South China Sea is the largest marginal sea in western Pacific, located at the junction of Eurasia, Indo-Australian and Pacific Plates. Its northern edge is the southern margin of South China Plate, where the landform is divided into shelf, slope and ocean basin. In Cenozoic, South China Sea underwent several tectonic events, including continental break-up, oceanic crust spreading, subduction of western Philippine Sea Plate and Manila Trench and uplift of Taiwan Island, as a result, a series of Cenozoic sedimentary basins with tremendous amount of hydrocarbon resources developed^{[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24]}.

The Baiyun Sag is located in the southern Pearl River Mouth Basin and the major part of it is in the deepwater area (water depth greater than 300 m), hence it is also called the Baiyun Deepwater Area. It is a massive Cenozoic sedimentary depression trending NE with an area of over 20 000 km². It is adjacent to Panyu Uplift on the north, Yunli Uplift on the south, Shenhu Uplift and western Zhu II Depression on the west, and Dongsha Uplift on the east. Due to differential subsidence and sedimentation, the Baiyun Sag is further subdivided into several smaller hydrocarbon generation sub-sags with different tectonic evolution and depositional sequences, namely, Main, East, West Sag and South subsags (Figs. 1 and 2).

The geology of the Baiyun Sag is complicated and special. Located at the weak spot of subduction zone in Mesozoic, thin ocean-continent transition crust and transform zone in Cenozoic, the basin has experienced unique tectonic evolution. With intense crustal thinning, the basin has high and variable geothermal gradient, which increases from 3.5 °C/100 m to 5.0 °C/100 m from north to south. The sag went through structural evolution from “rift to fault depression and to depression”. The strata from bottom to top in the sag: Eocene Wenchang Formation is non-marine rifting lacustrine sediment; Upper Eocene Enping Formation is massive lacustrine sediment during fault-depression period; Mid-upper Oligocene Zhuhai Formation is transitional deltaic deposits, and shelf edge delta to slope deepwater deposits in Zhujiang, Hanjiang, Yuehai and Wanshan formations since mid-Miocene. These strata have formed three sets of excellent reservoir-seal assemblages. It has been subsiding continuously due to the spreading of South China Sea since Oligocene, so it had a staircase type transgressional seal level changes opposite to the global change of sea level, received a depositional fill sequence with normal grain size order. The Baiyun Deepwater Area was in shelf shallow sea environment with the deposition of transitional deltaic sediments in Oligocene. Due to a major geological event at the end of Oligocene (23.8 Ma ago), the basin turned into deepwater slope environment, when large deepwater fan system developed. This geologic setting controlled and influenced the tectonic evolution, sedimentation, source rock development and hydrocarbon accumulation in the Baiyun Deepwater Area^[24].

The Baiyun Sag has been proved a hydrocarbon prolific basin by years of exploration. Discovered in 2006, Liwan 3-1 gas field, the first deepwater giant gas field China, is the milestone in the history of China petroleum exploration, opening the era of deep sea exploration in China, and further grew into the first giant gas field integrated project. By the end of 2017, the sag has discovered gas reserves in place of nearly 3 000× 10⁸ m^{3[25,26,27,28,29,30,31,32,33,34,35]}. From 2010 to 2015, oil exploration has made a breakthrough in the Baiyun Sag, with over 1×10⁸ m³ of oil reserves found^[35] , breaking the traditional understanding of poor oil potential due to high heat flow and large burial depth^[31,33,36]. Currently, the distribution of hydrocarbon in the Baiyun Sag has the following features: “rich in the northeast, poor in the southwest, concentrated vertically, more gas than oil”; moreover, most of the oil and gas (90%) is accumulated in lower Zhujiang Formation, while deeper and shallower strata have few discoveries. All of these indicate the Baiyun Sag has generation and accumulation conditions different from other deepwater oil fields in the world. Many researches have been done on the particularity of the area^{[24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54]}. Aiming at the particularity, for the first time, this work systematically sorted out the hydrocarbon distribution pattern and summarized the accumulation features of oil and gas in the sag by utilizing large quantity of 3D seismic, boreholes, geochemistry and other petroleum geology data. Through analyzing the variation of hydrocarbon generation and accumulation in the Baiyun Sag, and discussion of the key factors controlling hydrocarbon accumulation, a better understanding of the petroleum geology in northern South China Sea is established to provide support for the exploration in deepwater area of Pearl River Mouth Basin and other similar basins.

