Geological Distribution of the Miocene Carbonate Platform in the Xisha Sea Area of the South China Sea, and Its Implications for Hydrocarbon Exploration

Abstract

The newly collected seismic data and the existing drilling data provide a good opportunity to better understand the carbonate platform distribution characteristics and the hydrocarbon resource potential in the Xisha sea area of the South China Sea. Based on the seismic data and the reflection characteristics of the carbonate platform’s edge, three boundary indicators were established: abrupt lithological interfaces, fault interfaces, and tidal channels. Combined with the regional geological settings, its spatial and temporal distribution was clearly identified for the first time. The development of the Miocene carbonate platform in the Xisha sea area is divided into six phases, which are further assigned to three evolutionary stages: the bloom stage, the recession stage, and the submerged stage. The sedimentary facies belt of the carbonate platform in each stage is well developed, and the reefs are mainly distributed on the west and southwest edges of the platform. The analysis of the data indicates that the area of the reef and carbonate platform reached 80,000 km² during the mature period, followed by a retreat period where the scale decreased with the platform’s decline. The Miocene carbonate rocks in the Xisha sea area are widely distributed. They have experienced multiple periods of exposure and infiltration, which further improved the quality of their physical properties for hydrocarbon reservoirs. According to the regional hydrocarbon geological conditions in this area—including the source rock, migration system and the capping layer—the hydrocarbon accumulation potential is preliminarily discussed in this paper. A reservoir model of the reef and carbonate platform is established, which is proposed as typical characteristics of “lower generation, upper accumulation”. It is pointed out that the carbonate platform in the Xisha sea area adjacent to the Huaguang Sag in the Qiongdongnan Basin and the northern Zhongjiannan Basin is a potential area for oil and gas exploration.

1. Introduction

Carbonate platforms are widely developed in low-latitude shallow-water areas, not only requiring suitable seawater temperature (17~28 °C) and water depth (0~200 m), but also closely related to waves, salinity, dissolved oxygen, sediment type, nutrients, regional wind direction, etc. [1,2,3]. Any change in these factors may change the development form or internal structure of the carbonate platform. Therefore, the evolution of carbonate platforms entails abundant ancient marine information, which is regarded as an important record of marine environmental change. Additionally, carbonate reservoirs are characterized by high porosity and permeability, accounting for 60% of the world’s total oil and gas production [1,2,3], which is mainly distributed in North America, the Middle East, and Southeast Asia.

The South China Sea is located in an enriched area of carbonate-type oil and gas reservoirs in Southeast Asia [4,5]. Since the Late Cenozoic, numerous carbonate sediments have been deposited in the northern South China Sea, which is rich in hydrocarbon resources. In particular, the Miocene carbonate rocks have an excellent reservoir capacity and are the most favorable resource for development in the Xisha area [6,7,8]. Studies of 2D seismic data from the region indicate five types of reefs with typical seismic reflection characteristics that developed during the Miocene in the Xisha sea area [9]. Moreover, the sedimentary facies belts of carbonate rocks in the region have also been preliminarily divided, and a sedimentary model of the carbonate platforms has also been established [10].

Although previous studies have covered some basic aspects of the Miocene carbonate platform in the Xisha sea area, there are still some gaps that need to be filled. For instance, there is a lack of data on the temporal and spatial distributions of the biological reefs and carbonate platforms in the literature. Moreover, to date, there have been no relevant studies on investigation of the hydrocarbon accumulation mode and prediction of the prospects of hydrocarbon exploration in this area through the spatial and temporal distribution of carbonate platforms.

Based on newly acquired seismic and drilling data, this paper aims to enhance the understanding of the carbonate platforms in the Xisha sea area, including the spatiotemporal distribution characteristics and hydrocarbon resource potential. The seismic marks of the carbonate platform boundaries are depicted in detail. In addition, the characteristics of various sedimentary facies belts are described, and the distribution area of each period is identified. According to the regional hydrocarbon source rock, migration path, cap rock, and other geological conditions, the hydrocarbon accumulation model of the carbonate platform in the Xisha sea area is established. Finally, a sweet spot for exploration and development in the study area is identified. Hence, the present study could be considered as a stepping stone for further exploration and development of this carbonate platform in the Xisha sea area in the near future.

