| The Neogene Shawan formation,the main hydrocarbon-producing layer in the Chepaizi uplift of the western Junggar basin,has multiple sources and complex depositional systems.Owing to limited well data,core data and testing materials,there is ongoing debate about the depositional systems.Previous researchers thought it was subaqueous deposit,and the main sedimentary facies were fan delta,braided delta and lacustrine deposit.However,recent studies show that it is subaerial sediment,including alluvial fan and fluvial deposit.Due to the lack understanding of sedimentary regularity,it results in frequent drilling failure,which is a big disadvantage in the exploration and exploitation of lithologic reservoirs in the study area.Thus,it is urgent to need a new method to depict the characteristics of the sedimentary system and sand body.Based on cores,logging,seismic and testing materials,together with the theory of sequence stratigraphy,sedimentology,reservoir geology,logging geology and seismic sedimentology,this dissertation aim to:(1)establish isochronous sequence stratigraphy framework combined with outcrop and seismic-logging technique;(2)elaborate the depositional systems,spatial distribution and temporal evolution on the basis of core data,log facies and seismic sedimentology,and then establish the sedimentary models;(3)summarize the stacking patterns of sandstone;(4)construct the quantitative models to predict the distribution of sand bodies based on Google Earth software and strata slices.The main conclusions are as follows:Based on the analysis of the ancient landforms,paleo-current,heavy-mineral analysis and zircon U-Pb dating,we think that:(1)the peak age of zircon in study area is 400 Ma,which is close to Zaire mountain with the value of 391 Ma,and has a great difference of Altai mountain with 505 Ma and Tianshan mountain with 313 Ma.Then it shows that the Zaire mountain in the northwestern provide the source materials of study area;(2)there are two main directions of source materials,namely northwest – north and northeast;(3)in the early depositional period,the source material from northeast is in the domain,however,it shifts to north in the late Shawan formation.Based on the principles of sequence stratigraphy combined with the reflections on seismic data and well data,the sequence stratigraphy architecture of the Shawn formation is studied in great detail.The structure unit of the Chepaizi area is a large-scale gentle slope without obvious slope breaks in Neogene.Thus,it is impossible to define the first flooding surface.We propose sequence stratigraphy divisions for the Shawan formation with the transgressive systems tract(TST)and highstand systems tract(HST).The Shanwan formation unconformably overlies the Anjihaihe formation and is unconformably overlain by the Taxihe formation.On the basis of the association of vertical facies and characteristics of sedimentary cycles,two third-order sequences,namely,NSQ1 and NSQ2 from base to top,and four fourth-order sequences are recognized.Sequence NSQ1,corresponding to N1s1,has the features of medium amplitude-medium continuity in the seismic profiles.The lithology consists primarily of the thick sandy conglomerate interbedded with thin mudstone;Sequence NSQ2,in accordance with N1s2 and N1s3,is composed mainly of pebbly sandstone,with medium sandstone and fine sandstone intercalated with thin mudstone.Seismic sedimentology has been proven to be a powerful and effective method for providing independent and useful information about sedimentary facies and the distribution of reservoirs,the depositional history and reservoir-scale depositional architecture for subsurface conditions.In this study,by 90° phase rotation,thin sandbodies are mostly related to trough seismic events,and the mudstone-dominated bodies corresponded to peak seismic events,making the interpretation of lithology more direct.Besides,it is essential to analyze the rock-physics relationship before the study of sedimentary characteristics.In this study,sandstone corresponds to lower velocities,whereas mudstone corresponds to higher velocities,which can meet the needs of sedimentary studies of the study area.Finally,we find that large,irregular,rhombus-like(oval-shaped),bowlike(circular arc-shaped)strong-amplitude anomalies,many tiny branches and amplitude zoning appear on the stratal slice,which represent braid bars,point bars and distributary channels,respectively.These are obvious evidences to define sedimentary systems.The sandstone of Shawan formation has the characteristics of higher lithic content and lower quartz content,indicating the lower maturity.There has difference between various well blocks.The sandstone of the well block in the northwest of the study area,is mainly composed of lithic sandstone,with angular-sub-angular minerals showing point contact to line contact,and poor sorting.It is the characteristics of proximal source with low maturity.The sandstone of the well block in the northeast,is made up of lithic feldspar sandstone and feldspar lithic sandstone with angular-sub-angular minerals of matrix supported,which is the characteristics of materials with low maturity.In the central part of the study area,sandstone mainly includes lithic feldspar sandstone and lithic sandstone,and the minerals are angular – sub-angular shaped with matrix supported,indicating the low maturity;the well block in the southwest,with lithic feldspathic sandstone and higher quartz content and angular – sub-angular shaped with matrix supported,has the feature of higher maturity.Based on the detailed description of core interval,7 lithofacies have been recognized,including mottled massive conglomerate,pebbled sandstone,medium-coarse sandstone,fine-grained sandstone and siltstone,pebbled mudstone,horizontal bedding mudstone and gypsum.The logging facies include box-type,bell-type,funnel-type,finger-type and straight-type.The seismic facies is parallel – sub-parallel,moderate-strong amplitude,moderate-dense frequency and moderate-high continuity.Based on the detailed analysis of lithfacies,logging facies and stratal slices,six sedimentary systems,including 24 sedimentary microfacies are recognized.The alluvial fan is composed of braided channel,debris flow and sheet-flood deposits;the braided river system is made up of channel-lag deposit,mid-channel bar,flood plain,channel and