Tectonic Evolution And Its Influence On Gas Hydrate Accumulation In The Deep Water Area Of Qiongdongnan Basin

Prophase investigations show that the gas hydrates(hydrates for short)are largely controlled by the geological conditions such as sags of the generating hydrocarbon,transport pathways,temperature-pressure changes,and stable zones in the deep water area of the Qiongdongnan Basin.These conditions were formed in the Cenozoic tectonic evolution of the basin,and tectonic activities control its early formation,mid-term development,and late finalization.Therefore,the study of tectonics is an important prerequisite for understanding the laws of gas generation,migration,and hydrate enrichment.However,compared to the rift stage with mature tectonic understanding,the Neotectonic period lacks detailed research on the multiple episodes of key tectonic changes.It is scientifically related to the establishment of complete basin evolution,and in the application,helpful to understand the secondary migration of hydrocarbon and distribution of hydrates.This study recognized that special geological events since the late Miocene contain important information about tectonic changes in the Neotectonic period,and then used seismic data to reveal the response relationship between geological events and contemporaneous tectonic movements,which can serve as geological evidence to determine the post-rift tectonic phase,and to divide the entire Cenozoic tectonic stages with previous research data.On this basis,the tectonic differences in different zones in the deep water area of the basin and their dynamic genesis are mainly discussed.Moreover,the response of geological conditions related to hydrate enrichment to tectonic differences and their dynamics are studied.Then the differences in geological conditions and their structural constraints were revealed.By clarifying the relationship between different types of geological conditions and hydrate enrichment types,a hydrate enrichment model under structural constraints was established to guide the prediction of favorable hydrate areas.The following important insights have been gained in this study:1.There are close genetic relationships between special geological events such as dense faults,magmatic intrusions and volcanic eruptions,Miocene mounds,central canyons,and mass transport deposits(MTDs)occurred in the deep water area of the Qiongdongnan Basin since the Late Miocene,reflecting important tectonic changes that occurred in the Neotectonic period.Since the late Miocene(～10.5 Ma),frequent magmatic intrusion and volcanic eruption activities were occurred,accompanied by the escape of a large amount of molten material and heat from the upper mantle.This was an important factor leading to the rapid attenuation of the dynamic support for the upper crust,leading to the acceleration of basin subsidence.The subsidence centers are located in the central depression zone(corresponding to the crustal thinning zone).Regional uneven subsidence is the essential reason for the development of dense faults,mounds Meishan Formation,central canyons,and MTDs.The subsidence difference since 10.5Ma caused the Cenozoic strata to undergo upward concave deformation along the northsouth direction,resulting in the derivation of dense extensive faults in the Neogene on both wings.At the same time,gravity detachment of unconsolidated sediments on the southern slope drove a series of shovel-like detachment faults and related rotating fault blocks,resulting in an undulation of the sea floor,which was later filled and leveled to form a mounded landform.In addition,the Changchang Sag and the Xisha Trough sink fastest,triggering gravity flows that cut the seafloor under the guidance of the central axial negative terrain to form the central canyon.With the increase of the subsidence difference between uplifts and depressions since the Pliocene,coupled with an increase in the supply of provenance from the north,slope break systems were formed,which triggered massive gravity collapse events with a greater degree and larger scale,namely,MTDs widely distributed in the deep water areas.The occurrence of these geological events indicated that the basin underwent an important tectonic transformation from tectonic stability to activation at the turn of the Middle and Late Miocene(T4,～10.5Ma).2.Based on the tectonic transformation surface(T4)demarcated by special geological events,combined with previous studies related to syn-rift,the basin evolution was divided into: the first stage of syn-rift(44-30 Ma,Tg – T7),the second stage of synrift(30-21 Ma,T7 – T6),the slow thermal subsidence(21-10.5 Ma,T6 – T4),and the accelerated subsidence(10.5-0 Ma,T4 – present).The accelerated subsidence was further divided into three episodes,the initial stage(10.5～5.5Ma,T4 – T3),the intermediate stage(5.5～1.64 Ma,T3 – T2),and the late stage(1.64-0Ma,T2 – seabed),based on the tectonic timing information revealed by the Miocene mounds,three episodes of post-rift faults,the dual structure of the canyon,interfaces related to effusive facies,temporal and spatial migration of MTDs.The differences in tectonic evolution between the eastern and western regions of the Basin and their dynamic mechanisms were also revealed.The first stage of syn-rift was mainly controlled by the subduction of the ancient South China Sea(<～66 Ma)and the extrusion of the Indosinian massif(<～36 Ma).The study area was under an NW-SE tensile stress field in the first stage,and mainly characterized by near NE-trending faults and related grabens.In the northwest of the basin and the eastern Baodao-Changchang area,grabens are developed in nearly EW direction under the influence of the NW-trending Red River strike-slip zone and the EW-trending spreadingridge of the northwest sub-basin of the