in an increase in water alkalinity. Elements such as Na, Mg, Ca, and Mn in the sedimentary medium precipitate at the bottom of the water; therefore, these elements are more likely to be enriched under arid climatic conditions. Sr is a typical dry element. A high Sr content can reflect arid climatic conditions, whereas a low Sr content indicates a humid climate [29]. Furthermore, diagenesis may have a profound impact on the mobility and quantity of these elements. However, in view of the complexity of this field and the large workload needed, we plan to conduct more detailed research in subsequent stages. Therefore, in the present study, diagenesis was not considered in the analysis of the paleo-sedimentary environment of the source rocks with elements.
Because Sr and Cu are very sensitive to changes in climatic conditions, Sr/Cu ratios are commonly used to reflect paleoclimatic conditions, with w(Sr)/w(Cu) > 5 indicating a cold and dry climate and w(Sr)/w(Cu) < 5 reflecting a warm and humid climate [28,29,30]. Low Mg/Ca ratios indicate a dry and hot climate, whereas high ratios indicate a relatively humid climate [31,32,33]. Most of the values of Sr/Cu in the early stages of Els3 and Eyc3 were less than 5, indicating a warm and humid climate, whereas the values of Sr/Cu in the late stages of Els2 and Els1 were greater than 5, indicating a cold and arid environment (Figure 7). In addition, there was an obvious decreasing trend in the Mg/Ca values from the Oligocene Yacheng Formation to the Lingshui Formation (Figure 8), which also indicates that the paleoclimate transitioned from warm and humid to cold and arid from the early to later Oligocene, suggesting that the early Oligocene was more favorable for organic matter enrichment.

4.3.2. Paleoproductivity

The nutrient elements such as phosphorus (P), copper (Cu), iron (Fe), and zinc (Zn) are commonly used as indicators of paleoproductivity, with higher values reflecting greater paleoproductivity [34,35,36]. To eliminate the influence of terrigenous detrital deposits, P/Ti was used in this study to determine the productivity, and high P/Ti values generally indicate high productivity [37]. The values of P/Ti for the source rocks of Els1 and Els3 ranged from 0.1 to 0.12, with an average of 0.11, and the values of P/Ti for the source rocks of Els2 ranged from 0.1 to 0.31, with an average of 0.14. These values are all lower than the P/Ti ratio of the UCC (0.17; [37]), suggesting that the productivity of the late Oligocene stage was moderate (Figure 8). On the other hand, the P/Ti values of the source rocks of Eyc3 range from 0.06 to 0.14, with an average of 0.08, much lower than the P/Ti of the UCC (0.17; [37]). This may be attributed to the fact that the source rocks of the Yacheng Formation are dominated by marine–continental transitional facies, which have more input of terrestrial organic matter.
In addition, the rate of elemental barium (Ba) accumulation was positively correlated with the organic carbon content and biological productivity. Therefore, Ba enrichment indicates high productivity [38]. There are various sources of sediment Ba, among which only biogenic Ba (Babio) accurately reflects the magnitude of primary productivity [38,39], with the following expression:
Babio = Batotal − Baalusilicate = Basample − Alsample × (Ba/Al)alusilicate
where Alsample and Basample are the Al and Ba contents of the measured samples, respectively, and (Ba/Al)alusilicate is a correction factor used to exclude the effect of Ba in terrestrial aluminosilicates [39]. The Babio thresholds for low, moderate, and high paleoproductivity were <200 ppm, 200–1000 ppm, and >1000 ppm, respectively [40]. The Babio values of Els3 range from 737.96 ppm to 1057.3 ppm, of which 66.7% of samples have Babio values less than 1000, indicating moderate primary productivity, which is consistent with the P/Ti index. The Babio values of Eyc3 samples were all greater than 1000 ppm, indicating high productivity in the early Oligocene (Figure 8). From the early to late Oligocene, the overall paleoproductivity showed a gradual declining trend.

