Basement buried hill reservoirs are one of the new growth poles for oil and gas reserve discovery in deep water areas in the northern South China Sea of China. The common features of buried hill reservoir formation under different basin types， different tectonic backgrounds， different basement lithology， and hydrocarbon geological conditions are analyzed through the exploration of typical buried hill reservoirs around the world to address the problem of unclear key elements of its formation. On this basis， a three-element coupling model of source-reservoir-cap hydrocarbon accumulation in buried hill is proposed. The formation of buried hill reservoirs requires a sufficient supply of oil and gas sources， effective reservoir groups， and good cap conditions. The effective configuration of the source-reservoir-cap relationship directly determines whether oil and gas can be efficiently filled and effectively preserved in reservoirs. There are four types of source-reservoir-cap configuration relationships in basement buried hills including source-reservoir overlay type for deep-buried hills， source-reservoir lateral docking type for middle-buried hills， source-reservoir proximity transmission type for shallow-buried hills， and source-reservoir remote transmission type for shallow-buried hill. The first three types of reservoir formation conditions are better. Based on the three-element coupling model of source-reservoir-cap， the Yunkai basement buried hill in the Pearl River Mouth Basin and the Songnan basement buried hill in the Qiongdongnan Basin are two important exploration directions for oil and gas in the deep water area of the northern South China Sea.
The main part of Songnan-Baodao Sag is located in eastern part of deep-water area， Qiongdongnan Basin， which has been proved by drilling to be a hydrocarbon rich sag. Miocene Sanya Formation is an important exploration target in the areas， and the scale and particle size of reservoir are the key factor to the hydrocarbon accumulation of Sanya Formation in Songnan-Baodao Sag. Based on 3D seismic and drilled data， combined with regional geological background analysis， the source to sink system and main controlling factors of the Sanya Formation in the area are systematically analyzed to predict the favorable reservoirs distribution. Research has shown that during the period of Sanya Formation in the area， two source-sink systems were developed： the northern system with strong sediment supply capacity， whose provenance is Hainan Uplift， with strong uplifting in the Miocene； the northern part of the sag developed a continental shelf slope break with "shelf delta+slope canyon+fan" sedimentary system. The sag transitions from a slope to Songnan Low Uplift， with no development of continental shelf slope breaks. The southern system controlled by Songnan Low Uplift is a multi-stage fan model controlled by short transport distance and converging channels. The factors such as transportation distance， tectonic activity， and bottom-current transformation control the development of the reservoirs in Sanya Formation. The southern edge of the sag is selected as the most favorable reservoir development area； the favorable reservoir development areas are the Songnan area and the eastern part of Baodao area.
The deepwater area is one of the frontiers of oil and gas exploration， and the distribution and genesis of deepwater reservoirs are the key problems for the reservoir formation research. Baodao Sag in Qiongdongnan Basin has been proved to be rich in hydrocarbon generation， but large oil and gas fields have not been found in ultra-deepwater area， and the distribution patterns of reservoirs in the deep water are unclear. Taking the southern slope area of Baodao Sag in Qiongdongnan Basin as an example， the distribution characteristics of submarine fans are studied by using seismic prediction methods including seismic reflection structure， seismic facies geometry， seismic attribute as well as source-sink theory. The results show that fan delta is flat oblique cross progradation reflection， the slumping submarine fan is lenticular reflection， and the submarine fan of Sanya Formation is sub parallel sheet reflection. The provenance came from the denudation area of Songnan Low Uplift and Southern Uplift area in the southwest. The sediments were transported by the two main incised channel systems in the north and south， and then flowed into the southern slope area of Baodao Sag. The application of seismic sedimentological prediction methods and source-sink theory has laid a geological foundation for oil and gas exploration and analysis of reservoir forming conditions in the deep-water area of Baodao Sag.