Hydrocarbon fluid phases in the Baiyun Sag include natural gas, light crude, volatile oil and condensate oil (Fig. 2).

Natural gas in the Baiyun Sag is largely mature to overma-ture (with R_o of 1.3%-1.7%) wet gas (with dry coefficient of 0.82-0.94). The natural gas in different parts of the sag differs somewhat in nature: it is a mixture of oil-type and coal-type in the northern and eastern Main Sag, while oil-type gas generated from lacustrine sapropelic source rock in the southern Main Sag.

Crude oil produced from the sag includes light crude, volatile oil and condensate oil.

The light crude has a density of 0.76-0.81 g/cm^3, oil-to-gas ratio of less than 300 cm³/cm³, paraffin content of less than 10% and R_o of approximately 0.9%. It is generated from Wenchang and Enping shallow and semi-deep lacustrine source rocks at the oil window in different sub-sags of Baiyun Sag.

The volatile oil features low density (<0.81 g/cm³, below 15 °C) and low paraffin content (1.6%-5.1%). Inside formation, the volatile oil has a density of only 0.50-0.65 g/cm³ and very high gas-to-oil ratio of up to 1 379 m³/m³. It has close dew point, bubble point and formation pressure in PVT samples. Rich in C₂-C₆ with R_o 0.9% to 1.2%, it is generated by Enping and Wenchang shallow lacustrine and deltaic source rocks.

In addition, there is a certain amount of condensate oil associated with natural gas. It is higher in maturity, with R_o between 1.3% and 1.7%. Its biomarkers have terrestrial higher plant features, suggesting the condensate oil was generated by the cracking of overmature kerogen at the wet gas stage in shallow lacustrine-swamp facies shale of slope zone. But in the southern Baiyun Sag, the condensate oil was generated by lacustrine sapropelic source rock.

Previous studies show that hydrocarbon accumulation in the Baiyun Sag has following general characteristics^{[25,39,43,46,48,53,54]}: multiple sources, late accumulation and effective migration. But there are some differences in different parts of the basin, which will be elaborated later in the article.

2.2.1. Multiple sources

There are multiple sets of source rocks and multiple kitchens in the Baiyun Sag. Wenchang and Enping Formations are the major source rocks. Four first-grade source kitchens have been identified, and hydrocarbon has been discovered in all of them except the South Subsag. Discovered hydrocarbon is generated by deltaic-swamp-lacustrine source rocks of Wenchang and primarily Enping Formations^{[25,43,48,54]}.

2.2.2. Late accumulation

According to fluid inclusion analysis, the hydrocarbon charging in this area can be divided into 2 stages, and the accumulation features “early oil late gas generation, simultaneous charging of oil and gas, and late accumulation”^[39,46,54]:

In the first stage of charging 13.1-7.3 Ma ago, the hydrocarbon was primarily mature oil with yellow fluorescence. In the second stage about 5.5-0 Ma ago, the hydrocarbon was largely over-mature oil and gas with blue fluorescence.

2.2.3. Effective migration

A complex migration system in the Baiyun Sag is composed of faults on structural ridges, sand bodies and unconformities^[43,46,53].

Due to the long distance between primary Zhujiang reservoirs and lower source rocks of Wenchang-Enping, late faults are the effective migration paths connecting the source rock and reservoir. After expelled from source rocks, the oil and gas migrated in vertical direction along the faults rapidly, with high transport efficiency Marine deltaic-shoreline deposition system developed during the depositional period of Zhujiang-Zhuhai Formation in the Baiyun Sag, when extensive and continuous high quality sandstone deposited laterally, providing long distance passages for hydrocarbon migration. Moreover, high quality deepwater sands such as shelf edge delta, turbidite channels and basin floor fans developed above sequence boundaries in the Zhujiang Formation and Zhuhai Formation in the middle and southern part of the sag, which together with faults allowed hydrocarbon migration.

The complex migration system not only made the oil and gas migration wide in range and long in distance, but also enabled vertical migration of oil and gas from the Paleogene to Neogene strata. Multiple sources and complex migration system are two major factors causing the mixed sources of oil and gas in the Baiyun Sag.

