Sequence Stratigraphy And Development Of Sand Bodies In The Permian Of The Eastern Slope Of Fukang Sag

The eastern slope of the Fukang sag is located at the eastern Junggar Basin.There was a large scale uplift in the south of the area in the Bogda Mountain at Permian.Due to this tectonic uplift the strata in the southeastern part of the study area has been overlapped along the slope.In the western part of studded area,complete Permian stratigraphic section has been developed,the strata is thinner towards the eastern part of the study area,and Pingdiquan Formation as well as the Wutonggou Formation are generally deposited at that time throughout the area.Overall,the thickness of the Permian strata varied throughout the study area due to the thrusts caused by the orogenic crumpling.There were five sequence interfaces that have been identified in the Permian age at the eastern slope of Fukang sag.On the basis of well log data,mud logging data&seismic data,combined with the tectonic evolution of the study area.From bottom to top,the first order interface was C/P,third order sequence interface was P₂j/P₂p,secondary sequence interface was P1/P2,first order sequence interface was P₂p/P₃wt,secondary sequence interface was P/T.The Permian of the Fudong slope was divided into three secondary sequences,which are named as SQP1,SQP2 and SQP3 respectively from bottom to top,corresponding to the lower Permian system,the middle system and the upper system.There were four third-order sequences that have been observed,which are named as SQ1,SQ2,SQ3 and SQ4 from bottom to top.SQ1 sequences corresponded to the Jingou Formation,SQ2 corresponded to the Jiangjunmiao Formation,SQ3 corresponded to the Pingdiquan Formation,and the SQ4corresponded to the Wutonggou Formation.Based on the maximum flooding surface,The SQ1,SQ2,SQ3 and SQ4 were divided into rise half-cycles and descend half-cycles in the four third-order sequences,correspond to the stages of tectonic compression and the springback tectonic event respectively.The characteristics of provenance evaluation system of eastern slope in Fukang sag is researched based on the residual paleo-geomorphic restoration map and the seismic reflection characteristics,by analyses of the weight mineral composition,the core rock characteristics,the ZTR value contour map and the sand percentage content contour map.It was believed that the slopes were mainly developed in four major provenance systems at Permian.There were the northwest provenance system,the northeast provenance system,the north Santai source system and the southern provenance system.The source system of the northwest was raised from the Baijiahai,and the sediments were transported from northwest to southeast and deposited in the northwest of the study area.The source system of the northeast was transported by the Bogda Mountains and the sediments were transported from northeast to southwest,which were deposited in the Shaqiu area.The source system of the northern Santai is Santai high itself,along which sediment were deposited along the northern part of the north Santai.In the latter part of the Middle Permian the southern provenance area began to provide sand after the large-scale uplift of the Bogda Mountains.Under the constraint of the characteristics of sequence stratigraphic framework and provenance system,research has been done by analyzing the geological signs.After that the single well sedimentation characteristics was analyzed.It was believed that the slope belt mainly developed fan delta,braided river delta,shore shallow lake and beach and bar sedimentation in Permian.Six main sections of eastern part of the study area were selected to analyze sand body development,based on the characteristics of sedimentary facies,characteristics of single well,percentage of sandstone and conglomerate,seismic attribute extraction and residual paleo geomorphic restoration.On that basis he clear distribution of the sand body of the Pingdingquan Formation and the Wutongsong Formation were evaluated.It was known that the northern provenance source area as well as the southern provenance area corresponded to the development of fan delta deposition,the northwestern Baijiahai source area as well as the northeastern part of the Kelameili mountain corresponded to the development of braided river delta deposition,in the shore shallow lake developed beach and bar sand and in the semi-deep lake developed turbidite fan.As the latter part of the Middle Permian corresponded to the first vigorous uplift of the Bogda Mountains,the development scale of the sand bodies in the Wutonggou Formation were larger than in the Pingdiquan Formation.It was believed that the rise half-cycles deposition period,as the short-cycle structure is squeezed,the water deepened,and the squeezing action lead to more supply of the source,the size of fan body was larger,and the semi-deep lake was larger,the turbidite were more developed;the descend half-cycles sedimentation time was in the period of tectonic rebound,decrease of the orogenic crumpling lead to less supply of sources,smaller size of fan body,shallower water,semi-deep lake range reduced,less turbidites sand body,shore-shallow lacustrine area was larger and beach bar deposition was more developed.