Deep-water Sedimentary Systems And Their Hydrocarbon Significance In The Miocene In The Beijiao Sag,the Northwest South China Sea

In the Passive continental margin,deepwater sediments documented plentiful of paleoceanographic,palaeoclimatic,deep-water sedimentary dynamic information,etc.Deepwater sedimentary system has attracted the worldwide attention of geologists,especially after findings of deepwater oil and gas field.However,on account of difficulty in drilling,sampling,and abundant and complex geological phenomena,geologists poorly understand deepwater sedimentary systems,which urgently makes us to research or focus on deepwater sedimentary system in detail.In this dissertation,in this circumstance,we fully utilized 2D,3D high-resolution seismic data and multi-drilling data from deepwater and shallow water area to analyze the characteristics of different deep-water sedimentary systems in detail,including mounds and channels on the top of Meishan Formation and geological bodies(channel sandstone)with high amplitude within Meishan Formation and at the bottom of Huangliu Formation,and reconstruct their formation of sedimentary hydrodynamic process.In addition,the author analyzed the characteristics of polygonal faults and their genesis.The significance of petroleum geology in study area were briefly discussed.1.The mounded reflections of the mid-Miocene upper Meishan Formation in the Beijiao Sag of Qiongdongnan Basin have been arousing widely the attention of the geologists.Previous studies inferred that the mounded reflection originated from various genesises such as reef,combined result of gravity flow and bottom current,and contourite.Based on drilling,2D and 3D seismic data,this paper researched on the mounds(remnant mounds)and channels in detail.In the Beijiao Uplift the remnant mounds and channels were not developed well,while in the mid part and high of the marginal slope they occurred well.The scale of them became smaller on the two blank of the highland and they are asymmetric.Mounds and channels with nearly E-W orientation are liner and sub-parallel with each others and locally conjunctive and bifurcate.The strikes of those have a small angle intersection with the strike of Beijiao Uplift.The mounds display mounded widths,heights,and length of 562-1233m,29-87m,and about 10m,respectively.There are some seismic reflections with the truncation of the blank of the mounds and the incision of the channel.The 3D seismic attributes showed that long strip strong amplitude infilled by the interbedding of sandstone and mudstone mainly occurred in the S-W part of 3D survey,however,the interior remnant mounds with mid-lower amplitude.Integrated drilling and seismic data,the results of this research showed that mounds were composed of mudstone and calcareous mudstone,which belonged to the bathyal depositional environment.The wave impedance(5.0×106-6.5 × 106 kg/m3·m/s)of the mounds were lower than that of carbonate rock and igneous rock,which also belonged the scope of the wave impedance of mudstone and sandstone.According the strong amplitude of the lower Meishan Formation channel became weaker,bifurcate,progradation from W to E and the contourite depositional-erosional characteristics in the vicinity of seamount,we inferred that paleo-current of the remnant mound and channel of formation flew from W to E.And the epoch of contourite deposition and erosion could ascend to the early later Miocene(11.6 Ma BP).Integrated analysis of various mounded reflections,we inferred that in study area mid-Miocene mounded reflections originated from the remnant underlying Meishan Formation incised by bottom current in the early later-Miocene.2.The mound reflections in the Beijiao sag were found and have attracted a widespread attention of the geologists,however the genesises of the mounds have being on debate.Based on previous studies and using the high-density grid 2D seismic covered the whole basin,drilling datum from deepwater and shallowwater area and high resolution 3D seismic data,this paper first attempt to analysis the sedimentary setting and the relation of paleo-morphology and action of bottom current in Beijiao sag.Research shows that,in the Mid-Miocene,Beijiao sag which of paleo-morphology presented nearly symmetrical funnels(middle narrower and two tips wider)was in the bathyal sedimentary environment.The paleo-morphology made for the acceleration of bottom current velocity,which contributed to intensifying the action of bottom current in early later Miocene and incising or undermining the Mid-Miocene to form bottom current channels and mounds(remnant mounds).The narrower the Beijiao sag is,the larger the scale of mounds and channels are.This paper first suggested that distribution of bottom current channels and mounds were controlled by paleo-morphology in Mid-Miocene in Beijiao sag of Qiongdongnan basin and it is great significant to mound reflections in other sags with funnel shape.3.The mutually symmetric trough-mound/hummock seismic reflections have not attracted the attention of the geologist.With the widespread employment of seismic exploration technology,an increasing number of these reflections were found and originated from various genesis,which should not be ignored.The paper,for the first time,summarized the five types of genesis for these seismic reflections,i.e.the conversation opal-A to opal-CT,the mudstone density inversion,the erosion and deposition controlled by bottom current,the differential compaction for the sand-prone fill channel in deepwater mudstone depositional environment,invasive magmatite sill,Comparing with the five types of genesis for these seismic reflections,we,for the first time,implemented the seismic reflections characteristics to explain the mutually symmetric trough-mound/hummock seismic reflections in Beijiao sag and infer that the genesis of these seismic reflections attributes to bottom current.Mid-Miocene strata were cut or incised by bottom current in the early Late Miocene and carved into trough and mound morphology.The early Late Miocene strata deposited into troughs and mounds which were located beyond mounds and troughs in mid-Miocene strata respectively.In the late Miocene,the trough of the new series with turbidite has obvious migration,incision,which was