Deepwater exploration has been developed for more than 40 years since 1975，generally，its exploration history can be divided into the beginning stage (1975-1984)，the early stage(1985-1995)and the rapid development stage(1996-now).Currently，deepwater areas have become the hotspot of global oil and gas exploration，and they are also one of the most important fields of oil and gas increase in reserves and production all over the world.In 40 years，global deepwater oil and gas discoveries are mainly distributed along five deepwater basin groups which are characterized by “three vertical and two horizontal”：(1)In deepwater basins of Atlantic Ocean，giant discoveries of oil are mainly concentrated in the Brazil，West Africa and the Gulf of Mexico of the southern part of Atlantic Ocean，and the significant discoveries of natural gas are mainly in the west coast of Norway of the northern part of Atlantic Ocean；(2)In deepwater basins of East African continental margin，a group of giant gas fields have been found in Rovuma Basin and Tanzania Basin；(3)In deepwater basins of Western Pacific Ocean，giant discoveries of oil and gas are mainly concentrated in the South China Sea and Southeast Asian waters;(4)The deepwater basins of Neo-Tethys Region are rich in gas，and the most important gas discoveries are mainly distributed in the northwest shelf of Australia and the eastern Mediterranean;(5)In deepwater basins around the Arctic Pole，major discoveries of oil and gas have been only found in deepwater area of Barents sea currently.Global deepwater oil resources are mainly concentrated in the middle and south sections of Atlantic Ocean.Deepwater gas resources are relatively widely spread，mainly distribute in northern part of Atlantic Ocean deepwater basins，the deepwater basins of East African，the deepwater basins of Neo-Tethys region and the deepwater basins around the Arctic Pole.There will be six domains for future oil-gas exploration of global deepwater basins which are characterized by “two old and four new”，specifically，“wo old” domains refer to the Atlantic offshore deepwater basins and offshore deepwater basins of the Neotethys structural domain，where exploration degree is relatively high，but the potential is still great；While the “four new” domains stand for presalt and ultra deepwater basins formations，offshore deepwater basins in the circum-North Pole area and West Pacific offshore deepwater basins，and the new fields will be the main fields of deepwater oil and gas exploration in the future.
On the basis of summarization and analyses of the distribution and main factors controlling hydrocarbon accumulation of oil and gas-rich deepwater basins in the world，it is found that the distribution of global oil and gas-rich deepwater basins is characterized by a basic framework of “double vertical and one horizontal” arrangements.The oil discovered in deepwater basins of the world are mainly distributed in the submeridional Atlantic offshore deepwater basins presenting a framework of “one vertical” arrangement，which is affected by the so-called “south-lacus and north-marine fault basins with north-south strike”.The gas discovered in deepwater basins of the world are mainly distributed in the Neotethys structural domain and East African continental marginal basins presenting a framework of “one vertical and one horizontal” arrangements，which is affected by the so-called “ocean and continent transitional facies fault basins”.Based on a systematic comparative analyses of accumulation factors of the major deepwater oil and gas-rich basins in the world，it is also found that the main factors controlling hydrocarbon accumulation of oil and gas-rich deepwater basins in the world can be summed up into the following five types：(1)reservoirs controlled by salt structure，pathway system and large turbidite fan coordinately；(2)hydrocarbon provinces controlled by source rocks，reservoir controlled by large-scale reservoirs；(3)hydrocarbon provinces controlled by source rocks and covers，reservoirs controlled by large-scale deltas；(4)hydrocarbon provinces controlled by source rocks and covers，reservoirs controlled by reef flats；(5)hydrocarbon provinces controlled by source rocks and geothermal heats，reservoirs controlled by pathway systems and traps.