4.3.3. Redox Conditions

Because trace elements such as vanadium (V), uranium (U), and molybdenum (Mo) are affected by terrigenous components, there is uncertainty in characterizing the redox properties of water bodies only by their absolute content [33,41]. The redox-sensitive indicators can be used to effectively identify the redox status of water bodies. Commonly used elemental ratios include U/Th, Cu/Zn, Ni/Co, and V/(V + Ni); smaller ratios indicate a greater degree of oxidation, whereas larger ratios reflect a greater degree of reduction [42].
A Ni/Co ratio greater than 7 indicates a reducing environment, a Ni/Co ratio ranging from 5 to 7 indicates an anoxic reducing environment, and a Ni/Co ratio less than 5 indicates an oxic sedimentary environment. A Cu/Zn value less than 0.21 indicates a reducing environment, a Cu/Zn value ranging from 0.21 to 0.63 indicates a dysoxic sedimentary environment, and a Cu/Zn value greater than 0.63 indicates a dysoxic environment [42]. The Ni/Co values of the mudstone samples in Els1 range from 2.68 to 2.8, with an average of 2.74. The Ni/Co values of Els2 range from 1.8 to 3.04, with an average of 2.47. The Ni/Co values of Els3 ranged from 1.46 to 1.97, with an average of 1.78. The Ni/Co values of Eyc3 range from 1.22 to 2.26, with an average of 1.77 (Figure 9). The diagram clearly shows that Ni/Co exhibited a decreasing trend from Eyc3 to Els2. The value of Cu/Zn in Els1 ranges from 0.09 to 0.15, with an average value of 0.11. The Ni/Co values of Els2 range from 0.03 to 0.21, with an average of 0.08. The Ni/Co values of Els3 range from 0.05 to 0.18, with an average of 0.12. The Ni/Co values of Els3 range from 0.04 to 0.21, with an average of 0.13 (Figure 9). The sedimentary environment of E3y is mainly characterized by suboxic conditions, while the water environment of E3l becomes more and more oxidized (Figure 8 and Figure 9). From the early to late Oligocene, the redox conditions changed from suboxic (semi-reducing) to an oxidizing environment.

4.4. Sedimentary Model of Source Rock

Overall, the climate in the northern Yinggehai Basin and Yacheng District shifted from a warm and humid environment to a cold and arid environment during the early to late Oligocene. Paleoproductivity tended to decrease during this period, and the redox conditions changed from a semi-reducing to an oxidizing environment. Therefore, it was conducive to the enrichment of organic matter during the early Oligocene. Statistical analysis of the TOC contents of the Oligocene source rocks in the Yacheng District and Hanoi Depression proved this point. From the early to late Oligocene and from deep to shallow depths, the overall TOC content clearly decreased, and the quality of the source rocks also decreased [43].
In addition, the early Oligocene source rocks (E3y) were deposited mainly during the marine—continental transitional facies, which were controlled by early deposition and tectonics, and relatively few rocks formed on the northwestern, eastern, and western sides of the study area. On the other hand, the upper part of E3l is mainly a shallow coastal marine environment; that is, the water body gradually deepened from the early to late Oligocene. Based on the study of the paleo-sedimentary environments of E3y and E3l, a sedimentary model of the effective source rocks of the Oligocene in the northern Yinggehai Basin was established (Figure 10). The Oligocene Els3 and Eys3 were deposited in a warm and humid climate, which was conducive to the growth and prosperity of plants and provided favorable conditions for coal seams. The cold and arid paleoclimate of Els1 and Els2 lowered the TOC and was not conducive to the enrichment of organic matter. The depositional environment of E3y was dominated by oxic—reducing conditions, which reduced the decomposition of organic matter and favored the preservation of organic matter. In the late Oligocene, the oxidation gradually became stronger, and the water became deeper, resulting in a reduction in salinity, all of which were unfavorable to the deposition and preservation of organic matter.
The Eocene source rocks were deposited in a faulted lake basin with a steep-slope zone in the eastern part and a gently sloping zone in the western part. From the paleodepositional environment of the northern Yinggehai Basin in the Oligocene, it can be inferred that the Eocene had a warm and humid climate, a reducing environment, and high paleoproductivity, which is consistent with the Hanoi Depression and Beibuwan Basin [5,6,12,13,21,23,25,26,27]. The enrichment of mudstone organic matter is the result of the combined effects of the paleoclimate, redox conditions, and paleoproductivity (Figure 11). Owing to the influence of the warm and humid paleoclimate, the species and number of paleontological organisms reached unprecedented levels, and large quantities of lake benthic organisms and plankton subsequently appeared and flourished, which promoted an increase in the primary productivity of the paleolake and provided a rich source of material for the enrichment of organic matter. In addition, reducing water decreases the decomposition of organic matter and promotes the preservation of more organic matter. This suboxic–reducing depositional environment provided good preservation conditions for organic matter, which led to the development of lacustrine oil shale and mudstone with more organic matter.