The distribution of hydrocarbon in Baiyun area has following features: rich in northeast, poor in southwest, vertically concentrated, more gas than oil, and differential accumulation.

(1) Hydrocarbon shows are abundant in the wells of Baiyun Sag and surrounding area. This indicates extensive charging of hydrocarbon. So far 35 oil and gas pools have been discovered, including 25 gas pools, 6 oil pools and 4 oil-gas pools. Except for the center and southwest part of Baiyun Sag, other areas all have scale discoveries, but the oil and gas discoveries are mainly concentrated in northern and northeastern part of Main Sag and East Sag. The overall oil and gas distribution features “rich in northeast, poor in southwest, and more gas than oil”.

(2) Vertically, hydrocarbon is concentrated in distribution. Only a few pools discovered are in Yuehai, Hanjiang, Zhuhai and Enping Formations, the rest 90% of pools discovered are in the sandstone traps of lower Zhujiang Formation.

(3) Fluorescence is abundant in the wells near Baiyun Sag. The gas pools usually contain some condensate oil, and oil layers were found in many wells drilled in the gas fields of eastern Main Sag. All of these indicate coexistence of gas and oil in Baiyun Sag.

(4) On the plane, oil pools are mostly located in the sub- sags and surrounding uplift area, such as East Sag, West Sag and eastern Main Sag. Gas pools are distributed largely in the center of Main Sag, East Sag and West Sag. The Main Sag and sub-sags all have an “inner gas, outer oil” distribution feature.

(5) The distribution of oil and gas shows in areas around Baiyun Sag shows the gas shows and scale accumulations are concentrated in lower Zhujiang Formation, while oil shows and layers increase towards the deeper Zhuhai and Enping Formations, taking on the feature of “upper gas, lower oil”, and this feature is most evident in the eastern Main Sag.

Different parts of Baiyun Sag have differences in hydrocarbon accumulation mechanisms and patterns, which are rooted in the diverse source rock types, complex migration system types, trap types and their configurations caused by distinct tectonic evolution and depositional sequences.

The major source rocks in the Baiyun Sag are the Wenchang and Enping formations. The Wenchang Formation is semi-deep lacustrine source rock deposited in a reducing environment with moderate total organic carbon (TOC), high hydrocarbon generation potential, high hydrogen index, and sapropelic organic matter. It is currently at the mature-overmature stage. The Enping Formation is shallow lacustrine and paludal source rock with variable TOC, high hydrocarbon generation potential, low hydrogen index, mixed more humic organic matter with more terrestrial organic matter. Depositing in widely changing environment, the source rock has abundant bicadinane and oleanane and is currently high in maturity^[33,55].

Based on the composition and carbon isotopes features, the natural gas in the Baiyun Sag is deemed to be mixing origins. The parent material is the mixture of sapropelic and humic organic matter with high proportion of terrestrial organic matter. The oil discovered in the sag is mainly light crude and condensate oil with minor volatile oil. Geochemical analysis suggests parent material of the oil is sapropelic type with high proportion of terrestrial plant.

Comprehensive analysis shows that the parent material of oil and gas in the Baiyun Sag is mixed sapropelic and humic type with high proportion of terrestrial organic matter. According to the source rock features in the region, it is concluded that Enping Formation is the major contributor of oil and gas in the Baiyun Sag, but Wenchang Formation made some contribution too.

In the Baiyun Sag with a huge area of over 20 000 km², the source rock has a maximum thickness of 7 000 m and maximum burial depth of 10 000 m. The massive source rock has tremendous hydrocarbon generation potential, and basin modeling indicates a total hydrocarbon generation volume of 2 000×10⁸ t oil equivalent, including oil and gas. Among all the sub-sags, the Main Sag has the largest hydrocarbon generation potential, and others also have considerable hydrocarbon generation potential, so all of them have the material base to form large oil fields (Table 1).

Due to the differences in depositional history, the scale, or-ganic matter abundance and type, thermal and hydrocarbon generation evolution of source rock in different sub-sags or even in different structure positions of the same sub-sag differ greatly. This is the root why oil and gas distribution and enrichment degree in different sub-sags or in different structural positions of the same sub-sag differ so widely.