2. Geological Settings

The South China Sea is considered a unique marginal sea in the western Pacific Ocean. The reason for this is that since the Cenozoic it has experienced multiple periods of seafloor expansion [11], forming typical geomorphic units, such as continental shelves, continental slopes, and deep ocean basins, which have controlled the evolution of carbonate platforms. The carbonate platform in the South China Sea began to develop in the Late Oligocene, evolved extensively in the Miocene, and began to decline after the Late Miocene [10]. The carbonate platform has variable scales, forms, and properties throughout the area of study [12,13]. Affected by the tectonic evolution of the South China Sea, the carbonate platform in the south developed earlier than in the north. As a special shelf–slope break in the northern South China Sea, the study area contains different geological units, such as the Qiongdongnan Basin, Xisha Trough, Xisha Uplift, Zhongjian Trough, Zhongjiannan Basin, and Zhongsha Trough (as shown in Figure 1), which determine the particularities of the development and evolution of carbonate platforms. A combination of well and seismic data studies indicate eight sequential boundaries from top to bottom in the area, as shown in Figure 2 and Figure 3. Among them, T6 (23.3 Ma) is an essential structural interface. This interface divides the structural evolution of the whole area into two stages: the rifting stage and the post-rifting stage [13]. Between these stages, the rifting period (65.0–23.3 Ma) was dominated by northeast-trending faults, forming rift basins around the uplift. For instance, the Qiongdongnan Basin and Zhongjiannan Basin contain massively thick Eocene and Oligocene strata, which are also the primary source rocks in the region [14,15,16]. The post-rifting period (23.3 Ma to present) has been dominated by thermal subsidence, which has promoted the deepening of the water in this area, leading to the gradual submergence of the Xisha Uplift and Guangle Uplift. During the submergence process, the Xisha Uplift and its periphery have had little terrigenous debris input; as a result, the ceasing of fault activities has created conditions for the development of reefs and carbonate platforms [9,17,18,19].

3. Materials and Methods

This study used high-resolution 2D seismic data of about 20,000 km collected from 2010 to 2015 by the Guangzhou Marine Geological Survey and reprocessed in 2017. The seismic acquisition parameters include shot spacing of 25 m, track spacing of 12.5 m, and a 2 ms sampling rate, while the maximum record length is 9 s. All seismic data were distributed mainly in the Xisha Islands and adjacent areas. The seismic acquisition grid reached 2 × 4 km in the main development areas of the carbonate platform and 4 × 8 km in the neighboring areas. According to the layer velocity analysis of the strata, the vertical resolution of the seismic data reached 30–50 m. The information on nine wells distributed in the development area of the carbonate platforms provides good support for the interpretation of formation age. In addition, well lithology, mineral composition, and fossils offer valuable information for the sequence division and origin of carbonate rocks. All seismic interpretations were completed using the GeoFrame 4.5 software developed by Schlumberger. Eight sequence interfaces were interpreted from bottom to top: T_g (~53.5 Ma), T₈ (35.4 Ma), T₇ (29.3 Ma), T₆ (23.3 Ma), T₅ (15.5 Ma), T₄ (10.5 Ma), T₃ (5.5 Ma), and T₂ (2.6 Ma), as shown in Figure 2. The ages of the seismic surfaces were based on previous research results [13]. The identification of the spatial and temporal distribution of the reefs and carbonate platforms was mainly completed by interpreting seismic data and analyzing drilling data.