During the past nearly 70 years，deepwater sedimentary theory has been developed in the disputes with disagree，and has always been a hot domain in sedimentology.Review of the research history of deepwater sedimentation，It can be divided into three stages: first is “turbidity current” and “turbidite” period in 1950s-1980s，second is the development of deepwater traction current and the reflecting period of the classical depositional model in 1980s-2000s，and the recent period of sandy debris flow and the deepwater slope sedimentation model.But，to date，there are still quite a lot of disputes and issues in deepwater sedimentary areas because of the complexity of the process itself.And the main concentration of these problems is pointed out，such as the complex and confusion of the gravity flow deposits terminology，the identification and evaluation of deepwater traction current deposits，the differences between fixed deposit model and multiphase transformation of fluid properties.In general，nowadays，the difficulties are precise identification of deepwater sedimentary process and flow pattern transformation and the establishment of a comprehensive sedimentary model.The urgent demands are effective prediction of the deep sandstone reservoir and find more deepwater oil-gas resources.Therefore，the future research of deepwater sedimentation should attach importance to unify and standardize terms，the application of seismic sedimentology，experimental simulation technology and multi-disciplinary intersecting，establish a deepwater sedimentation pattern considering “provenances-channel-lobe” integrated factors and the orderly transformation of sediment flow，improve the predictability of deepwater deposits and provide scientific guidance for deepwater oil-gas exploration．
Deepwater channel deposits are one of the main depositional types in petroleum exploration and development.Based on high-resolution 3D seismic data，logging data and drilling data，a deepwater channel system was discovered in the Zhujiang Formation，Baiyun deepwater area，and the distribution character and genetic model of the system were identified by integrated with geological analysis and geophysical data interpretation.This system is NS-trending in a low sinuosity meandering form，and three types of deepwater gravity-flow deposits were identified，i.e.mass-transport，channel-levee complex and lobate deposits.The mass-transport deposit is developed in the northeast of Baiyun Sag，and has dome shaped appearance，internal chaotic seismic reflection with basal erosion scratches.The channel-levee complex deposit is the main pathway of the deepwater fan system，and can be classified as erosion，erosion/accretion and aggradations according to the origin.The channel deposit has the development feature of generation and segment controlled by source and geomorphology characteristics，and is a typical turbidite channel complexes configuration within complex superposition and migration relations.The lobate deposit which is located in the front of the channel is developed in the south of the Liwan Sag，and comes in a lobe-like shape on plane within a high-amplitude parallel to sub-parallel seismic reflection.And then the genetic model of the deepwater channel system was discussed by integrated with the ancient water depth，palaeotopography，triggering mechanism and the ancient source.The ancient Pearl River delta was speculated as its main source，and the carbonate deposition of Dongsha Massif as a secondary source.The research is of the theoretic significance in studying various depositional processes of deepwater gravity flow.
The Baiyun Sag is an acknowledged hydrocarbon generating sag with the advantageous geological condition in the deep-water area of the northern South China Sea.There are different conclusions on structural geology，sedimentary fillings and dynamic mechanism in previous research.In recent years，the high quality seismic data collected in Baiyun Sag provide us an opportunity to recognize the seismic reflecting horizons and structure characteristics，and study in the sedimentary fillings and structural evolution.As a consequence，Baiyun Sag is chosen as an application example of detachment faulting in uniform lithosphere deformation.On the basis of 3D seismic data，we determined the detachment faults，established the theoretical model，and finally pointed out the influence of structural evolution on petroleum exploration.Owing to the special tectonic position，landward dipping detachment faults were developed in Baiyun Sag as a ductile deformation area in continent-ocean transition between South China plate and South China Sea.11 control faults could be recognized as the detachment faults.And the structural evolution and sedimentary fillings might be controlled by the deformation activities of these detachment faults.The evolution history of Baiyun Sag included three stages，i.e.，detachment faulting stage，faulting depression stage and regional subsidence stage.The detachment faulting and the sustaining regional subsidence lead Baiyun Sag to be the largest and deepest basin in deep-water area of northern South China Sea，and controlled the distribution of main source rocks in Wenchnag Formation and Enping Formation.
This paper has applied thin sections，casting thin sections，fluorescence thin sections，X-ray fluorescence，scanning electron microscope，mercury porosimetry measurements，oxygen and carbon isotope characteristics to analyze the petrologic features，physical properties，pore structure characteristics and the effect of diagenesis on reservoir quality of sandstone reservoir from Enping Formation in Baiyun Sag，Pearl River Mouth Basin.Then demonstrate formation mechanism of relatively good reservoir.The results showed that reservoir sandstones are classified as litharenite.Detrital grains are grain-supported and cemented by authigenic quartz，ferrocalcite and clay minerals.Pores are mainly intragranular dissolved pore from feldspar and feldspar-bearing volcanic debris，some intergranular dissolved pores，the secondary porosity zones can be found near the coal bed.Reservoir properties in general have low porosity and permeability，compaction is the main reason for the loss of intergranular pores in the burial history.During mechanic compaction，the loss rate of porosity of the sandstone is 12.51%-29.6% in the study area，with an average of 23.99%，which were taking 33.53%-79.34% of the initial porosity and the average value is 64.30%，the strength of the compaction decrease porosity is 39.20%-91.05% with a high average value of 81.17%；The bad reservoir quality of the calcareous sandstone bed results from the mid-late cementation.During cementation，the loss rate of porosity is 2.91%-19.40% with an average of 5.61%，the strength of the cementation decrease porosity is 8.95%-60.80% and the average value is 18.83%；The formation of secondary porosity is caused by the organic acid.Organic maturity and acidic water in which CO2 is released by decarboxylation of organic matter in the process of diagenesis were the main reason.The feldspar particles and feldspar-bearing volcanic debris are dissolved by organic acid.The proportion of secondary porosity takes up to 3.19%-8.54% with an average of 5.66%，which were taking 59%-90% of the total porosity and the average value is 76%.
