The Sedimentary Characteristics And Its Control Factors Of Nearshore Subaqueous Fan-Turbidite Fan In The Shahejie Formation Of Chengbei Fault-step Zone

Chengbei fault-step zone of Dagang off-shore area is located in the slopeband from Qikou Depression to Chengning Uplift in the centre of HuanghuaDepression. There are three main faults,namely Yang'erzhuang Fault,ZhaobeiFault and Zhangdong-Hai4 Fault,with the NE or NEE strike and north dip,thesecondary fault with the north dip too,and the main fault described aboveconstituted the fault-step framework,falling downwards step by step fromChengning Uplift to Qikou Depression.The major target Shahejie Formationinclude the frist member,the second member and the third one.Nearshoresubaqueous fan-turbidite fan sourced from Chengning Uplift to the south isdeposited in the Chengbei fault-step zone, with the favorable reservoir conditionand good assemblage of reservoir and cap rock.The studied area is a slope stripwith successional development in a long period and is near oil-generating centerof Qikou Depression.Therefore,oil and gas tend to migrate to this area.Nearshoresubaqueous fan-turbidite fan of Shahejie Formation in this aera is advantagableexploration area based on the former exploration. There is higher explorationpotential in the area ,and it is one of exploration hotspots in recent years.Butexploration practice indicates that the reservoir vary quickly in transversedirection as this area lies in the fringe of basin,so it is necessary to strengthenresearch to the nearshore subaqueous fan-turbidite fan sedimentary system.The lithofacies of nearshore subaqueous fan is characterized by rockconstituent of quartz content 4-60%,average 22.87%,feldspar content21-58%,average 44.66%, rock debris content 17-63%,average 32.47%.Degree ofsorting is poor to moderate. Degree of rounding is subangular. Six rock types is seenincluding mottled conglomerate,grain support sandy conglomerate,seriatesandstone with gravel, grain support sandstone,siltstone,as well as darkmudstone .Reformed bedding and blocky structure are identified in the area. Thecharacteristics of well logging phase are follows. The logging representsbox-shaped with medium amplitude,dentoid bell-shaped, dentoid form and low-amplitude finger-shaped.The shape of well logging of from fan-root till fan-endcurved vary from moderate amplitude box-shaped, bell-shaped to low-amplitude finger-shaped.Seismic phase include lentoid facies,wedge shapedfacies,filling facies,disorder facies and so on.Accordinging to the data of core,rockdebris, logging and seism, the nearshore subaqueous fan in this area can bedivided into three subfacies and seven micro-facies:in which fan-root includesmain channel,zone between main channel;fan-middle includes braidedchannel,zone between braided channel and fan-middle front;fan-end includefan-end mudstone and sheet stone.Clastic constituents of turbidite fan includes quartz(4-76%,averagely38.31%),feldspar(3-77%,averagely34.93%), rock debris(7-79%,averagely 26.76%).Thesorting is moderate. The rounding is subangular to subrounded. The types of rocksinclude matrix-support sandy conglomerate,grain-spport sandy conglomerate,gradedbedding fine conglomerate, grain-spport sandstone,siltstone,dark mudstoneaccording to rock characteristics,lithofacies,sedimentary sequcnce and structure.Thesedimentary structure is found such as ball-and-pillow structure, load cast,sandstonevein, slump structure horizontal bedding,parallel bedding, gradedbedding,interference bedding and reformed bedding et al . The logging featurerepresent dentoid box-shaped and bell-shaped upward with high amplitude to thelow . Seismic facies show imbricate foreset facies, filling facies,disorder faciesand so on. According to the basis of core,rock debris,logging,seism, turbidite fanin this area can be divided into three subfacies and six micro-facies:inner fanincludes main channel;middle fan includes braided channel,zone between channeland channel front;outer fan include outer fan mudstone and sheet sandstone.Synthesized the data of core,rock debris, logging,seism,Paleontology andgeochemistry , three unconformity surfaces are identified in ShahejieFormation.Two sequences is divided that is sequenceI(S3 sequence) andsequenceII(S2+1 sequence).In sequence stratigraphy framework, turbidite fanmainly distribute lowstand system tracts and transgressive system tracts ofsequenceI and sequenceII in Zhangdong area through single well and linking wellfacies analysis;nearshore subaqueous fan distribute lowstand system tracts andtransgressive system tracts of sequenceII in Guanjiapu area,Zhangdongdong areaand Zhaodong area, besides nearshore subaqueous fan distribute lowstand systemtracts and transgressive system tracts of sequenceI in Zhangdongdong area.During the Shahejie Formation,the whole lake basin is in expansion anddeepening,so abundant provenance circumference terrigenous deposit andquickly subsiding basin make-up special bockground,detrital sediment withcharacter of proximal source,torrent and directly entering lake has quicklyaccumulate to form series of nearshore subaqueous fans and turbidite fan in thedownthrow wall of Yang'erzhuang Fault.Its formation are controlled bypaleostructure,paleoclimate,paleo-provenance and base level,thereinto, paleostructureis decisive factor, nearshore subaqueous fan in this area belong to retrogressivetype,upwards grain vary small and sandstone thickness vary thin, nearshoresubaqueous fan is made up of fan-root→fan-middle→fan-end from bottom totop.Nearshore subaqueous fan in the downthrow wall of Yang'erzhuang Faultbecome astable due to succession upgrowth and easily slide,slump,move anddeposit in Zhangdong area forming turbidite fan, upwards grain vary small andsandstone thickness vary thin, turbidite fan is made up of inner fan→middle fan→outer fan from bottom to top controlled by base level.