3.2.1. The “inner gas, outer oil” pattern controlled by spatial differentiation of hydrocarbon generation

(1) Source rock evolution in different sub-sags

Comparing the thickness, basement depth and evolution of source rocks in the four sub-sags shows the source rocks in them have different evolution situation. Among them, the Main Sag is the largest and most mature hydrocarbon generation sub-sag with the most extensive, thickest and deepest source rock formations. The source rock there has reached condensate gas window since 16 Ma ago. The potential of oil and gas are similar but most of the oil was generated in the early period (prior to 16 Ma), so mainly gas has been discovered in this sub-sag due to late accumulation. In the other sub-sags, the source rocks are much smaller in area, shallower in burial depth and lower in thermal evolution degree. Entering mature, some overmature stage since 16 Ma, the source rocks in them produce oil primarily, so the discoveries in them are mainly oil. Therefore, the overall hydrocarbon distribution in the sag features “inner gas, outer oil” (Fig. 3 and Table 1).

To better characterize the contribution of primary exploration target, Zhujiang Formation, in different sub-sags, we proposed a term called “active source rock”. Under the crust thinning and detachment background, the geothermal gradient in this area increased dramatically since 23 Ma, making the thermal evolution of source rock accelerate. Hence the source rocks became mature much earlier: the Wenchang Formation reached hydrocarbon generation threshold 49 Ma ago and was in major hydrocarbon generation window from 33.9 Ma to 16.0 Ma; the Enping Formation entered hydrocarbon generation threshold 23 Ma ago and was in major hydrocarbon generation window from 23 Ma to 10 Ma ago. The best quality reservoir in the Baiyun Sag is the early-mid Miocene sandstone deposited 23-16 Ma ago, so the effective reservoir-seal was formed about 16 Ma ago to present. Therefore, considering the timing of reservoir-seal formation and source rock in high thermal setting, the source rock generating and expelling hydrocarbon between 16 Ma to the present is termed “active source rock”. This source rock has the most contribution to the hydrocarbon discovered in this area.

The resource distribution of active source rock indicates that the Main Sag has the most resources, and more gas resources than oil; hence the discoveries so far are mainly gas. But in the East and South Sag, the timing of hydrocarbon generation is rather late and the oil resource is larger than gas, with oil-gas ratio at 5.00 and 3.91, respectively, so the sag has an oil and gas distribution feature of “inner gas, outer oil” (Table 2).

(2) Differential hydrocarbon generation at different structure positions of the same sub-sag

Due to the differential subsidence and deposition at the center and surrounding slope area of a basin, the hydrocarbon generation history and thermal evolution of source rocks at the center and surrounding area are different. The larger the area the sag, the bigger the differences will be.

Take the Baiyun Sag as an example. In early Oligocene (33.9 Ma ago), the source rock in the center of the Main Sag is buried deeper and higher in maturity, first started hydrocarbon generation. The majority of this source rock reached overmature stage in mid-late Miocene, and generated gas primarily. In the surrounding slope area, the source rock, lower in thermal evolution degree, generated very small amount of oil and gas in the early stage, more and more hydrocarbon later, and entered major oil window in mid-Miocene. Therefore, the source rock since mid-Miocene is mainly located in the slope belt, which generated a large amount of oil in late period. Hence, multiple late charging light crude and condensate oil pools have been discovered in the eastern part of Main Sag, and the oil and gas distribution pattern of “inner gas, outer oil” came about.

3.2.2. “Upper gas, lower oil” pattern controlled by timing differential of hydrocarbon generation

Timing differential of hydrocarbon generation is the contribution of source rock in different periods. The source rocks in the Baiyun Sag started hydrocarbon generation early and continuously, and generated oil first and gas later, leading to differential accumulation features in different periods and affecting current oil and gas distribution.

In the early peak stage of oil generation from 33.9 Ma to 23.0 Ma ago, the source rocks mainly generated oil, and the oil generated was preserved in the Enping and Wenchang formations, forming early oil plays. This is why more oil layers discovered in the deeper strata (Fig. 4).