4. Results

4.1. Sedimentary Characteristics from Drilling Wells

Previous studies on drilling wells in the Xisha Islands have mostly used the sedimentary facies of a single well and the unique combination of fossil communities to divide the sequence, analyze the sedimentary environment, and discuss diagenesis [6,20]. This paper uses the drilling data from Wells XY-1 and XC-1 on the uplift, combined with the latest data from Wells XK-1 and 120-CS-1X [21] on the Guangle Uplift. A detailed analysis of the sedimentary facies of the two uplifts was carried out, as shown in Figure 4. In addition, a comparison of wells in the deep-water area in Figure 5 shows that different structural units have different depositional environments during the same period. In the Early Miocene, the carbonate rocks of the Sanya Formation were directly deposited on the metamorphic rock basement, and the fossil content was relatively low and contained mainly algae and coral fragments. These carbonate rocks were dolomitized in the later period. Well YL2-1-1 in the deep-water area shows that the Sanya Formation is dominated by sandstone and marl, which are littoral clastic shoal deposits. Wells LS33-1-1 and YL19-1-1 show that the Sanya Formation is dominated by mudstone and marl deposits. Well YC35-1-1 also contains large amounts of marl, sandstone, and sandy limestone. These results indicate that the peripheries of the three wells are sedimentary slope facies mixed with carbonate fragments and mudstone. The above phenomena indicate that carbonate platforms developed during the Early Miocene, but most of them were modified by late deformation and partly dolomitized. The Xisha Uplift and Guangle Uplift continuously received carbonate sediments during the Middle Miocene. Wells XK-1 and XY-1 reveal that algae dominate the reef-building organisms in this section. In the Late Miocene, almost all of the carbonate rocks in the Xisha Island reef area were dolomitized, as shown in Figure 4, which may have been caused by the evaporative environment [22]. During this period, the water bodies on the peripheral slopes of the Xisha Uplift were relatively deep, making them unsuitable for the development of reefs and carbonate platforms, as shown in Figure 5. In the early Late Miocene, the carbonate platform of the Guangle Uplift was exposed and eroded, possibly due to the structural uplift of the regional basement [21]. In the Late Miocene, the rapid rise in the relative sea level resulted in no more carbonate platform development on the Guangle Uplift. From the Late Miocene to the present, the water bodies in the area deepened further, causing the carbonate platform to enter the submerged stage. Only the islands on the Xisha Uplift have exposed carbonate platforms, represented by the Xuande Islands and Yongle Islands [23].

4.2. Morphological Characteristics from Seismic Data

The seismic identification of carbonate platforms is mainly based on their external geometry, internal reflection structure, and contact relationship with the surrounding rock formations [5,24,25,26]. Carbonate rocks are more seismically pronounced than the overlying mudstone or siltstone, and the impedance difference is noticeable. Hence, the top surface of carbonate rock presents a noticeable strong reflection and a good continuity. Most carbonate platforms are directly developed on early sandstone and other rigid basements. The difference in wave impedance is not significant, and the reflection energy of the bottom interface is not vital. Therefore, the seismic response characteristics of the bottom interface are of medium–weak amplitude and medium–low continuity, as shown in Figure 3 and Figure 6. Based on the seismic profiles, the drilling data, the slope type, and the seismic characteristics of the carbonate platform boundary in the adjacent area [27], three identification marks of the platform boundary in the Xisha sea area have been established: abrupt lithological interfaces, fault interfaces, and tidal channels, as shown in Figure 6. Among them, a lithological interface is a sudden change in lithology between the carbonate rock and the mudstone. Its seismic response is characterized by a sudden change from the carbonate area’s strong amplitude to the mudstone area’s medium–weak amplitude, as shown in Figure 6A. This kind of interface is common in the northern Guangle Uplift and the western periphery of the Xisha Uplift. The sedimentary environment gradually transitioned from a shallow-water continental slope to a bathyal continental shelf. Fault interfaces are mostly located at faults, steep cliffs, or platform margin slopes. The existence of platform margin faults caused a large number of carbonate rock fragments to collapse, and the strata overstepped along the fault plane—as shown in Figure 6B—and later developed into bordered platforms. Such platform boundaries are mainly distributed in the southern Guangle Uplift and the southern Xisha Uplift. In addition, eastern tidal channels are mostly located on the gentle slopes of carbonate platforms. The characteristics of the channels are apparent. The beginning of the channel is the outer boundary of the carbonate platform. Seawater enters and exits the platform along the channel, forming a channel at the platform’s edge that is consistent with the descent direction of the gentle slope of the platform. The changes in the platform range and the migration of the channels to land or to the sea form a vertical superposition of multiple channels. The seismic profile shows an abrupt phase transition between the strong continuous reflection of the carbonate platform and the cluttered reflection of the tidal channel, as shown in Figure 6C. The cross-section shows that these tidal channels appear as “V”-shaped undercuts with widths of 1.5~3 km, as shown in Figure 6D. These features are similar to the current tidal channels around the tongue bay of Grand Bahama Beach [28], and they are also channels for transporting debris from the platform shallows to the deep-water areas.