A number of large and medium-sized oil and gas fields have been found from the Upper Miocene Huangliu Formation to the Pliocene Yinggehai Formation in the Qiongdongnan Basin. Several years of rolling development have confirmed that the exploration potential of the above strata is gradually declining. Therefore， it is urgent to find favorable replacing layers and exploration blocks in Qiongdongnan Basin， and the Meishan Formation is the preferred layer for favorable replacing layers. This paper takes the turbidite fan of Meishan Formation in the northern Qiongdongnan Basin as the target area. The sand body composition， structural characteristics and source system were analyzed by using rock thin section observation， laser particle size and heavy mineral analysis， trace element/rare earth element test in combination with the analysis of sedimentary background data， surrounding water system of the northern basin， and its rock characteristics. The results show that there is a large denudation area of Meishan Formation in the northern part of Qiongdongnan Basin， and the source supply is sufficient. A wide range of submarine fan deposits are developed on the edge of the continental slope with a sedimentary pattern of good in the west and poor in the east. The source system of Meishan Formation in the northern part of the basin is characterized by a paleogeographic pattern of “multi-point source and geomorphological constraints”. Among them， the Ningyuan River provenance of Hainan Island affects the Ledong Sag， Yanan Sag and Yacheng Uplift of Meishan Formation in the northern part of the Central Depression of Qiongdongnan Basin， which is a near-source supply of submarine fan deposits. The Lingshui River provenance of Hainan Island supplies to the Lingshui Sag and Songnan-Baodao Sag of Meishan Formation in the northern part of the central depression of Qiongdongnan Basin. The dual sources of Lingshui River and Wanquan River in Hainan Island jointly affect the uplift of Meishan Formation in the northern Central Depression of Qiongdongnan Basin and the northeast of Songnan-Baodao Sag. Two major source systems of Honghe River in the west and eastern Vietnam have regional influence on Meishan Formation in the well area of Ledong Sag.
In recent years， buried hill traps have become hotspots for offshore oil and gas exploration. Many large-scale oil and gas fields have been discovered. The Songnan Low Uplift buried hill has achieved good oil and gas discoveries， and the Lingnan Low Uplift buried hill has great exploration potential， whose exploration has just begun. Buried hills in Lingnan Low Uplift are difficult to drill. In order to improve the success rate of exploration， it is urgent to strengthen the research on reservoir characteristics. On the basis of 3D seismic data， wall cores， logging， FMI imaging logging， thin section， and elemental logging are used to analyze reservoir characteristics of the lithology and zoning. The results shows that the rock types in buried hills of Lingnan Low Uplift are rich， which includes granite， metamorphic rock and later intrusive dikes. The felsic content of granite is more than 85%， which has the material basis for the formation of large-scale buried hill reservoirs. According to the differences in reservoir pore types and physical properties， the granite buried hill can be vertically divided into four zones：weathered residual zone，weathered fracture zone， dense zone，and internal fracture zone.Multiple tectonic activities have controlled the formation of buried hill fractures， which can be further divided into three stages： the Indosinian northeast compression fracture formation， the Yanshanian northwest compression shear fracture network segment， and the Himalayan extension reactivation. In addition， atmospheric fresh water can effectively improve the physical properties of reservoirs in weathered fracture zones， while deep thermal fluid activity can effectively improve the physical properties of weathered zone reservoirs， while deep thermal fluid activity can improve the physical properties of internal zone reservoirs. Therefore， buried hill in Lingnan low uplift has favorable conditions for good rock types and strong post transformation， and has good exploration potential.
After more than 60 years of exploration， commercial breakthroughs have not yet been achieved in the northern part of the Yinggehai Basin， and the proportion of structural lithologic or lithologic trap evaluations in this area is gradually increasing. One of the key to evaluating such traps is the prediction of sand rich areas. The prediction of sand rich areas is the foundation of reservoir distribution prediction， and sand rich areas are often favorable areas for reservoir development and also favorable locations for drilling. Sedimentary facies identification and favorable sand body distribution prediction are hard to be achieved with large number of cores and well log data， due to insufficient or no drilling in offshore exploration new areas. Due to the limitation of seismic resolution， it is difficult to directly conduct sedimentary microfacies research with seismic data， resulting in the limited prediction of favorable sand body enriched areas. Based on the analysis of sea level change and sedimentary system tracts division， identification of complex lobes and their sedimentary periods were conducted within the isochronous stratigraphic framework， and sand body enriched lobes were found. By analyzing the seismic internal structural characteristics and micro paleogeomorphology of the sand body enriched complex lobe， the favorable sandstone accumulation areas within the complex lobe were predicted. It has shown that the Huangliu Formation of the A structure in the northern section of the Yingdong Slope of Yinggehai Basin developed delta sand bodies deposited in the late stage of the high system tract， and five stages of delta lobes can be identified. The complex lobe developed in the last stage is a delta progradation complex lobe developed during sea level decline， which has the highest sandstone content among the lobes. It is predicted that the central north-south strip in the lobe is the sandstone concentrated area. The results can provide guidance for predicting the distribution of favorable reservoirs and deploying exploration wells.
