Study On Sedimentary Facies Of The Middle-Upper Jurassic In The Northern Margin Of Windorah Depression,Cooper-Eromanga Basin

As an important inland petroliferous basin in eastern Australia,Cooper-Erumanga Basin has developed a large amount of oil and gas,and is an important gas source in southeastern Australia with great potential of petroleum resources.However,the characteristics of sedimentary facies and sedimentary evolution model of clastic rocks for the northern margin of Windorah Depression in studied area(Cooper-Erumanga Basin)have been rarely reported up to now,which are restricting the exploration and development in this area.In this study,varieties of methods were employed(e.g.,logging facies analysis,single well sedimentary microfacies analysis,seismic facies analysis and the comparing of interwell sedimentary facies),coupled with geological data,cuttage logging,well logging and seismic data in this region to summarize the characteristics of sedimentary facies and sedimentary models for four groups within Middle and Upper Jurassic in the northern margin of Windorah depression.Four types of logging facies of the Middle and Upper Jurassic in the studied area are identified,such as the toothed box type,bell type,finger type and funnel type.Six types of seismic facies are also recognized.The types of sedimentary facies in braided river,meandering river and delta in studied area are summarized,which can be divided into seven sub-facies and11 microfacies.It has been found that the braided river sedimentary is widely developed in Hutton group,middle-Jurassic series in studied area via the analysis of single well and comparing of interwell sedimentary facies.There also exits coarse sandstone,fine sandstone and siltstone.The hydrodynamic is strong in sedimentary environment,which is characterized by medium-high amplitude of gear box in logging facies and weak continuous phase with sill-like sub-parallel medium-weak amplitude in seismic facies.The Birkhead Formation is overwhelmed with the meandering river and delta sedimentary environment,with fine sandstone,siltstone and shale outcropped.This group displays medium-low amplitude of bell shape,finger shape and funnel shape in logging facies,and continuous phase with sill-like parallel medium-strong amplitude,relatively continuous phase with sill-like(sub)-parallel medium-weak amplitude and filled facies in seismic facies.Braided river deposits are also developed in Adori Formation,which seems to be similar to Hutton Formation.Nevertheless,the hydrodynamic conditions of sedimentary environment are relatively weaker than Hutton Formation.The Lower Westbourne Formation is characterized by meander river sedimentary environment with medium fine sandstone and siltstone.The sedimentary environment is medium-amplitude bell shape,toothed box shape and digit shape in logging facies,and continuous phase with sill-like(sub)-parallel medium-weak amplitude and filled facies in seismic facies.The Upper Westbourne Formation is characterized by delta sedimentary environment with siltstone and shale,and presents low amplitude of bell shape,finger shape and funnel shape in logging facies,and continuous phase with sill-like parallel medium-strong amplitude in seismic facies.The distribution of planar sedimentary facies in Hutton Formation,Birkhead Formation,Adori Formation and Westbourne Formation within the Middle and Upper Jurassic were carried on by using four seismic attributes,which are called root mean square amplitude,maximum magnitude,maximum amplitude and mean amplitude,combined with seismic facies profile analysis.The Hutton Formation is surrounded by a braided river sedimentary and it is characterized by the development of large core bank;Whereas in Birkhead Formation the hydrodynamic conditions of the sedimentary environment declined sharply,and delta and meandering river deposits developed;When it reached the Adori Formation,the high-energy environment returned briefly,and braided river grew;the sedimentary environment returned to be stable and transformed to meandering river-delta environment in Westbourne Formation.In conclusion,the cycle trend is displayed by braided river,meandering river,delta,meandering river and striated river in the middle and upper Jurassic of the studied area.Finally,the "sand-mud encrusted" structure of braided river,the "dual" structure of meandering river and the "sand-mud" sedimentary model of fluvial-delta facies in studied area are established based on the characteristics of planar sedimentary facies,interwell facies and seismic facies.