South China Sea.The second syn-rift was mainly affected by the expansion of the South China Sea(<32-30 Ma),and the stress field was converted to a nearly N-S direction.The post-rift thermal subsidence superimposed the influence of NNE–SSE trending tension.The rheology of the lithosphere decreases in the eastern basin,resulting in a large number of EW-trending faults that are orthogonal to the tensile stress.In the eastern part of the basin,the lithosphere has relatively high rheological properties,with fault activation yielding to preexisting NE-trending faults,and EW-trending faults are rare.The difference in lithospheric rheology also affected the styles of faults and related depressions.The western basin was mainly featured by large shovel-like faults that controlled semi-grabens(single shear deformation),while the eastern area was mainly featured by grabens,with the symmetrical distribution of multilevel faults(pure shear deformation)on both sides of the graben.The accelerated subsidence stage is an important period that further led to structural differences between the east and west of the basin.The vertical up-down movement of the eastern basin with a relatively thin crust was more sensitive to deep thermal anomalies.The eastern basin has a higher subsidence rate compared to the western part,and is mainly characterized by the development of a large number of Neogene faults,magmatic diapirs,and eruptive structures;In the western region,the crust is relatively thick,the structure is relatively stable,the scale of magmatic diapir is small,the distribution is limited,and new faults are not developed.3.Based on the analysis of the structural differences in the deep-water areas of the Qiongdongnan Basin,the geological conditions associated with hydrate enrichment(hydrocarbon generation depression,natural gas migration system,formation pressure,hydrate occurrence conditions)in the east and west of the basin were revealed.At first,stretching and thinning of the lithosphere and large-scale magmatic activity resulted in a high thermal background in the deep-water areas.The Ro of the Yacheng Formation and its underlying strata are more than 2.0%.The source rocks are located in the center of Lingshui,Songnan,Baodao,Changchang,Huaguang,Ganquan,Beijiao,and other depressions.The No.2 fault at the northern boundary and the No.4 fault at the southern boundary of the basin were segmented in the early stage and controlled the center of each depression and the extent of fine deposits,which further controlled the range of the source rocks.Secondly,differential tectonic evolution resulted in distinct structures and related migration pathways in the eastern and western parts of the deep-water area.In the western basin,the seepage pathways in the lower structural layers were mainly basement faults,uplifts,and diapir structures,while the upper structural layers were mainly microcracks,gas chimneys,listric faults,sand injectites,and canyons.They can be combined into three types of vertical pathway systems:(1)middle-low paleo-uplifts and diapiric structures(bottom)+ fractures/faults(top),(2)middle-high paleo-uplifts or diapiric structure(bottom)+ gas chimneys(top),(3)deep faults(bottom)+ canyons/sand injectites(top).In the eastern basin,basement faults developed in the lower structural layers,and dense faults,canyons,and other seepage pathways in the upper structural layers can be combined as two types of pathway systems:(1)basement faults(bottom)+ Neogene faults(top),(2)deep faults(bottom)+ canyons(top).Thirdly,the dense faults and magmatic intrusions formed in the eastern area in the late resulted in the release of pressures,which led to relatively low residual pressure(< 40 MPa).On the other hand,the western area in the late was tectonically stable and had a rapid deposition in Pliocene and Quaternary.Therefore,the pressure increased significantly and the residual pressure was relatively high(< 110 MPa).The tectonic pathways are strongly correlated with the release of overpressure.In the context of strong overpressure,relatively low-amplitude overpressure zones indicate potentially efficient hydrocarbon leaks.Finally,the theoretical thickness of the gas hydrate stability zone is lower than that of the other areas in the South China Sea due to the high thermal background caused by magmatic activities.Rapid sedimentation in deep-water areas triggers multi-stage sedimentation of MTDs,resulting in weak overpressure in the underlying layers,which can slow the rate of freegas leakage and facilitate hydrate enrichment.4.Based on the correlation between the differences in geological elements of hydrate mineralization and the characteristics of hydrate enrichment,it is believed that the diapiric activities of the paleo-uplift between sags and fault activities are conducive to hydrocarbon generation,expulsion,and secondary migration of hydrocarbon,which restrict the distribution of hydrates.Areas with strong tectonic activity in the late stage have a better potential for hydrate accumulation,while areas with few structures lack gas source supply.Based on the study of tectonic differences between e east and west of the basin,three types of hydrate enrichment models are proposed:(1)leakage type related to paleo uplift/diapir,(2)diffusion type related to faults,(3)diffusion type related to fracture.Moreover,based on the coupling relationship between structural type,pressure distribution,information from fluid migration and accumulation,and hydrate distribution,three favorable areas were divided for the deep water area of the Qiongdongnan Basin,including(1)favorable hydrate mineralization zone in the Lingnan low uplift,(2)favorable mineralization zone of hydrate in Songnan low uplift,and(3)favorable hydrate mineralization zone in the Baodao–Changchang Sag.