Baiyun Sag has become the most important deepwater exploration target in the Pearl River Mouth Basin. However， its complex high-variate geothermal characteristics have severely constrained further oil and gas exploration and resource evaluation. In this study， the present-day geothermal field and tectono-thermal evolution histories of Baiyun Sag were systematically studied based on measured rock thermal conductivity and heat generation data， borehole temperatures， low-temperature thermochronometer and geodynamic methods. The thermal conductivity of 251 core samples ranges from 1.131 to 4.478 W/（m·K）， with an average of 2.258 W/（m·K）， while the heat generation rate of 106 samples ranges from 0.868 to 1.735 μW/m3， with an average of 1.499 μW/m3. The thermal conductivity in Baiyun Sag gradually increases from Wenchang Formation to Hanjiang Formation， while the heat generation rate increases with depth. The present-day heat flow in Baiyun Sag ranges from 66.6 to 139.1 mW/m2， with an average of 89.7 ± 14.7 mW/m2， showing a gradual increasing trend from northwest to southeast. The formation temperature at depths of 1-5 km increases with depth. The thermal inversion with the low-temperature thermochronological data of six basement samples revealed that each tectonic units in Baiyun Sag experienced unique temperature path， which was mainly related to regional tectonic uplift-subsidence and basement heat flow variation. Geodynamic simulations further indicate two extensional events in Baiyun Sag occurred in the Eocene and Middle Miocene， which resulted in a rapid increase in the basement heat flow. This study systematically elucidates the present-day geothermal field characteristics and tectono-thermal evolution history of Baiyun Sag， which is of great significance for regional tectonic evolution and future deepwater oil and gas exploration.
To explore the control of faults on the formation of oil and gas reservoirs in inversion anticlines in the Xihu Sag， East China Sea Shelf Basin， and to explore potential exploration directions for the next step， this paper selects the typical inversion anticline structure G as a research target. Based on 3D seismic and drilling data， through structural interpretation and fault activity analysis， structural evolution profile and fault sealing analysis， the fault system’s characteristics， its controlling factor on traps formation， hydrocarbon migration and reservation were analyzed. The results show that， there are three groups of faults developed in structure G， including two main control faults， secondary faults forming flower like structure and roughly EW fault in the Miocene series. The main control fault was formed in the Early Cenozoic， the secondary fault of flower like structure was formed in the sedimentary period of Eocene Pinghu Formation， and the roughly EW fault was formed in the Longjing movement in the Late Miocene. The main body of the structure G is characterized by a fault anticline that complicated by faults and volcanic structures， which is controlled by multiple tectonic movements such as extensional faulting， magmatic activity， and compression inversion， and finally formed in the end of the Longjing movement； The “stamen” hydrocarbon source fault at the high point of the anticline has a small fault displacement， and its activity does not match the main hydrocarbon expulsion period， resulting in weak hydrocarbon supply capacity； The lateral sealing ability of the control fault is poor in the shallow part， and the high point of the anticline is destroyed by the roughly EW fault； The above factors collectively lead to unsatisfactory exploration results of the structure G at high points in the anticline. We believe that the inversion anticline in the Xihu Sag is located in or close to the hydrocarbon-rich sag， has sufficient oil and gas supply， and has good exploration prospects. The structural and structural-lithologic composite traps in the inversion anticline flanks and the sub-high point without EW-trending faults are favorable areas for further exploration.
