Study On Shale Gas Geological Conditions Of Qiongzhusi Formation In The South Of Sichuan Basin And Its Periphery

The organic-rich shale covers large areas of the Lower Cambrian Qiongzhusi Formation in the southern part of the Sichuan Basin.It is a very important source rock in the Upper Yangzi region of southern China.Taking the Qizhuzhusi Formation in southern Sichuan for research object,this thesis aims to conduct in-depth study on the shale reservoir properties of the Qiongzhusi Formation and make a division of favorable areas of shale reservoirs in the study area,on the basis of full use of sedimentology and geological conditions theoretical methods.Following achievements have been gained in this study.Divides sedimentary facies in the study area was divided based on the rock type and sedimentary structure and other phase marks.The south of Sichuan and adjoining areas can be divided into the coastal and continental facies from west to east.It can be further divided into near-shore,shallow-water shelf and deep-water shelf.The main rock types are paleolithic siltstone,sand debris,argillaceous siltstone and silty mudstone.The shallow shelf mainly develops argillaceous siltstone.silty mudstone and calcareous siltstone.According to X=ray diffraction analysis indicates that the content of brittle minerals in Ganluo region,Leibo region and Zunyi region is generally high,mostly between 50%and 70%,clay content between 25%and 35%,and carbonate content below 10%.The shale gas geological conditions and reservoir characteristics of the shale in the study area were forther studied.The overall performance of the Qiongzhusi Formation was characterized by low porosity and low permeability.The porosity was mainly concentrated in the range of 0.2%to 2%,and the permeability was mainly concentrated in the range of 0.004 to 0.012×10-3?m2.Shale pore types in the study area include organic pores,intergranular pores,intragranular pores,dissolved pores,and microfractures.Among of them,the pores of organic matter are the most developed which are mostly honeycomb and the sizes range from 3 nm to 300 nm.The organic carbon contents range from 0.33%to 8.66%,organic carbon content ranged from 0.5%to 2%,and 55%accounted for more than 2%of the sample,accounting for 40.74%.Most of these were concentrated in the deep-water continental shelf deposition environment.The type of kerogen is dominant,and the degree of thermal evolution is high.Ro is between 2.04%and 4.96%.Under the conditions of multiple conditions,such as under the condition of the development of rich organic shale,the structure activity is not severe,there is proper fracture,the reservoir is overpressure state,the rock brittle mineral content is high,and the shale reservoir has good development potential.This paper analysis the main controlling factors,make shale gas reservoir evaluation criterion and optimize the favorable area.Combined with the shale gas reservoir classification criteria in China and abroad,the shale burial depth,TOC,Ro,etc.conditions were used to divide the favorable area of the Qiongzhusi Formation shale gas reservoir.Among them,the best favorable areas locate in the east of study area,and the west of study area and partly of east area comes second.