Paleo-morphologic Restoration Of Paleogene In Qiongdongnan Basin And Its Control On Sequence Architecture And Sedimentary Characteristics

Qiongdongnan Basin is an important hydrocarbon bearing area in the northern margin basinof the South Sea.It is one of the large nearshore hydrocarbon-rich basins in China,with a totalsquare of 4.5×10~4km~2.Up to 2008,the hydrocarbon estimation of Qiongdongnan Basin is about3.5 billion m~3 volume of natural gas,and 0.22 billion m~3 volume of third-order reserves.Thediscovered resources amount is up to 15.38% of the total in the basin,and the undiscovered isabout 84.62%.The basin is of low exploration degree,huge surplus resources,and largeexploration potential.It is provided with the basic petroleum geological conditions fordeveloping a large oil-gas field,which is in the phase of preparation for exploration breakthroughand shows a great development potential.Qiongdongnan Basin is located in the intersection place of Eurasian plate,Indochina plate,and Pacific plate (the northern continental shelf of the South Sea in the northeast margin ofEurasian plate),belongs to Pacific tectonic domain.It can be divided into northern depressionzone,northern uplift zone,central depression zone,and southern uplift zone.There are 12secondary sub-sags in the basin,with the characteristics of"zones distributed along north-southand blocks distributed along east-west".During 52 years of exploration work in Qiongdongnan Basin,important reservoirs (mainlygas reservoirs) have been found in Yacheng uplift and et al.The proven degree of resources inshallow area has been increasing,which indicates a margin for development and also a crisis afterprosperity.In exploration,the deep area should be a key point to break through.But thecomplicated geological conditions in deep sea,high geothermal and overpressure,difficulties inreservoir prediction are the three hard problems that need to be solved.The proven rate ofresources is increasing;the difficulty of discovering large and intact reservoirs is getting bigger;the exploration of single tectonic trap can no more meet the needs of the industrial production ofthe oilfields.We are in dire need of strengthen the study on sequence stratigraphy in this area,especially bring in the thought of seeking subtle reservoirs from sequence stratigraphic point anduse it to guide the hydrocarbon exploration work in Qiongdongnan Basin.At present,the keyfactor that whether can get a breakthrough or not in Qiongdongnan Basin is the breakthrough intechnology and innovation in research thoughts. From the point of theoretical study,the control factors of sequence development havealways been the emphasis and the difficulty in sequence stratigraphy study.Different allocationrelations between paleo-morphology and subsidence history can affect the sequence architectureand distribution of sedimentary system.In an important hydrocarbon-rich basin such as Qiongdongnan Basin,to develop thecomprehensive study on sequence stratigraphy,sedimentary system,and controlling factors,it isof great theoretical value and practical meaning for the subtle reservoirs exploration inQiongdongnan Basin.Based on seismic,logging,borehole data,etc.,sequence stratigraphy of the paleogenesystem in Qiongdongnan Basin has been studied in this paper.Following are the Abstract:1,The fault depression activities more in the stage of Paleogene system in QiongdongnanBasin,and the uplift-sag pattern had a huge changes because of the enormous influence of thetectonic activities.Based on the faults distribution and influence on structure district,Qiongdongnan Basin can be divided into"four belts and ten sags":North depression belt,Northuplift belt,Zhongyang depression belt,South uplift belt,Yanan sag,Yabei sag,Songxi sag,Songdong sag,Ledong sag,Lingshui sag,Songnan sag,Baodao sag,Changchang sag,andBeijiao sag.2,Seven third-order sequences (SQEsx,SQEyc3,SQEyc2,SQEyc1,SQEls3,SQEls2,SQElsl ) are identified in Paleogene system of Qiongdongnan Basin based on the typicaltruncation,onlap,downlap toplap and the features of well-logging.Based on the seismicreflection,the incised valleys,low-stand fans,low-stand progradation,lake expandingretrogradation and high-stand progradation can be identified.For each third-order sequence of thekey formations,it can be divided into three systems tracts:low-stand systems tracts (LST),lakeexpanding systems tracts (EST) and high-stand systems tracts (HST).3,According to the observation of rock core,analysis of mud well -logging,and associatedwith the analysis of seismic data,alluvial systems,fan delta depositional systems,(lacustrine)delta depositional systems,coastal plain systems,coastal systems and (semi enclosed) submarinesystems are identified in Paleogene system of Qiongdongnan Basin.The analysis of seismicfacies which is based on the reflection abeyance pattern (such as progradation,onlap anddownlap ) can identify the fan delta depositional systems,lacustrine delta depositional systems,coastal systems and (semi enclosed) submarine systems.4,The evolution characteristics of depositional systems of Paleocene of Qingdongnan basinas follows::(1) in temporal,there is a huge transgression process from Eocene to lingshuiformation,it presents the basin margin and ancient uplift of basin are submerged gradually.Meanwhile,with the enhance of mainly transgression effect,it gradually transition fromcontinental of Eocene,bay-restricted sea of Yacheng formation (Ya-3 section),restricted sea ofYacheng formation (Ya-2,Ya-1 section) to open sea of coastal plain which is effected by tide(ling-3 section) and tide-controlling open sea (ling-2,ling-1 section),the time area distribution ofclastic system is also influenced.(2) in spatial,the main source (main water system of basinmargin) have inheritance at Paleocene,especially fan delta injected Yanan depression from YA 13-1 Structure on the northwest,fandelta of basin margin inject Beijiao depression fromnortheast,fan delta of basin margin inject Baodao depression and Beijiao depression from southto north on the northeast of basin.They all have relative stability when every sequences ofLingshui formation and Yacheng formation developing.Isolated ancient uplift inject clastic toperiphery depression with changefulness.The location and scale changed with the change oftemporal,especially low-stand fan.5,Three typical tectonic backgrounds develop in Qiongdongnan basin.