In this work, using a combination of regional drilling data, sedimentary and tectonic environmental data, detailed interpretation of seismic data, and the three identification marks of the platform boundary, the carbonate platform in the Xisha sea area was systematically identified.

Drilling data indicate that the development of carbonate platforms in the Xisha sea area began in the Early Miocene as the Sanya Formation. These isolated platforms mainly developed at the structural high points of ridge-shaped fault blocks but were partially modified by late diagenesis, volcanic activity, and tectonic processes [29]. In the early Middle Miocene (lower Meishan Formation), a stable tectonic environment, a relatively slow-rising relative sea level, and very few terrigenous clastics promoted the full development of the platform [10,19]. The two phases of the platform were identified in the seismic profile. The boundary of the first phase of the platform is on the gentle slope in the northern Guangle Uplift, where tidal channels are the most common, as shown in Figure 3, while faults dominate the southern part [10]. As the relative sea level slowly rose, the second phase of the platform began to migrate to the structural high. On the slopes of the northern Guangle Uplift, two types of platform boundaries are common: tidal channels and lithological interfaces, as shown in Figure 3 and Figure 7. Faults and lithological interfaces are the main platform boundary types around the Xisha Uplift during the second phase. The boundary characteristics of the two stages of the platform are apparent, and Well XY reveals that the biological components in the bottom Miocene limestone changed from mixed bioclastic to mainly corals, reflecting the changes in the sedimentary environment [23] and showing the presence of the Xisha sea area in the Early Miocene. Two different depositional periods occurred. Carbonate fragments were transported to the deep water through the channel and deposited with the simultaneous argillaceous components, forming mixed deposits of mudstone and marl. This interpretation is consistent with the interbedded marl deposits in the early Meishan Formation of Well YL19-1-1.

In the late Middle Miocene (upper Meishan Formation), the carbonate platform began to decline, and the third and fourth phases of the platforms were identified in the seismic profile. As the relative sea level rose, the edge of the third phase of the platform retreated to the junction of the uplift and the slope. The platform boundary in the northern Guangle Uplift is dominated by lithological mutation interfaces, as shown in Figure 3 and Figure 8, while the topographical gap between the southern Guangle Uplift and the eastern Xisha Uplift is large, and the platform boundary is difficult to identify. The western and southern areas of the Xisha Uplift are dominated by abrupt lithological interfaces. The large-scale water channels in the Zhongjian Trough are passages that transported carbonate clastics to the deep-water area, as well as locations where carbonate clastics were deposited, as shown in Figure 8. In the fourth phase, the platform continued to migrate to the high terrain, the edge of the platform retreated to the edge of the uplift, and faults dominated the platform boundary. The terrain above the Guangle Uplift is relatively flat, and the intra-platform depressions above the Xisha Uplift were gradually filled up. The depressions are small in scale, and the channels around the uplift are fully developed. The third and fourth phases of the platforms were recorded in Wells 120-CS-1X and XY-1 and mainly resulted in changes in the types of reef-building organisms. The response of Well XC-1 to this process was that the grain size of carbonate debris changed from coarse to fine. The depositional environment in the deep-water area is relatively stable, inheriting the marl deposition of the previous phase, as shown in Figure 5. The evolution of the platform in the fourth stage of the Miocene is more obvious in the seismic profile and, overall, the boundary of the platform gradually moved from the slope to the uplift, as shown in Figure 3, Figure 9 and Figure 10.