natural levee microfacies;the meandering river system includes channel-lag deposit,point bar,flood plain,channel,natural levee and abandoned channel microfacies;the shallow water delta consists of delta plain and delta front,of which the delta plain is composed of distributary channel,abandoned channel,inter-channel and natural levee microfacies,and the delta front has underwater distributary channel,inter-channel,abandoned channel and sheet sand microfacies;the fan delta includes underwater distributary channel,inter-channel and sheet sand microfacies;the lacustrine system is shore-shallow lacustrine deposits.The characteristics of lithology,logging,seismic reflection and sandbody thickness of each microfacies are also analyzed.There is significant difference of sandbody in each source direction,of which the sandbody almost disconnect to each other,because of the mudstone in the middle part acting as a barrier.Besides,the sandstone thickness shows heterogeneous vertically,that is,it is decreasing and has lower connectivity upwards.The distribution of sedimentary microfacies is complex.The braided river system is the main sedimentary system in the N1s1 with a band-like pattern extending from the northeast to the southwest.The alluvial fan develops only in the northwest with a small scale,corresponding to the tectonic belt of paleogully.In the depositional period of N1s2,the braided river shifts to meandering river gradually,thus,point bars form.At the same time,braided delta plain develops at the end of south part of study area.A small scale alluvial fan still develops in the northwest.As the lake level rising and the source direction shifting,fan delta develops in the north with shallow-water delta front in the south.At the end of Shawan formation,alluvial develops in the north with flood plain in the north as lake level desending.Then,two sedimentary models,namely,alluvial fan-braided river model and fan delta-shallow water delta model,are built.The types of primary sandbody includes mid-channel bar,point bar,distributary channel,underwater distributary channel and sheet sand.The mid-channel bar which is characterized by gentle down-flow progradation,multi-stage vertical accretion and pan communicated sandbody,develops in the N1s1.The main lithology is massive sandy conglomerate,and the average thickness of the single sandbody is about 20 m,formed by the multi-stage sandbodies vertically superimposition.There are channel lag deposits at its bottom and falling-silt seams on the top.The main logging facies is medium-high amplitude box-shaped.It shows as strong amplitude and weak continuity in seismic profiles,and oval-shaped,rhombus-like and dumbbell-shaped on the stratal slices.The point bar forms in the N1s2,which is the result of lateral migration and accretion by rivers.The main lithology is pebbled fine sandstone with moderate sorting and rounding,and higher shale content.The range of single sandbody thickness is from 3 m to 15 m.The main logging facies are medium-high amplitude-serrated box-shaped,bell-shaped and finger-shaped.The seismic reflection shows strong amplitude,moderate-high continuity or worm-shaped in profiles,and irregular,arc-shaped on stratal slices.The distributary channel is composed of pebbly fine sandstone and fine sandstone with good sorting and roundedness,and higher shale content.The seismic reflection shows strong amplitude and moderate-high continuity or lenticular shaped in profiles,and narrow stripped-shaped on stratal slices trending as NE to SW.The logging facies shows finger-shaped and serrated box-shaped with commonly single sandbody thickness ranging from 3 m to 10 m.The underwater distributary channel which is composed of medium to fine sandstone,is the main sandstone of shallow water delta front.The seismic reflection shows moderate-strong amplitude,moderate-high continuity and sub-parallel pattern in profiles and narrow stripped-shaped on stratal slices with average sandbody thickness about 5m.The sheet sand which is composed of fine sandstone,is the result of lateral migration by wave.The single sandbody thickness is about 3 m with logging facies showing finger-shaped.The seismic reflection shows moderate amplitude,moderate-high continuity and sub-parallel pattern in profiles and sheet like on stratal slices.Based on the study of sandbody anatomy,four types of plane configuration are recognized,namely,stripped-shaped,branch-like,lenticular shaped and sheet like.Besides,we conclude four types of lateral contact pattern,including lateral superimposition-tangency contact,interdistributary contact,horizontal overlap contact and vertical separated;and three vertical superimposition patterns,those are vertical stacked,lateral stacked and vertical separated,of which vertical stacked pattern includes vertical superposition and vertical tangency shaped,the lateral stacked pattern is composed of lateral superposition and lateral tangency type.As lake level rising and A/S increasing,the sandbody configuration pattern shifts from vertical stacked,lateral stacked to vertical separated.On the basis of satellite photos from Google Earth software,logging and seismic data,the quantitative relations of main sandbodies are studied in great detail.The measured data of Maqu river,including the length and width of mid-channel,the width of braided river,the width of meandering river,and the length of point bar,is analyzed and then the quantitative models between the length and width of mid-channel,the width of mid-channel and the width of braided river,the width of meandering river and the length of point bar are built and tested by the data from strtal slices.What's more,the reflection of distributary channels on stratal slices are clear,thus the width of the channel can be measured on them,meanwhile,the thickness of the same distributary channel can be obtained from logging data.Then we analyze the width and thickness of the channel and establish the quantitative model with correlation coefficient 0.82.Thus,this provides another method for sandbody prediction where well data are rare.By detailed description of sand body genesis,distribution,quantitative characterization,and pinch out zones,combined with the rules of hydrocarbon accumulation and seismic characteristics,we find three favorable target areas. |
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