High-resolution Sequence Stratigraphy And Sedimentary Evolution Of The Shahezi-Yingcheng Formation In The Southern Part Of The Changling Fault Depression

The deep faulted depression in Longfeng Mountain-Dongling area has great potential for natural gas exploration.In order to clarify the distribution law of highprecision sequence stratigraphy and sedimentary microfacies of Shahezi-Yingcheng formation in Longfeng Mountain-Dongling area,the sequence stratigraphy and sedimentary evolution are studied by using drilling,core,thin slice and other data.The research results are of great significance for deepening the sequence division and sedimentary evolution model of the fault depression in the continental lacustrine basin,and provides geological support for the further exploration and development of the deep fault depression in the study area.Sequence frames were constructed by comprehensive analysis of seismic,core,and well logging data,incorporating well shock and removing clear sequence schemes.Shahezi-Yingcheng formation mainly identified four third-order sequence boundaries,to divide the strata of the study area into three third-order sequences.It also identified three maximum flooding surface to defined each sequence structure.Each third-order sequence was further divided into three fourth-order sequences.SQ1 is mainly distributed in Longfeng Mountain area,SQ2 is the most widely distributed in the study area,and SQ3 is basically distributed in Dongling area.According to the stratigraphic division of Shahezi-Yingcheng formation in Longfeng Mountain-Dongling area,Ying Ⅴ-Ⅶ sand formation is divided into Sha 2 member,and Ying I-IV sand formation is divided into Ying 1 member.By finely analyzing core,well logging,and seismic data,we can define the types of depositing microphase,and establish single well and connected well microphase microphase studies,which eventually make clear the development of the depositing microphase in the Shahezi-Yingcheng Formation in the vertical direction.Fan delta facies,lacustrine facies,Nearshore Subaqueous Fan facies and volcanic facies are mainly developed in Shahezi-Yingcheng formation,and several subfacies and microfacies are identified.The maximum flooding surface were developed During the sedimentary periods of SQ1-2,SQ2-2 and SQ3-2.The lower part of the fourth order sequence mainly develops tributary interbay and pre delta mud deposits,and the lithology is mainly mudstone mixed with sandstone.The lower part of the fourth order sequence in Dongling area develops explosive facies,airborne subfacies and volcanic sedimentary facies.The upper part of the fourth order sequence mainly develops distributary channels and braided channel deposits,with occasional occurrences of the lower upper subfacies of the effusive facies and the thermal clastic subfacies of the explosive facies,showing an overall trend of progradation and aggradation.By compiling a series of contour sheet,such as stratigraphic thickness,sand ratio and gravel ratio,and combining with the analysis of paleogeomorphological background during the sedimentary period,Sedimentary facies distribution map of each section of fault depression is prepared,and finally the sedimentary evolution law in the area is summarized.The distribution of delta blossoms and main channel is controlled by the growth faults in the same sedimentary period.There are many NE-trending fans in Longfeng Mountain-Dongling area,and the fan bodies are superimposed and merged during the SQ2-HST deposition period.During this period,the fan body size reaches the maximum,and then the fan area decreases gradually.The sedimentary evolution of the fault depression in the study area is simultaneously restricted by three main factors:tectonic activity,volcanic activity,lake basin evolution and lake level change.The distribution pattern of residual strata of Yingcheng formation was formed in the uplift and contraction period at the end of Yingcheng formation.The initial stage of the fault depression is mainly characterized by the uneven filling of volcanic rocks and clastic rocks of the Huoshiling formation;during the strong peak of the fault depression,the lake basin rapidly expanded and deposited the mudstone and fan delta of the Shahezi formation;during the duration of the fault depression,the Yingcheng formation was further deposited and filled;during the uplift and contraction period,the Yingcheng formation on the east and west sides of the study area was strongly eroded.