In the Late Miocene (upper Huangliu Formation), the carbonate platform entered the submerged stage, and the fifth phase of the platform is identifiable in the seismic profile at this time. During this period, the platform’s edge retreated to the island’s periphery above the Xisha Uplift, forming a number of isolated platforms of varying sizes. The platform boundary is dominated by abrupt lithological interfaces and is represented by lithological mutations between strongly reflecting carbonate rocks and weakly reflecting mudstone, as shown in Figure 10. A small amount of carbonate debris was deposited on the slope of the platform’s edge. Data from Well 120-CS-1X revealed that in the early Late Miocene, the platform on the Guangle Uplift was exposed and eroded, leading to a period of missing sediment from 10.5 Ma to 7.0 Ma. The subsequent input of terrigenous clastics caused carbonate platforms to stop growing and become submerged in the Guangle Uplift. The dolomitization of carbonate rocks above the Xisha Uplift was the second most important dolomite event in the Xisha sea area [22]. The dolomitization lasted until the end of the Late Miocene. Since the Pliocene, only large-scale atolls have remained on the periphery of the island, such as Yongxing Island and the Xuande Islands.

5. Discussion

5.1. Distribution of the Carbonate Platform

Based on the detailed interpretation of high-resolution seismic profiles and welling data, the spatial and temporal distribution of the Xisha carbonate platform is presented here for the first time. The evolution of the Xisha carbonate platform is also discussed according to the number, scale, and distribution characteristics of the carbonate platform. Since the Middle Miocene, the sixth phase of the platform can be depicted as shown in Figure 11, which can be further divided into three evolutionary stages: the bloom stage, the recession stage, and the submerged stage. The bloom stage consists of the first and second phases of the platform in the early Middle Miocene (lower Meishan Formation), as shown in Figure 11A,B. The recession stage is composed of the third and fourth phases of the platform in the late Middle Miocene (upper Meishan Formation), as shown in Figure 11C,D. The submerged stage is the fifth phase in the late Miocene (upper Huangliu Formation), as shown in Figure 11E. Today, the platform survives in the Xisha sea area, as presented by some isolated platforms in Figure 11F.

5.1.1. The Bloom Stage

In the early Middle Miocene (lower Meishan Formation), two phases of carbonate platforms were identified: the first and second phases of the Meishan Formation, as shown in Figure 11A,B. During this period, the whole area generally received sedimentation with various sedimentary facies belts, and carbonate platforms were widely distributed. In the first phase, the platform flourished over 80,500 km², and the surface area of the reefs was approximately 1010 km². The platform began to retreat in the second phase, and its distribution was approximately 63,500 km². Carbonate platforms were mainly distributed around the Xisha and Guangle Uplifts, forming the Guangle Platform and the Xisha Platform, which were separated by the Zhongjian Trough. The carbonate rock fragments were mainly deposited in these depressions. The reefs were mainly developed on the northern Guangle Platform and the southwestern Xisha Platform. Channels were developed on the slope of the carbonate platform and transported fragments to the basin. The central channel in the Xisha Trough also began to develop and flowed to the Shuangfeng Basin.

5.1.2. The Recession Stage

In the late Middle Miocene (upper Meishan Formation), the carbonate platform entered the recession stage. Two phases were identified in the seismic profile: the third and fourth phases of the Meishan Formation, as shown in Figure 11C,D. As the sea level rose, the platform migrated to structural high area. The platform area of the third phase was approximately 40,300 km², while the scale of the fourth phase of the platform shrank to 25,500 km², reflecting the rapid decline of the carbonate platform. However, the reefs shrank relatively slowly, measuring approximately 800 km² in the third phase and nearly 660 km² in the fourth phase. As the Zhongjian Trough deepened, the Xisha and Guangle Platforms gradually separated and formed two large isolated platforms. The volcanoes in the Zhongjian Trough gradually evolved into isolated small platforms. The reefs, represented by point reefs and platform-edge reefs, were mainly distributed on the edges of the uplifts. With the continuous deposition of carbonate debris on the uplifts, the intra-platform depressions gradually filled up. In addition, the platform slopes and channels in the basin were well developed. The highlands on the east side of Zhongjian Island formed the watershed for the channel in the Zhongjian Trough. One branch flowed south to the Zhongjiannan Basin, while the other branch flowed north into the Huaguang Sag.

