Sequence Stratigraphy And Sedimentary Facies Of The Upper Karamay Formation In Junggar Basin

Eastern Wu 2 Area of Karamay Oilfield is located in Baijiantan area,Karamay City,and the footwall of Karamay fault on the northwest edge of the Junggar basin.The Triassic Upper Karamay Formation,which is in comfortable contact with the underlying strata,is a set of oil-bearing rock which consists of medium-coarse sandstone,sandy small conglomerate,sandy mudstone and mudstone.Under the guidance of high resolution sequence stratigraphy,the sequence stratigraphic framework of the whole region is established through 27 well-tie sections.Based on the log response of different sedimentary microfacies,the plane distribution character of the sedimentary facies within the sequence stratigraphic framework is researched,and the facies sequence and pattern of this area are summed upThree categories of seven subclasses short term cycles are divided from the upper Karamay formation,which are,the rising asymmetric type: low accommodation(A1,A2),high accommodation(A3);the falling asymmetric type: low accommodation(B1);symmetric cycle sequence : rising half-cycle based and incompletely symmetric type(C1),almost symmetric type(C2),descending half-cycle based and incompletely symmetric type(C3).Based on the pattern of short-term base-level cycles,the characteristics of the interface and the variation of phase sequence,five medium-term base-level cycles(SQ1-SQ5)are identified.Based on the stacking pattern of medium-term base-level cycles,two long-term base-level cycles(SSQ1-SSQ2)are recognized.A complete transgression and regression is formed.High resolution sequence stratigraphic framework has been established in the region.Under the control of high resolution sequence stratigraphic framework and source direction,the characteristics of microfacies' vertical distribution are well analysis.During the period of SQ1-SQ3 cycle,the long-term base level keeps rising to the apex,meanwhile the accommodation space is increasing continuously,which is in the stage of undercompensation.The vertical sedimentary sequence is: fan delta plain-fan delta front-prodeltafan delta front-fan delta plain.The study area develops braided distributary channel,overbank sand,underwater distributary channel,and frontal delta mud.The thickness of the sand body from base to top is thinning gradually,which shows an obvious retrograde sequence.During the SQ4-SQ5 medium-term base-level cycle,the long-term base level reaches the highest point and begin to decline gradually.In the meantime,the accommodation space is in the over-compensation stage.The research shows the sedimentary microfacies include underwater distributary channel,braided distributary channel and channel bar.Inside the fan delta plain subfacies,the braided channel is mostly banded and divided into several waterways by channel bar.Channel bar is the main body of the delta plain,has potato-like and lump-like shape.The overbank sand is scattered in the area,and the interchannel is less developed,and mainly distribute like ellipse.Inside the fan delta front subfacies,the underwater channel shapes like strip band.The overbank sand distributes steadily over the whole area.