During late peak stage from 23 Ma ago to present, evident subsidence of Baiyun Sag (Baiyun Movement) and the rise of sea level resulted in the retreat of shelf edge sediments. The slope break shifted from southern (Zhuhai period) to northern part (Zhujiang period) of Baiyun Sag, forming a tectonic framework similar to the present time. Under this influence,the majority of early oil pools had been adjusted or destroyed. Only those on the long term uplifting zone less affected by the Baiyun Movement are preserved. In the mean time, during this period, the source rock mainly generated gas, which migrated vertically to upper Zhuhai and lower Zhujiang along diapirs and late intensely active faults. Regional flooding shale around T50 dividing lower and upper members of Zhujiang Formation is the top seal for lateral migration of gas, resulting in massive gas accumulation in shallow Zhujiang Formation. The deeper oil pools are less affected and forming “upper gas, lower oil” distribution.

This kind of hydrocarbon distribution pattern can only occur in long-term uplifting zones and surrounding areas with traps formed in early stage, excellent reservoir-seal assemblage and weak tectonic reformation later. Eastern Main Sag meets all these criteria. This area is a long term inherited uplift, with little late subsidence, the seal of Zhuhai and Enping formations was good and complete traps developed in the early stage, and gas largely gathered into pools in Zhujiang Formation in the late stage, leaving the deeper oil pools intact, hence the “upper gas, lower oil” pattern is most evident in the eastern Main Sag (Fig. 4).

Differential accumulation is a common rule for hydrocarbon migration and accumulation. This theory suggests that under hydrostatic condition, if there is a series of traps with ascending spill points along the primary migration path, when the source and top seal are competent, hydrocarbon can enter the lowest trap and due to density differences between oil, gas and water, water would be at the bottom, oil in the middle and gas at the top of the trap. When the first trap is filled and more gas is still coming, the original oil filled in this trap would be displaced out and migrated to higher traps through the spill point. If the oil and gas source is sufficient, the above process continues at the higher traps. If the source is not sufficient, hydrocarbon cannot reach the further traps and the formation water would be preserved there^[56]. Due to differential accumulation, the traps close to source are usually filled with late gas, while the further traps are filled with oil, hence comes the “inner gas, outer oil” pattern on the planar view^[57]. However, the conditions are strict for the formation of this accumulation pattern: there must be inter-connected traps with ascending spill points, abundant hydrocarbon supply, no precipitation of dissolved gas, and conditions for regional long distance migration. It also requires good permeability and connectivity in the reservoirs^[55,57].

In Baiyun East Sag, a single source kitchen provided sufficient oil and gas, and the oil and gas came from the downdip direction of the reservoirs. The reservoirs were filled with formation water and under hydrostatic condition. They have homogeneous lithology with good permeability. A series of traps with similar reservoirs and ascending spill points developed along the migration pathways. All of these meet the conditions required for forming “inner gas outer oil, differential accumulation” pattern. Therefore, following the laws of buoyancy and gravitational differentiation, gas accumulated in the trap with the lowest spill point and closest to source; oil-gas pools or oil pools accumulated in further traps with higher spill points; and the furthest traps may contain water. Under the action of this model, gas pools (G7 and G8) are discovered inside and near the center of Baiyun East Sag and oil pools (O1-O5) are discovered on surrounding slope zones. No hydrocarbon is encountered in the further traps (Fig. 5).

3.4.1. Late fault activity and diapirs controlled the trap formation and efficient migration of hydrocarbon

In the late tectonic stage of Pearl River Mouth Basin (mid-late Miocene, 10-5 Ma ago), Dongsha Movement resulted in block shifting, denudation of uplifts, fluid activities, and the reactivation of faults formed in rifting and Nanhai periods. A new series of en echelon faults more active in eastern part developed, which have had crucial impacts on the trap formation, migration, accumulation, spill and redistribution of oil and gas (Fig. 6).

(1) Late fault activity controlled the trap formation and distribution in Baiyun Sag

Influenced by Neotectonics, a series of NWW trending transtensional faults developed in Baiyun Sag, hence a large number of structural traps controlled by the faults came about, or the faults enlarged the storage capacity of existing structural traps. More late faults developed in the northeastern part than the southwestern part. Large quantity of half anticline traps formed on the hanging walls of these faults. Most of these traps are antithetic roof ridge shaped, with massive overlying marine shale acting as effective lateral fault seals, these traps can provide reservoir space for hydrocarbon accumulation in lower Zhujiang Formation^[53].