5. Conclusions

(1) Through the comprehensive analysis of the geological characteristics and the analogy with the Hanoi Depression in Vietnam and Yacheng District in Qiongdongnan Basin, the northern Yinggehai Basin has favorable geological conditions for the development of effective source rocks in terms of tectonic, sedimentary, and paleoenvironmental conditions. Specifically, the tectonic movement not only shaped the sedimentary deep depression but also drove the dynamic migration of the center of sedimentary depression of source rocks. In addition, there is a diversified sedimentary source supply system in the northern Yinggehai Basin, including lacustrine facies, neritic facies, and delta plain swamp facies, which are conducive to the deposition of effective source rocks.
(2) From the early to late Oligocene, the paleoclimate transitioned from warm and humid to cold and arid, and the water depth gradually increased, evolving from a marine–continental transitional facies to a coastal shallow sea. As the water body deepened, the redox conditions shifted from a semi-reducing to an oxidized environment, and the paleoproductivity decreased. From the depositional environment of the Oligocene and the analogy with the Eocene in the Hanoi Depression, it is inferred that the Eocene lacustrine mudstone developed in a warm and humid paleoclimate under a reduced environment and high paleoproductivity. The paleodepositional environment suggests that the Eocene and early–middle Oligocene periods promoted organic matter enrichment source rocks, which means that there is great exploration potential for oil and gas. Some exploration activities are worth pursuing in the future in the northern Yinggehai Basin.
(3) Based on the comprehensive analysis of paleo-sedimentary environments and sedimentary conditions including sedimentary tectonic setting and sedimentary facies, the sedimentary model of coal-measure source rocks of marine–continental transitional deltas and mudstone of shallow seas was established in the Oligocene. At the same time, the sedimentary model of Eocene lacustrine mudstone deposited in the faulted lake basin has also been established. These studies make up for the serious deficiency of previous studies and play a role in encouraging the study of the deep Eocene and Oligocene source rocks in the northern Yinggehai Basin. As the deep wells are drilled and the seismic data quality is improved, the deep Eocene and Oligocene source rocks in the northern Yinggehai Basin will be studied systematically and meticulously and be recognized more accurately in the future.

Author Contributions

Literature search: J.P., G.H. and Y.C.; Figure: J.P., Z.C., H.L. and G.C.; Study design: J.P. and Z.H.; Data collection: G.H., Z.H., Y.C., W.W., H.L. and G.C.; Data analysis: J.P., Y.C., X.F. and Y.W.; Data interpretation: J.P.; Writing: J.P.; Supervision: G.H., Z.C., W.W., H.L., J.L. and G.C.; Writing—Review & Editing: J.P., Z.H. and X.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the 14th Five-Year major national science and technology projects, Deep and Ultra-Deep Oil and Gas Accumulation Conditions and Mechanisms in Complex Edge Sea Basins, Yinggehai–Qiongdongnan Basins grant number KJGG2022-0404.

Institutional Review Board Statement

No prior ethical approval was necessary for the study.

Informed Consent Statement

No human subjects were included in the study. Thus, consent was not needed.

Data Availability Statement

The authors have included all relevant data and the sources of freely available data in the manuscript.

Acknowledgments

Four anonymous reviewers are thanked for their insightful comments and suggestions that help to significantly improve the clarity of this manuscript. Gest Editor Mianmo Meng and Wenming Ji are also acknowledged for their patient editorial work.