(1) Slope position ofevery mono-faulted depression in Qiongdongnan basin (Yanan depression,Songxi depression,Songdong depression,Beijiao depression etc.) is typical representative with gentle slope break,itdevelops gentle slope break sequence style.This style has similar structure characters with classsequence stratigraphy sequence I.Typical low-stand bodies like slope fan and low-stand wedgedevelop in this place,which is from slope break to the center of lake.This area also is thefavorable exploration area of lithological reservoir.The platform that is on the slope breakdevelops small incised valley,thin expansion system tract and high-stant system tract.(2) Thethrown side of No.5 fault,No.2 fault,No.3 fault is steep slope controlled by fault,developsequence architecture with steep slope controlled by fault.Large fan delta,underwater fan andbasin floor fan formed by transportion of distal underwater fan develop in low-stand systemstracts.Fan delta and underwater fan systems with a small extent develop in high-stand systemstracts,fan delta and underwater fan develop slightly in local area,which reflect the scarceprovenance or the provenance can't reach the sag.In general,the sequence architecture is withthe cyclicity of progradation-retrogradation- progradation.(3) The thrown side of No.2 fault andthrown side of Changchang depression margin take on multi- fault-terrace,develop fault-terracebelts sequence architecture.The upper fault always be boundary of the low stand system tractdistributing.It develops low-stand fan and low stand wedge on flat of depths of depression.Thedistribution of high stand system tract does not controlled by fault,it represent slope thinning andtop truncation to basin margin.6.Before the Paleogene of Qiongdongnan Basin deposited,the palaeo-geomorphology tookon a structure characteristic which is north-south zoning distribution and west-east divisibleblock.The range of Deposition progressively expanded,terrain gradually gentled,and thedifference between uplift and depression gradually weakened.The evolution was from west-eastzoning distribution to north-south.It was that sediment source provided by the North and theSouth and uplift in the Depression evolved into provided by the the North and the South,and thesediment source of uplift disappeared.And the subsidence characteristic of Qiongdongnan Basin was matched with itspalaeo-geomorphology structure characteristic.the center of subsidence was changing andtransferring during each depositional stage,and the rate of settlement changed with the degree oftectonic activity.the central of the depression zone is the first echelon of settlement,and thenorthern part of the depression zone is the second echelon the settlement,and there are threenon-settlement areas on the edge.The above high-rates of subsidence areas are highly coincidedwith the areas where the gully developed in the geographical position. 7.The different configuration relationship of the palaeo-geomorphology and the rate ofsettlement would produce different patterns of sequence and different sedimentary systems.Thecontrol to the sedimentary systems' distribution represents that:When fault depression exercisesthe primary control function,three big provenance around the basin and the bulge provenancebuild the great strength in the deposition of sand body in the basin.The many stages of the faultterrace effects of the second fault zone and the boundary fault terrace of the Changchangdepression are obvious,and form broken stage wise sequence structure model.Both of the lowstand system tract and the high stand system tract develop delta system and fan delta system.Themany stages transportation channels of the step-fault zone cause the development of the gravityflow fan body in the fault flat in the deep place of the depression.Meanwhile the third,the fifthand the eleven fault control the deposition of the steep slope of the depression.These areasdevelop break controlling steep slope zone.The gentle slopes of each depression develop slopebreak.The low stand system tract of break controlling steep slope zones is quite growth,which isthe main centralized spot of the sand body.But the gentle slope sand body is not well developed.Because of the influence of the ancient landform of the depression edge,the low stand systemtract mainly develop low stand delta,the distal of which form basin floor fan.The sand body isenrichment in the low stand system tract where is the mainly prospecting area.the high standsystem tract mainly develop high stand delta.As the fault activities decreased,the control effectof fault gradually decreased.With the transgression of Lingshui Group,the provenance of salientdisappeared.The basin presented that the north and south provenance dominated.The controleffect of fault terrace decreased.The slope of the break controlled steep slope and the slope breakbegin to slow.The palaeo-geomorphology and the rate of settlement were the main controlling factorswhich controlled the patterns of sequence and the pattern of sedimentation when the high-ratefilled activities happened in the QiongDongnan basin.Firstly,the provenance system controlledthe position of third-order sequences and sedimentary system,while the palaeo-geomorphologyand the rate of settlement controlled the distribution of third-order sequences and sedimentarysystem,and the interaction amont the three controlled the types of sequence and sedimentation.Innovations of the thesis are as follows:1.Build the sequence stratigraphic framework in the whole Qiongdongnan area.For the firsttime identify and close boundaries T71,T72,and divide Yacheng Formation into three third-ordersequence.2.For the first time,systematically analyze syn-depositional paleo-morphology,subsidencecharacteristics,response coupling relations between sequence and sedimentary of the paleogenein Qiongdongnan Basin,and build sequence developing architecture and sedimentarycharacteristics under the control ofpaleomorphology.