5.1.3. The Submerged Stage

In the Late Miocene (upper Huangliu Formation), the carbonate platform began to enter the submerged stage. This phase of the carbonate platform was identified in the seismic profile as the fifth phase of the platform, as shown in Figure 11E. With the rapid sea level rise, the platform quickly shrank to the island’s periphery of the Xisha Uplift. The total area of the platform was almost 5450 km² during this period, represented by Dongdao and Yongxing Islands. The channel inherited its original properties and developed around the carbonate platform. Large-scale atolls developed on the edge of the platform, and lagoons developed inside the atolls. The area of the biological reefs was almost 350 km². In addition, large-scale block transport systems developed in the northern Guangle Uplift and the western Xisha Uplift [30].

5.1.4. Present

The current distribution of carbonate platforms was mainly derived from previous studies [6,10,17]. Hemipelagic sedimentary environments dominate the whole area, and the carbonate platform is limited to the Xisha Islands, such as Yongxing Island and the Xuande Islands, as shown in Figure 11F. The total area of the platform is less than 870 km², and the area of the reef is approximately 110 km². Large atolls dominate these reefs, and lagoons are developing on the tops of the large atolls. Many types of lagoons are present. The Yuzhuo Reef lagoon is fully enclosed, the Huaguang Reef lagoon has three passages to the open sea, and the Xuande Reef lagoon is crescent-shaped and fully open.

5.2. Significance of Hydrocarbon Exploration

5.2.1. Physical Properties of the Reservoir Rock

Carbonate rocks are considered to be high-quality reservoirs for oil and gas exploration [1]. In the study area, the reefs are mainly distributed in the western and southwestern slopes of the Xisha uplift, with a spatial distribution over 1000 km². The point reefs and platform-edge reefs flourished on east of the Huaguang Sag during the Middle Miocene and were regularly arranged in space to form a spectacular group of reefs, as shown in Figure 11A–C. The single-point reef is about 1 km², and its thickness is over 50 m, providing an ideal space for oil and gas accumulation, as shown in Figure 12. In addition, platform-edge reefs are also well developed north of the Zhongjiannan Basin. Most of those reefs show predominantly lateral growth, with a single reef over tens of square kilometers that primarily developed on the hanging wall of the fault.

During the diagenetic process, these reefs experienced multiple cycles of growth–exposure–regrowth–submergence–death with the sea level fluctuation, further increasing the carbonate rocks’ secondary porosity. Several drilling wells on Xisha Island provide valuable data on the reservoir properties. The porosity test results of Well XK-1 show that the average porosity of the Meishan Formation and Huangliu Formation carbonate rocks exceeds 25%, and the average permeability exceeds 500 × 10⁻³ μm² [31], which is beneficial for hydrocarbon reservoirs.

5.2.2. Regional Source Rock

High-quality source rocks are basic and necessary conditions for the formation of large hydrocarbon deposits. Recent studies have shown that three main sets of source rocks developed in the South China Sea and Southeast Asia, i.e., the Eocene lacustrine source rocks during the Early Paleogene rift stage, the Oligocene lacustrine and marine–continental transitional source rocks during the Middle–Late Paleogene rift stage, and the Lower Miocene marine source rocks during the Neogene depression stage [11,32]. The source rocks in the Qiongdongnan and Zhongjiannan Basins of the Xisha sea area also have similar characteristics. The Oligocene and Miocene source rocks developed in the Qiongdongnan Basin. The Oligocene Yacheng and Lingshui Formations’ mudstone constitutes the main source rock, with high thickness and wide distribution [33]. The organic matter abundance of these source rocks is high, with an average organic carbon content of 1.05%. In particular, the measured organic carbon abundance of coal source rocks is much higher, with an average of 48.4%. The kerogen types of the source rocks are II₂-III, which mainly generate gas. The Miocene source rocks were rapidly deposited in the neritic–bathyal sedimentary environment, with thick argillaceous rocks and an average organic matter abundance of 0.71%, and mainly humic organic matter [14,16,33,34].