Conflicts of Interest

Jianxiang Pei, Gaowei Hu, Zhihong Chen, Yabing Chen, and Haiyu Liu were employed by the Research Institute of Exploration and Development, Hainan Branch of CNOOC (China) Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Geological map showing the location and sampling wells of the northern Yinggehai Basin, Hanoi Depression, and Yacheng District.
Figure 1. Geological map showing the location and sampling wells of the northern Yinggehai Basin, Hanoi Depression, and Yacheng District.
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Figure 2. Stratigraphic column of the northern Yinggehai Basin.
Figure 2. Stratigraphic column of the northern Yinggehai Basin.
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Figure 3. (a) Tectonic evolution profile of the Hanoi Depression of seismic line YGH01 [22]. (b) Tectonic evolution profile of the northern Yinggehai Basin of the 2D seismic line 913393.
Figure 3. (a) Tectonic evolution profile of the Hanoi Depression of seismic line YGH01 [22]. (b) Tectonic evolution profile of the northern Yinggehai Basin of the 2D seismic line 913393.
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Figure 4. Sedimentary paleotopography of the Eocene–Oligocene in the northern Yinggehai Basin (the water body gradually gets deeper from yellow to blue). (a) Sedimentary paleomorphology of the Eocene; (b) sedimentary paleomorphology of the Yacheng Formation; (c) sedimentary paleomorphology of the Lingshui Formation.
Figure 4. Sedimentary paleotopography of the Eocene–Oligocene in the northern Yinggehai Basin (the water body gradually gets deeper from yellow to blue). (a) Sedimentary paleomorphology of the Eocene; (b) sedimentary paleomorphology of the Yacheng Formation; (c) sedimentary paleomorphology of the Lingshui Formation.
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Figure 5. Typical seismic facies and sedimentary facies identification of seismic line 5600 in the northern Yinggehai Basin. T80–T60 represent the seismic (stratigraphic) interface (see Figure 2).
Figure 5. Typical seismic facies and sedimentary facies identification of seismic line 5600 in the northern Yinggehai Basin. T80–T60 represent the seismic (stratigraphic) interface (see Figure 2).
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Figure 6. Sedimentary facies of source rocks of the Oligocene Yacheng Formation in the northern Yinggehai Basin, mainly including neritic facies and coastal plain, which are conducive to the sedimentation for mudstone and coal-measure source rocks.
Figure 6. Sedimentary facies of source rocks of the Oligocene Yacheng Formation in the northern Yinggehai Basin, mainly including neritic facies and coastal plain, which are conducive to the sedimentation for mudstone and coal-measure source rocks.
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Figure 7. Diagram of the Sr/Cu and Mg/Ca ratios used to discriminate paleoclimatic conditions of the northern Yinggehai Basin and Yacheng District.
Figure 7. Diagram of the Sr/Cu and Mg/Ca ratios used to discriminate paleoclimatic conditions of the northern Yinggehai Basin and Yacheng District.
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Figure 8. The paleodepositional environment reflected by the major and trace elements in the well YC13-X2 in the Yacheng District. Paleoclimate indicators (Sr/Cu and Mg/Ca), paleoproductivity-related indicators (Babio and P/Ti), and redox indicators (Ni/Co and Cu/Zn).
Figure 8. The paleodepositional environment reflected by the major and trace elements in the well YC13-X2 in the Yacheng District. Paleoclimate indicators (Sr/Cu and Mg/Ca), paleoproductivity-related indicators (Babio and P/Ti), and redox indicators (Ni/Co and Cu/Zn).
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Figure 9. Diagram of Ni/Co versus Cu/Zn ratios of the source rock samples from E3l and E3y.
Figure 9. Diagram of Ni/Co versus Cu/Zn ratios of the source rock samples from E3l and E3y.
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Figure 10. A sedimentary model of source rocks in the marine–continental transitional facies of the Oligocene in the northern Yinggehai Basin.
Figure 10. A sedimentary model of source rocks in the marine–continental transitional facies of the Oligocene in the northern Yinggehai Basin.
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Figure 11. Sedimentary model of the Eocene lacustrine mudstone in the northern Yinggehai Basin.
Figure 11. Sedimentary model of the Eocene lacustrine mudstone in the northern Yinggehai Basin.
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Pei, J.; Hu, G.; Huo, Z.; Chen, Z.; Chen, Y.; Fu, X.; Wang, W.; Liu, H.; Wang, Y.; Luo, J.; et al. Geological Conditions and Sedimentary Models of Oligocene and Eocene Effective Source Rocks in the Northern Yinggehai Basin. J. Mar. Sci. Eng. 2025, 13, 100. https://doi.org/10.3390/jmse13010100

AMA Style

Pei J, Hu G, Huo Z, Chen Z, Chen Y, Fu X, Wang W, Liu H, Wang Y, Luo J, et al. Geological Conditions and Sedimentary Models of Oligocene and Eocene Effective Source Rocks in the Northern Yinggehai Basin. Journal of Marine Science and Engineering. 2025; 13(1):100. https://doi.org/10.3390/jmse13010100

Chicago/Turabian Style

Pei, Jianxiang, Gaowei Hu, Zhipeng Huo, Zhihong Chen, Yabing Chen, Xiaofei Fu, Weihong Wang, Haiyu Liu, Yanan Wang, Jingshuang Luo, and et al. 2025. "Geological Conditions and Sedimentary Models of Oligocene and Eocene Effective Source Rocks in the Northern Yinggehai Basin" Journal of Marine Science and Engineering 13, no. 1: 100. https://doi.org/10.3390/jmse13010100

APA Style

Pei, J., Hu, G., Huo, Z., Chen, Z., Chen, Y., Fu, X., Wang, W., Liu, H., Wang, Y., Luo, J., & Chen, G. (2025). Geological Conditions and Sedimentary Models of Oligocene and Eocene Effective Source Rocks in the Northern Yinggehai Basin. Journal of Marine Science and Engineering, 13(1), 100. https://doi.org/10.3390/jmse13010100

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