Sequence Stratigraphy And Sedimentary Characteristics Of Jurassic System In Wulungu Depression

With the decrease of oil and gas exploration in eastern China,more and more attentions have been paid to exploring oil and gas reservoirs in the western areas.Wulungu Depression,characterized by deserts,is located in the northeast of Junggar Basin,which were mainly divided by three secondary structure units-Hongyan step-fault zone,Suosuoquan sag and Shibei sag.Wulungu Depression is expected to be potential oil and gas reservoirs,with the development of structural,stratigraphic-lithologic traps and signs of oil and gas.In spite of the importance,very few progresses have been achieved on sequence stratigraphy of this depression.Thus,it's necessary to establish the isochronous sequence stratigraphic framework,understand the sedimentary facies distribution characteristics and provide useful hypotheses for potential oil and gas reservoirs.We mainly research on the Jurassic of Wulungu Depression and mainly work on three targets in this study:identify the sequence stratigraphy boundaries to establish the isochronous sequence stratigraphic framework using logging and seismic data,recognize the types of sedimentary facies to understand the sedimentary facies distribution characteristics using drill core and logging data,predict potential oil and gas reservoirs using fluorescence.In the thesis,the sequence stratigraphy boundaries have been recognized and the Jurassic of this area can be divided into one first-order sequence,two second-order sequences,four third-order sequences and nine systems tracts.Those sequences were recognized by observing unconformity surfaces,lithologic interfaces and abrupt change interface of log curves.For example,obvious unconformity develops between Triassic and Jurassic in Wulungu Depression because of the Indosinian movement occurred in late Triassic.The unconformity surface can be traced across the Depression as medium amplitude reflection and continuous seismic reflection events on the seismic sections and abrupt change of lithology from sandy conglomerate to mudstone intercalated with siltstone.The unconformity surface is labeled as the base of the first-order sequence(JSB1).Also,obvious regional unconformity develops between Cretaceous and Jurassic in Wulungu Depression due to the Yanshan movement occurred in late Jurassic.The unconformity surface can also be easily traced through seismic sections and as abrupt change of lithology.The unconformity surface is labeled as the top of the first-order sequence(KSB1).SQ1 firstly thickens and then reduces from north to the south;SQ2 uniformly thickens from south to the north;and SQ3 reduces from north to the south,firstly thickens and then reduces from west to the east.Six sedimentary facies,including braided river,braided river delta,alluvial fan,fan delta,lake and river-floodplain,have been recognized and sedimentary facies distribution characteristics has been established using drill cores and log curves.Of these,SQ1 is mainly characterized by braided river and braided river delta facies;SQ2 is mainly characterized by braided river,braided river delta and lake facies,braided river delta front and shore-shallow lake sub-facies;and SQ3 is mainly characterized by braided river and braided river delta,braided river delta plain sub-facies.SQ1-LST sand body has been proposed to be potential oil and gas reservoir.The thick sand bodies of SQ1-LST have good connectivity,which is necessary for accumulating oil and gas.