The high CO2 content have profound impacts on oil and gas accumulation in Bozhong 21-22 structure， Bohai Bay Basin. And， but the relevant research is weak. Based on the geochemical data of natural gas components， carbon isotopes and helium isotopes， the genesis of CO2 and hydrocarbon gas is revealed. The relationship between CO2 and hydrocarbon accumulation and its influence on hydrocarbon accumulation in Bozhong 21-22 structure area are analyzed. The results show that the content of hydrocarbon gas and non-hydrocarbon gas in Bozhong 21-22 structural area is approximately the same. The CO2 is the volcanic mantle-derived inorganic origin， and the hydrocarbon gas is mainly the partial humic kerogen pyrolysis gas from the third member of Shahejie Formation. The main charging period of CO2 was the volcano-magmatic period during the deposition of the Guantao Formation in the Neogene. The main accumulation period of oil and gas was the Minghuazhen Formation in the Neogene， and the charging and accumulation period of CO2 was obviously prior to that of hydrocarbon. The accumulation model of the Bozhong 21-22 structural area is firstly charged by CO2， subsequently by hydrocarbons. The early-charging CO2 will infill the effective storage space first and inhibit the continuous filling and accumulation of late crude oil and hydrocarbon gas. Only part of hydrocarbon gas can continue to fill and mix with CO2 to form the present-day high-CO2 gas reservoir.
Seismic amplitude contains abundant geological information， and plays an important role in structural interpretation and target evaluation. In deep-sea exploration with limited logging information， accurate amplitude attributes will conducive high-precision lithology identification and fluid detection. Recently， Baodao Sag in Qiongdongnan Basin is an important field for gas exploration in the deep water area of the South China Sea. However， its seismic amplitude is influenced by multiple factors， restricting the development of geophysical evaluation seriously. To improve the accuracy of reservoir prediction and fluid detection， we proposes a high-precision seismic amplitude decoupling technique in this paper. Firstly， we use forward modelling to clarify the formation coupling amplitude response law， and adopt the amplitude ratio method to identify the impact of fluid properties on amplitude. Then， semi-quantitative matching decoupling technology is used to eliminate the influence of formation thickness， and the effect of cap rock on the amplitude is reduced by the adaptive wavelet decomposition technology. Finally， the gas bearing capacity of the formation is accurately predicted. The practical application indicate that the proposed method helps to guide significant exploration breakthroughs and discoveries in the deep water area of Baodao Sag in Qiongdongnan Basin.
The Huangliu Formation in Dongfang area of the Yinggehai Basin in the northwest of the South China Sea has a large composite submarine fan system， which is widely distributed and has many lobes. Under the influence of high temperature and overpressure， multiple source intersections， channel erosion and alteration， and diapir activity， in the development of multi-stage superimposition， chaotic sedimentary configuration and complex seismic reflection structure make it difficult to evaluate the reservoir. Starting from the geological model and based on 3D visualization technology， the large-scale submarine fan is anatomized step by step from macro to micro in multi temporal and spatial scales. The large-scale submarine fan is described by integrated analysis of coherence， slicing， sub body carving， seismic attributes， paleogeomorphology and other technologies， and its structure are studied. Regional amplitude anomaly bodies， large-scale geological submarine fans， lobes or channels， sand bodies， gas formations are depicted step by step， and single sand bodies are carved reservoir prediction and hydrocarbon detection， etc. Finally， 11 lobed sand bodies in the four stages of the submarine fan were identified， and further detailed research was conducted on the sand body configuration and gas formations during the development stage. Combined with comprehensive analysis of geological laws， a comprehensive understanding of the multi-scale and comprehensive aspects of the submarine fan system was obtained. From exploration to development stages， the reservoirs of the composite fan system were effectively evaluated， and drilling confirmed the discovery of large gas fields and smooth development. The 3D visualization technology has broad application prospects for the analysis of complex geological bodies by breaking them into parts.