(2) Late fault activity (diapirs) controlled the efficient vertical migration of hydrocarbon

In a hydrocarbon bearing basin, hydrocarbon is often generated in a relative stable environment. But to make oil and gas migrate, the dynamic equilibrium must be broken to create a migration system. This system usually consists of unconformities, faults, sand bodies and diapirs. They destabilize the balance in temperature, pressure, geostatic stress and fluid, resulting in the reactivation and redistribution of hydrocarbon and other fluids in the basin, promoting the accumulation of hydrocarbon. Among all these factors, fault is the most important and active. With different activity styles and configurations in different geologic periods, faults connect source rocks, sand bodies and unconformities, forming multiple types of migration systems. Eventually, the latest fault movement controlled the final distribution of hydrocarbon^[53].

Influenced by late Dongsha Movement, faults developed in the northeastern part of Baiyun Sag. Meanwhile in the basin center, there was undercompaction due to rapid deposition and over-pressure caused by hydrocarbon generation, which caused the formation of a series of diapirs. Some of the diapirs and late faults connected the source, forming the major vertical pathways for hydrocarbon migration. The hydrocarbon then migrated along the sand bodies and unconformities laterally, and finally accumulated in Neogene structures.

The late faults and diapirs not only controlled the formations of structural traps, but also acted as the vertical passages for hydrocarbon migration. Coupled with lateral migration system, they ultimately controlled the hydrocarbon accumulation in Neogene. The faults are more active in the northeast than southwest; hence the Neogene hydrocarbon accumulation is “rich in the northeast, and poor in the southwest”.

3.4.2. Inherited structural ridges controlled the oil and gas enrichment in Neogene

A structural ridge is the hinge of a normal structure, such as the core of an anticline and the hinge of a nose structure^[46]. During the secondary migration, hydrocarbon would occupy the high points of the migration paths - structural ridge, under buoyancy force. So a structural ridge is the direction for hydrocarbon migration. Zou et al. pointed out that the traps on the structural ridge had higher rate of exploration success^[58]. Hao et al. proposed that migration path was controlled by the style of structural ridge even in heterogeneous strata and the distribution of ridges was the controlling factor for hydrocarbon secondary migration^[59].

The Baiyun Sag has its unique reservoir-seal assemblage: high quality marine sandstone layers in extensive distribution in Zhuhai and lower Zhujiang are the primary reservoirs; overlying top seal is the flooding shale several hundred to thousand meters thick. Therefore, the hydrocarbon mainly migrates laterally along structural ridges after reaching Neogene strata, making the structural ridges in lower Zhujiang sandstone the primary factor controlling oil and gas accumulation. Unsurprisingly, many oil and gas pools discovered in the Baiyun Sag are distributed along the structural ridge in the lower member of Zhujiang Formation, and 90% of discoveries are in this section (Fig. 7).

Faults and ridges are important to the hydrocarbon accumulation in Neogene strata of Baiyun Sag. Controlled by these factors jointly, Neogene oil and gas pools are abundant. The discoveries are primarily on the nose structure ridges surrounding the sub-sags with matching late fault activity (Fig. 7). The joint control of faults and ridges also suggests that under buoyancy force, hydrocarbon migrated vertically from Paleogene source rocks to Neogene reservoir through faults, then laterally to higher structures through ridges. Faults are very effective vertical pathways and ridges controlled the accumulation.

Three factors controlled the hydrocarbon accumulation and distribution in the Baiyun Sag: (1) As a broad and deep rifting basin, the massive source rock is the material basis for forming large oil and gas fields and the sub-sags all have good potential. (2) Influenced by the differential evolution of source rock in space and time, mainly gas is generated in the center of Baiyun Sag and oil is generated in surrounding area; and the generation process features “early oil and late gas”. All of these resulted in the orderly distribution of “inner gas-outer oil, upper gas-lower oil”. Furthermore, the differential accumulation of hydrocarbon controlled “inner gas, outer oil” distribution in Baiyun East Sag. (3) Late fault activity, diapirs and inherited structural ridges controlled the migration in Neogene. Under the joint control of faults and ridges, the majority of discovered pools are located on the nose structure ridges surrounding the sub-sags with late active faults.