| Carbonate strata are remarkably important potential petroleum reservoirs.Petroleum production from the carbonate reservoirs occupies ~60% of the whole amount in the world,indicating the carbonate reservoirs are of great value in exploration.Apart from the significance in petroleum exploration,carbonate sediments are also excellent palaeoceanographic and palaeoclimatologic archives,recording plentiful information about the environmental changes in the ancient times.Scientists around the world,therefore,have paid much attention to the research on the carbonate system deposits.This paper focuses on the study on the reef-bank system in well XK-1 located in the Xisha Islands,South China Sea.The aim of this paper is twofold.The first is to describe the Miocene-Holocene reef and bank system in details.The second is to clarify the respective role of sea level,tectonic movement,temperature and nutrient level in shaping the tropical carbonate factory turnovers since the Miocene in the northern South China Sea.A new dynamic model of the reef-bank system evolution in the northern South China Sea is also built on the basis of these factors controlling processes.The findings in this study are not only theoretically significant for the future research about the tropical reef-bank system evolution,but also practically important for the carbonate reservoir prediction in the South China Sea.The important research outcomes in this paper are listed as follows.1.Five sedimentary facies are differenciated within the carbonate strata since the Miocene in well XK-1,including reef,bank,open bank,mixed carbonate-siliciclastic deposits,and aeolianite deposits.Reef facies can be further divided into coral reef microfacies,coral-algal reef microfacies,and reef cap microfacies.Bank facies can be further divided into inner bank microfacies and outer bank microfacies.Coral reef microfacies primarily occurred from the Early Pleistocene to the Late Pleistocene.Coral-algal reef microfacies mainly existed during the early Late Miocene.Reef cap microfacies occurred during the late Early Miocene,early Late Miocene,Middle and Late Pleistocene.Inner bank microfacies mainly existed during the early Middle Miocene,Late Pliocene,and Early Pleistocene.Outer bank microfacies mainly occurred during the late Late Miocene and Early Pliocene.Mixed carbonate-siliciclastic deposits occurred only during the Early Miocene.Aeolianite deposits existed only after the Late Pleistocene.2.A total of 12 carbonate associations are identified within the carbonate succession in well XK-1 on the basis of the development of hermatypic corals.The Miocene-Holocene carbonate deposits are divided into 6 photozoan associations,4 heterozoan assciations,and 2 heterozoan-domianted associations,with the photozoan association containing large amount of hermatypic corals,the heterozoan association comprising almost no hermatypic corals,and the heterozoan-dominated association containing few hermatypic corals.Photozoan carbonates occurred during the Miocene,and from the Early Pleistocene to the Late Pleistocene.Heterozoan carbonates occurred during the Early Miocene,Middle Miocene and Late Pleistocene.Heterozoan-dominated carbonates occurred during the late Middle Miocene,and from the late Late Miocene to the Early Pleistocene.3.The relative sea level fluctuation,the combined effect of global sea level change and local tectonic movement,proves to be an important factor influencing the carbonate development in the Xisha area.From the Early Miocene to the early Late Miocene,the relative sea level was low,exposing the carbonate platforms frequently.Between the late Late Miocene and the early Early Pliocene,the relative sea level rised quickly due to the fast basement subsiding in Xisha area.Heterozoan carbonate factory occurred in the study area during this period,forming the outer bank deposits in deep water settings.During the late Early Pliocene,the tecnoic subsiding activity ceased,allowing the global sea level to be the dominant factor.The eustatic sea level was high during the Pliocene,and dropped gradually afterwards.In company with the global sea level drops,the carbonate factory in study area turned from heterozoan to photozoan,causing the change in deposition from outer bank,inner bank to coral reef.The thickness of reef units varied as a result of changes in global sea-level fluctuation frequency.Prior to the Middle Pleistocene,the global sea level was generally high,fluctuating at high frequency.Under this condition,the reef units were thin.In contrast,the global sea level was generally low,fluctuating at low frequency after the Middle Pleistocene.This kind of environment favored the development of thick reef bodies.Sea level fluctuation amplitude increased after the Middle Pleistocene.The large amplitude in sea level falls resulted in the frequent exposures of reef units,forming the reef cap deposits.After the Late Pleistocene,the eustatic sea level further dropped a lot,making the Xisha Islands exposed in the subaerial environment.Aeoliantes developed during this period.4.The changes in nutrient level had also affected the Xisha carbonate platform evolution since the Miocene.The nutrient level in the study area was generally high within the period of 21.2-17.3Ma,15.5-11.6Ma and 3.0-1.7Ma,based on the changes in geochemical indexes(i.e.Ln(Cu/Ti),Ln(Ba/Ti))as well as the variations in the abundance of bryozoans,rhodoliths,Discorbis,Bolivina,and Ammonia,which prefer mesotrophic-to-eutrophic conditions.The high nutrient level in the study area during the late Early Miocene and Middle Miocene was related with the East Asian Summer