Characteristics Of Paleogeomorphology And Distribution Of Sandbodies Of The Liushagang Formation In The Weixinan Depression

The Weixinan Depression is a sub-unit of the Beibuwan basin and is a rift basin developed on the Paleozoic basement.The Weixinan Depression has the best conditions for reservoir and the large amount of remaining resources,the Paleogene of the Beibuwan Basin was affected by multiple tectonic movements,and it was strongly reconstructed.At present,the paleogeomorphology of the basin is not exactly determined,which seriously restricts the future oil and gas exploration.In this paper,we use the seismic data and log data to reconstruct the Weixinan Depression of key surfaces of paleogeomorphology,analyze the characteristics of different geomorphic units,and combine the abundant core data and a variety of analytical laboratory data to study the genetic types of sandbodies to summarize the control of paleogeomorphology on the distribution of sandbodies.In this paper,the relative paleogeomorphology of each surface before sedimentation was reconstructed by the method of surface flattening.The erosion thickness is reconstructed by the method of well-seismic combination.In the process of reconstruction,the unconsolidated areas can be reconstructed by the method of surface trend extension after the surface contact conditions are clarified based on seismic data,while the well-to-well areas can be reconstructed by the mudstone acoustic time difference.After reconstruction,the well-seismic mutual correction should be carried out to finally determine the erosion thickness.Based on the principle that the volume of the rock skeleton is unchanged,the inversion method is adopted and the compaction correction is carried out by using Basinmod software.Because different sedimentary environments correspond to a certain depth range,the ancient water depth of the depression is reconstructed.Based on the relative paleogeomorphology,the erosion thickness,compaction correction thickness,and ancient water depth are superimposed to finally obtain the paleogeomorphology of Weixinan Depression.The depression is distributed in NE-oriented,and the original shape is elliptical.The overall landform pattern is a single-breaking step,a half graben depression,which is characterized by multiple sedimentation centers.The third member of the Liushagang Formation in the Weixinan Depression shows the characteristics of shallow water,while the sedimentation center is small.The valleys in the steep slope zone are relatively developed.The faults are of shovel type,with small ditch and gentle slope.They are the characteristics of shallow valley and gentle slope,and large-scale fan delta are developed.The first grade slope break zone is developed in gentle slope zone,braided river delta is developed in the first grade slope break zone,and fault transition zone is not developed in the center of depression.The second member of the Liushagang Formation of the basin is a deep-water feature.The depression has evolved into a central depression.The steep slope zone has deep valleys,developed plate type and broken step type faults,with large ditch and steep slope.It is a feature of steep deep slope of the trench.A small-scale nearshore subaqueous fan is developed,and the gentle slope zone develops the first grade slope break zone.The first member of the Liushagang Formation of the basin,which is characterized by wide water and multiple depressions.The valleys in the steep slope zone gradually become shallower,the faults develop shovel style,and ladder style fault structure in middle part,which changes from shallow slope to steep slope,and sandbodies evolved from fan delta to sublacustrine fan.The second grade slope break zone is developed along the long axis,characterized by delta-sublacustrine fan sediments.The platform area on slope break is dominated by plain deposits,the first grade slope break slope area is dominated by braided river delta,and the sublacustrine fan is developed under the second grade slope break.The gentle slope zone develops a fault slope break zone,and a sublacustrine fan develops below the fault.The central fault transition zone in the depression belt is extremely developed and consists of a series of echelon faults,which control the distribution of sublacustrine fan.