Explorations have proved that coal-bearing series and related mudstone of Enping Formation (Early Oligocene) are the main hydrocarbon source rock in Baiyun Sag，Pearl River Mouth Basin.According to seismic profiles and well data，there is a large pro-graded delta in the north of Baiyun Sag in Early Oligocene，which can be divided into three stages.The Rare Earth Element study shows that most of the 1st stage delta’s source rocks are sedimentary rock，which comes from Panyu low-uplift by braided rivers；because of the effect of peneplanation，Enping Sag and Baiyun Sag are connected by meandering rivers，and source rocks of the last stage delta is mostly granite，which is distributed to the north of Enping Sag.The increasing area of provenance and catchment causes the increasing supplementation of the delta，while it is one of the crucial reasons that the pro-graded delta can develop and extend bigger and wider constantly.Composition of sandstone has a critical influence on the quality of the deep-burial reservoirs，and feldspathic-rich sandstone in Baiyun Sag gets great dissolution potential to develop much more secondary porosity.upper Enping Formation in north of Baiyun Sag could be great exploration target in which channels，bars and other kinds of sandstones of delta are all great potential reservoirs.
The Zhuhai Formation is an important oil-gas reservoir and migration layer in Baiyun Sag，and the provenance analysis of Zhuhai Formation is a key basis for the studies of reservoir development and oil-gas migration conduits.Based on large amounts of 3D seismic and well data analyses，this paper proposed that there were three provenance areas which could offered clastic materials for Baiyun Sag during the process of Zhuhai Formation deposition，and Paleo Pearl River，which situated in north of Baiyun Sag，carried most of the deposits，then the South Uplift，the clastic originated from the Dongsha Uplift were relatively limited.We focused on the study of South Uplift provenance in this paper utilizing seismic facies，petrologic fabrics of cores in wells，X-ray analysis of clay materials and the ages of clastic zircons.Imbricate foresets of Zhuhai Formation prograded northward and the incised river channels elongated eastward or northeastward in Yunkai Lower Uplift implying the transportation processes of clastic materials from south to north.Also，the apparent differences between granularity in sandstones，content of kaolinite in mudstones and ages of clastic zircons in the wells situated in northern and southern parts of Baiyun Sag indicate the clastic materials in Zhuhai Formation originated from different areas and different mother rocks.After comprehensive analysis，it was realized that southern provenance provided so abundant clastic materials，and these materials covered the Yunkai Lower Uplift from south to north and arrived southern parts of Baiyun Sag during deposition of Zhuhai Formation，and the widely distributed sandstones originated from South Uplift also provided effective reservoirs and benefited the oil-gas migration from Baiyun Sag to Yunkai Lower Uplift.
Deepwater channel system is one of the main fields in deepwater hydrocarbon exploration.The exploration practice in the Baiyun deepwater area indicates that the deepwater sandstone is the main reservoir type.Based on high-resolution 3D seismic data of the study area，this paper presents a set of comprehensive geophysics methods that can be used to recognize and characterize deepwater channel sandstone reservoirs.First of all，the distribution of the deepwater channel system in the upper Zhujiang Formation was identified according to seismic-sedimentary facies，seismic coherence and seismic attribute，as a result the favorite facies distribution and sandbody distribution lows of the reservoir were qualitatively understood.And then petrophysical analysis and post-stack inversion reservoir prediction research were conducted，so the reservoir distribution lows and the reservoir thickness were half quantitatively or quantitatively delineated.Recognition and characterization of the deepwater channel might provide some insights for future explorations in the Baiyun Sag.
The deepwater oil and gas resource is abundant and has a great potential of exploration.Baiyun Sag is a hot area of deepwater oil and gas exploration in Pearl River Mouth Basin，north South China Sea.Due to technology and cost conditions，the method of using diagenetic facies logging response characteristics to study reservoirs is one of the effective measures from shallow to the deepwater in offshore oil and gas exploration at present.With the methods of thin sections，casting thin sections，scanning electron microscope，oxygen and carbon isotope analysis，this paper analyzes the basic characteristics of reservoir，typical diagenesis types，diagenetic facies assemblage types and its logging response marks.The study shows that the main diagenesis developed in the medium-deep reservoirs of the north slope of Baiyun Sag is compaction，cementation and dissolution，which are key factors to affect the reservoir physical property.The diagenesis facies of reservoirs in the study area is qualitatively divided into four types，including strong cementation-weak dissolution facies，weak compaction-strong dissolution facies，moderate compaction-strong dissolution facies and strong compaction-weak dissolution facies.Then,based on the analysis of conventional logging curves，such as acoustic transit time log，gamma log and resistivity log，which are sensitive to diagenetic facies，this paper establishes the logging identification standard of deagenetic facies.The final aim of this study is to provide a scientific basis and reference approach for the further oil and gas exploration in deepwater of Baiyun Sag.
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