According to the analyses of paleogeography and sedimentary facies, Eocene–Oligocene lagoons, shallow lakes, moderately deep and deep lakes’ facies mudstone, and the Lower–Middle Miocene neritic and bathyal facies mudstone developed in the Zhongjiannan Basin [35]. During the Eocene–Oligocene, most of the basin was in a closed, shallow lake or a deep-water environment in the prodelta zone. These environments primarily resulted in mudstone deposits and local swamp mudstones. Terrestrial sources were abundant and suitable for forming kerogen types II or II–III. The bottom of the lake was mostly in an anoxic reducing environment, conducive to the preservation of organic matter. Therefore, thick lacustrine deposits in the graben and half-graben of the basin were the primary source rocks during the Eocene–Oligocene rifting period [36,37]. This set of source rocks can be compared with the coastal plain swamp coal seams, carbonaceous mudstones, and mudstones that developed during the Yacheng and Lingshui Formations of the Qiongdongnan Basin. Related data suggest that the average organic carbon contents are 0.47% to 1.16% in argillaceous rocks and 13.26% to 21.43% in the coal and carbonaceous mudstone. The chloroform bitumen “A” contents are 1.78% to 4.16%, with medium-to-good quality. The hydrocarbon generation threshold values of the source rocks are 1900–3100 m, having now essentially entered the gas-generation stage and even the mature stage of oil generation in most of the western region [38]. Another set of source rocks of the Lower–Middle Miocene developed during the continuous transgression stage. During this period, a set of neritic and neritic–bathyal facies mudstones were deposited as the secondary source rocks of the basin, which developed organic matter of kerogen types I–II.

5.2.3. Migration System and Capping Layer

During the Paleogene, northeast-trending basement faults developed in the Xisha sea area when the middle Xisha block rifted from the South China block, and most of the faults were active until the end of the Oligocene (23.3 Ma). These faults not only controlled the distribution of tectonic units but also linked the Eocene–Oligocene and Miocene sequences as migration conduits, as shown in Figure 12. Moreover, T6 (23.3 Ma) was an essential tectonic unconformity interface that divided the tectonic evolution into a rifting stage and a post-rifting stage in this area [13], resulting in a sound hydrocarbon migration system.

The capping layers on the western slope of the Xisha sea area are extremely well developed, and the Yinggehai and Huangliu Formations are widely distributed and relatively stable as regional capping layers; they consist of Meishan Formation carbonate rocks, calcareous mudstones, and fine siltstones, as shown in Figure 12. According to the seismic profile, the displacement pressures of mudstone are 7~10 MPa, and the thicknesses of a single layer are 2.5~10 m. The inside of the Meishan Formation is a closed abnormal overpressure system (49–66 MPa), which has the double-sealing ability of displacement and overpressure. In addition, the regional capping layers of the Yinggehai and Huangliu Formations have cumulative thicknesses of 300 to 1400 m of mudstone, constituting a very effective closed system.

5.2.4. Hydrocarbon Accumulation Potential

Through the analysis of drilling data from nine wells in the Xisha sea area and the detailed interpretation of nearly 2000 km of high-precision seismic data, the results show that the Miocene reefs and carbonate platforms in the Xisha waters are widely distributed and large in scale. During the process of development, as the relative sea level declined, the area experienced multiple periods of exposure and infiltration. The secondary porosity of the carbonate rocks further increased, providing a prerequisite for the formation of high-quality reservoirs. For the accumulation system, the source rock is crucial. Currently, the study area mainly consists of two sets of source rocks: Eocene lacustrine mudstone, and Oligocene half-enclosed marine mudstone, which are mainly distributed in the Huaguang Sag of the Qiongdongnan Basin and the northern Zhongjiannan Basin [16,33]. Since the Late Miocene, the region has experienced a rapid heat sink. During this period, large amounts of hydrocarbon generation and expulsion began [33,34,39]. The generated oil and gas began to migrate vertically and laterally on a large scale by communicating with the sequence’s Paleogene faults, unconformities, and internal transport layers. The oil and gas accumulated in traps at the edges of the Miocene carbonate platform. The reef is the primary trap type at the platform’s edge, and it developed in multiple phases as the platform’s edge gradually migrated to the higher sections of the structure, as shown in Figure 13. The mudstones of the overlying Huangliu Formation and Yinggehai–Ledong Formation are 500~1500 m thick. The very thick mudstone formation provides good cap-rock conditions for the preservation of oil and gas reservoirs and generates large displacement pressure, resulting in the formation of an abnormal overpressure system within the Miocene strata, which is conducive to the migration of oil and gas. In summary, the organic reefs in the Xisha waters close to the edge of the depression platform have formed a typical accumulation model characterized by “lower generation, and upper accumulation”.