Weizhou Formation is the main reservoir in Haizhong Sag of Beibuwan Basin. The study of fine characterization of sand body is of great significance to the reservoir prediction in this region. Weizhou Formation reservoir is characterized by small thickness and poor continuity as well as low main frequency seismic data， which incapacitates the use of conventional thin sand recognition technology. This paper presents the fine characterization of sand body in delta front at Weizhou Formation in Haizhong Sag with frequency division analysis method been adopted which is based on forward modeling technique and actual drilling data in the studied area. The results show that the top and bottom interface of the sand body target layer is trough and peak， respectively. The amplitude and energy of seismic waves are positively correlated with the thickness of the sand body. The longitudinal resolution of the sand body gradually increases with the increase of frequency. The comparison between original data body and the frequency division data body of the response of the actual drill sand body in the same profile indicates that the latter has significant advantages in identifying sand bodies of different thicknesses. Based on various frequency division data bodies， the sand bodies of delta front of the third member of Weizhou Formation are delicately depicted， and the results are in line with the distribution law of the delta front fan.
The deep-water area of the Pearl River Mouth Basin is rich in oil and gas resources. It is of great significance to study the geochemical characteristics and group types of crude oil in this area for the complex oil and gas sources， accumulation process and oil and gas exploration. Based on the systemic geochemical experimental analysis results of 30 crude oil samples in the Baiyun Sag， the effectiveness of different analysis methods in oil classification was compared and two oil groups were identified. The analysis results show the composition characteristics of light hydrocarbons， carbon isotopes of oil components， and stable carbon isotopes of n-alkanes in oils from the Baiyun Sag are similar. δ13C values range from -28.3 ‰ to -27.2 ‰， indicating that they all originate from humic source rock and seem to belong to the same oil group. Multiple higher plant-derived biomarkers were detected in the oils from the Baiyun Sag， including oleanane （OL）， taraxastane （C）， and bicadinanes （T）. On the basis of biomarker parameters， oils from the study area can be divided into two groups， i.e. Group-A and B. The Group-A oils are rich in bicadinane-T and rearranged oleanane Ⅰ， Ⅱ and Ⅲ and poor in oleanane and taraxastane， which occur in the northern of the Baiyun Sag and western of East Sub-sag. The Group-B oils have relatively high content of oleanane and taraxastane and low content of bicadinane-T and rearranged oleanane Ⅰ， Ⅱ， Ⅲ， which are mainly distributed in the eastern part of East Sub-sag， the East Low Uplift and Eastern Main-sag. Delta mudstones and shallow marine rocks of the Enping Formation have been confirmed as the effective source rocks. Moreover， the distribution of the two oil groups divided by biomarker parameters is consistent with that of these source rocks. Therefore， in the deep-water area of the Pearl River Mouth Basin， above biomarkers with strong specificity can reflect subtle differences in organic matter input， and have more effective application effect for crude oil classification with similar original organic material compositions.
Thick mudstone layer is widely deposited in Dongying Formation of Paleogene system in Bohai Sea. The lacustrine mudstone in the Third Member （E3d3） has good hydrocarbon generation potential. The hydrocarbon generation simulation experiment by sealed gold tubes shows that the E3d3 source rock has the characteristics of “oil generates first and gas generates later” in two stages： the oil generation stage ranges from 0.6%EasyRO to 1.1%EasyRO and oil production peaks from 0.75% to 0.86%， with a maximum oil yield of 8.11 mg/g. Gas generation stage ranges between 1.1%EasyRO and 1.6%EasyRO， and the increased gas production accounts for 55.3% of the total hydrocarbon production， of which kerogen cracking gas contribution accounts for 29.6% and oil within source rock cracking gas contribution accounts for 25.7%. The study shows that the total oil production rate of the E3d3 mudstone with medium organic carbon content is not high， and the generated oil is difficult to discharge to form an effective supply. After entering the gas generation stage， the fluidity of oil is improved， and the generation of hydrocarbon gas promotes the internal pressurization of the source rock， which is conducive to the discharge of highly mature oil and gas. Thermal evolution has a significant effect on hydrocarbon generation and hydrocarbon expulsion. Medium to high maturity may be a necessary condition for effective hydrocarbon supply of the E3d3 mudstone.
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