Monsoon intensification,which caused the presence of a large-scale upwelling at Xisha area.The summer-monsoon-induced upwelling brought great amounts of nutrient resources upwards from the deeper waters in the Xisha area,and therefore stimulated the bloom of phytoplankton in shallow waters.The phytoplankton bloom made the water bodies turbid and environment dim,fostering the development of heterozoan carbonate factory at shallow settings.Hermatypic corals could not survive in such a turbid environment and the facies turned out to be dominated by open bank deposits.The high nutrient level in the study area from the Late Pliocene to the Early Pleistocene(3.0-1.7 Ma)was probably linked to the increases in ice sheet volume in polar zones,which led to the transportation of trophic resources from the Southern Ocean to the South China Sea via intermediate water connections.In company with the high sea level,the high nutrient level in the study area resulted in the dominance of heterozoan factory during this period.After the Early Pleistocene,the sea level and nutrient level declined synchronously.Photozoan carbonate factory became dominant in the study area,forming the thick reefs in this region.5.A normal reef and bank model in oligotrophic environment and an open bank model in mesotrophic-to-eutrophic setting are respectively proposed in this study.Under oligotrophic conditions,reef and bank system will develop in the Xisha area,and water depth is the primary factor controlling the development of reef-bank system.Large numbers of hermatypic corals grow in the shallow and well-lit environment,building thick coral reef microfacies.Encrusting red algae gradually thrive in slightly deeper waters.These red algae are capable to encrust the coral frameworks and bind the nearby skeletal debris,forming coral-algal reef microfacies.In relatively deep environment,the coral abundance shows a further decrease,while the abundance of red algae and benthic foraminifers exhibits increase.The inner bank microfacies develop at this water depth.In the deep waters,corals do not exist,whereas planktonic foraminifera and calcareous nannofossils flourish.The outer bank microfacies occur in such a deep environment.In the normal reef and bank system,the foraminifera progressively turn from shallower-water species into deeper-water species as the water depth rises gradually.Under conditions of monsoon-induced upwelling,open bank system will develop in the study area.In the mesotrophic-to-eutrophic setting,plenty of phytoplanktons bloom in the surface waters in the Xisha area,prohiting the light penetration in water body and creating a dim environment in the shallow zone.Hermatypic corals can't survive in such an oligophotic setting,and the atoll will thus not develop in the shallow waters.Insead,the open bank facies dominate.The oligophotic environment fosters the growth of benthic foraminifer species,adaptive to less-lit conditions,at shallow zone.Bryozoan,rhodoliths and some benthic foraminifer species,preferring high nutrient level conditions,occur frequently in mesotrophic-to-eutrophic waters.6.The carbonate system evolution is controlled by relative sea level,nutrient and temperature.During the initial period of the Early Miocene,the East Asian Summer Monsoon was weak,and the temperature was high.The initial rise in relative sea level,caused by local basement subsidence,allowed the development of normal reef and bank systems on top of the Xisha Uplift.The continuing relative sea level rise forced the reef migration towards highlands.Between the Early Miocene and the Middle Miocene(21.2-11.6 Ma),the relative sea level dropped and the study area was generally shallow.During most parts of this period,the East Asian Summer Monsoon prevailed,resulting in the presence of upwelling activity at Xisha area.Such a monsoon-induced upwelling turned the reef-bank deposits into open bank deposits.The normal reef-bank system survived only within three short episodes of weak monsoon activity.From 11.6 to 10.5 Ma,a further fall in relative sea level occurred,which exposed the study area and resulted in the erosion.After the Late Miocene(10.5 Ma),the summer monsoon weakened rapidly,and the Xisha area got rid of the upwelling influence since that time.From the early Late Miocene(10.5 Ma)to the Early Pliocene,a fast and continuing subsidence in platform basement happened,which helped change the environment in study area from shallower water to deeper water progressively.The low relative sea level during the early Late Miocene permitted the thriving of the corals as well as the development of reef facies in the study area.The deep water setting during the late Late Miocene and Early Pliocene caused the domination of outer bank microfaices.After the Middle Pliocene,the basement tectonic subsidence ceased,and the global sea level started to dominate the role.Between the Late Pliocene and the Early Pleistocene(3.0-1.7 Ma),the combination of elevated nutrient level and high sea level prohibited the coral growth as well as the reef acceration.After the Early Pleistocene(1.7Ma),the global sea level and nutrient level droped at the same time,allowing the thriving of hermatypic corals at shallow and oligotrophic settings.From the Early Pleistocene to the Late Pleistocene,the sea level fluctuation frequency decreased while the fluctuation amplitude increased.The longer short-term highstand after the Middle Pleistocene favoured the rises in reef unit thickness,and the larger amplitude in sea level drop caused the esposure of the thick reef units.Reef cap microfacies thus developed frequently. |
PDF Full Text Request