5.2.5. Favorable Exploration Zones

In the Xisha area, the distribution of effective source rocks, migration channels, and the spatial distribution of biological reefs and carbonate reservoirs are the three most important factors controlling the formation of oil and gas reservoirs in reef facies. The reefs of the Huangliu and Yinggehai–Ledong Formations are mainly distributed in the islands and reefs above the Xisha Uplift. This area is far from the source rock and has not yet been sealed, making it impossible to form oil and gas reservoirs. The drilling data of Well XY-1, Well XC-1, and Well XK-1 provide the most powerful reliable evidence. The biological reefs developed in the Huaguang Sag of the Qiongdongnan Basin and the platform margins of the northern Zhongjiannan Basin are abundant and of different types and scales. The area of a single-point reef is within 1~2 km², and the thickness is almost 50 m. The area of the reef edge of the platform can reach 150 km², and the thickness is more than 200 m. These reefs are mainly developed on the hanging wall of the fault, and the thicker (lower) Oligocene Yacheng Formation in the lower part is the main interval for the development of source rocks. In addition, during the separation of the Xisha block from the South China block, a large number of northeast-oriented faults developed in this area during the Oligocene [40]. These faults connect the Paleogene and Neogene and are migration conduits for hydrocarbon migration [41]. These geological conditions of oil and gas accumulation in this area resulted in the spatial configuration of source rocks, migration of hydrocarbons, and formation of biological reefs and carbonate reservoirs. The abovementioned results indicate that the Huaguang Sag in the Qiongdongnan Basin and the northern Zhongjiannan Basin are two favorable exploration zones for the biological reefs and carbonate platforms in the Xisha sea area.

6. Conclusions

(1)

Three identification marks of the boundaries of the carbonate platforms in the Xisha waters have been established: lithological mutation interfaces, fault interfaces, and tidal channels. Among them, the lithological mutation interfaces constitute the lithological mutations of the strongly reflecting carbonate rock and the weakly reflecting mudstone, the fault interfaces are mostly the interfaces between faults and steep cliffs, and the tidal channels are on the gentle slopes of the platform and facilitate exchange between the platform and the sea. For the passage, the cross-section of the seismic section is shown as a “V”-shaped undercut.

(2)

The Miocene carbonate platform in the Xisha sea area is divided into six phases and three evolution stages. In the early Middle Miocene, the carbonate platform was fully developed and large in scale. The biological reefs were mainly distributed on the steep slopes from the western to the southwestern edge of the platform. In the Late Miocene, the carbonate platform began to decline and mainly developed in the Xisha Uplift. After the Late Miocene, the platform entered the submerged stage. The platform was located on the periphery of the islands and reefs above the Xisha Uplift, represented by the Dongdao and Yongxing Islands. Since the Pliocene, the platform’s scale has further decreased, and atolls that grow vertically are the main platforms, which remain on the periphery of the islands and reefs.

(3)

Based on the analysis of the development characteristics of carbonate reservoirs and oil and gas accumulation factors in the study area, it is proposed that the Miocene carbonate platform in the Xisha sea area has a typical accumulation model of “lower generation, upper reservoir and upper cap”. The hydrocarbon source rock, migration path, and other oil- and gas-related geological conditions indicate that the Huaguang Sag in the Qiongdongnan Basin and the northern Zhongjiannan Basin are two favorable exploration zones for carbonate rocks.