fundamentally different with others.It is significant meaning to infer the genesis of trough and mound occurred in other sags,South China Sea.4.Nowadays,geologists have mainly focused on fan bodies originated from gravity currents,which are perpendicular to continental margin slope,the field of which is now mature.The field of the anomalous fan body,however,presented along the continental margin slope is least known.In the paper,an anomalous cone-shape fan body with high amplitude was discovered using high resolution 3D seismic data,2D seismic data and drilling from deepwater and shallow water region.The anomalous fan body deposited in bathyal environment was presented at the right of nearly asymmetric funnel-shape paleotopography,genesises of which were discussed,which was a contourite fan.The result about discussing the genesis of the anomalous fan illustrated that the anomalous fan was originated as a result of the erosion and deposit of contour current.Mid-Miocene contour currents(bottom current)flowed through a narrow nearly asymmetric funnel-shape passageway,velocity of which were accelerated,and eroded underlying strata in the right of the passageway under the effect of the Coriolis force.During this process,fine grained clastics were taken away and coarser grained clastics were deposited under the function of the contour current.Coupled with contour current flowing through widen passageway and its velocity decreasing gradually,the coarse grained clastics were unloaded from contour current with deceasing velocity and then presented cone-shape fan with progradation seismic reflection.Based on the research,the characteristics of anomalous fan were summarized in the paper.Sandy contourite fan(i.e.the anomalous fan)found first in South China Sea is high quality reservoir and has a great implication for the Paleooceanography and paleoclimate.It not only enriches the types of deep-water sand bodies in China,but also is a potential new area for deepwater oil and gas exploration in the South China Sea5.There is a set of strong amplitude geologic bodies(turbidite channels)in the middle Miocene and the late Miocene in the western part of the 3D seismic survey.Turbidite channels are straight,but have their own characteristics.The turbidite channels in Meishan formation are perpendicular to the northern slope of the southern uplift.The left sides(concave bank)of individual turbidite channels are steeper than the right sides(convex bank)of those.The amplitude of turbidite channels gradually becomes weak from concave bank to convex bank.In the early period,the negative geomorphic unit(channel or sub-canyon)originated from the incision of turbidity current is smaller and shallower than canyon,which contribute to different interaction of bottom current and turbidity current and hydrodynamic process.Under such circumstances,bottom current erode concave bank,but deposit in convex bank,impel turbidite channel to migrate to concave bank.However,the turbidite channels in the bottom of Huangliu formation are oblique to the northern slope of the southern uplift and present nearly EW direction distribution,which were controlled by mounds and channels in the top of Meishan Formation.The turbidite sandstone were distributed between mounds.The amplitude of the geological bodies becomes weak gradually from one side to the other side.Clockwise bottom currents in early later miocene flowed above the mounds and channels and then formed helical bottom current on the channels.When paroxysmal turbidity current occurred,the helical bottom current interfered the low density and low velocity upper turbidity current and then formed overflow turbidity current to flow to two banks.During this process,Sediments were deposited from coarse grained debris to fine grained debris,which were the cause of the gradual high-to-weak change in amplitude.The southern source,clockwise mid-water mass,submarine bottom shape and paleoclimate and sea level changes together controlled the special geological bodies.6.Based on 3D high-resolution seismic data,polygonal fault system(PFS)discovered in the Beijiao sag of the Qiongdongnan(QDN)Basin was studied through fine seismic interpretation and coherent attribute analysis.PFS occurred in study area from Shanya Fm to Huangliu Fm.The characteristics of the PFS are diverse in the different local area and lithology area.The mounds and bottom current channels on the top of Meishan Fm were of nearly W-E orientation.The bottom current channels filled with turbidity sands were of nearly W-E orientation,while the turbidity channels with sands in the lower Meishan Fm were of nearly N-W orientation..In the northern mudstone area,the fault concentrated belts with EW orientation occurred in the bottom Huangliu Fm above the top of mound and the faults traversed bottom current channel over the channel with SN orientation.The larger the mound is,the wider the fault concentrated belt is,vice versa.the fault concentrated belts mainly occurred in the bottom of Huangliu Fm and the upper Meishan Fm.Compared with the fault with upward extension traversed bottom current channel,the faults with downward extension traversed bottom current channel are more longer and extended downward to the Meishan Fm or even Sanya Fm.A small number of Omni-directional faults began to occur in the upper Meishan Fm.The PFS gradually present Omni-direction in the Meishan Fm and Shanya Fm.In the southwest sandstone area,Compared with the faults traversed bottom current channel and concentrated belt in the northern mudstone area,the magnitude of the faults in the southwest area is smaller.The magnitude of faults in the lower Meishan Fm traversed in turbidity current channel is smaller and less.The faults adjacent to channel with strong amplitude filled sandstone were not developed or less developed.Gravitational spreading controlled the genesis of the fault concentrated belt and the fault traversed bottom current channel.The faults within Meishan Fm and Shanya Fm were mainly dominated by overpressure syneresis,with gravitational spreading.The faults traversed channel and concentrated belt above the top of mound are fist found.The faults in area have great significance in hydrocarbon migration,strong amplitude geologic bodies and mounded reflection.