Research On Structural Characteristics And Shale Gas Preservation Conditions Of Wufeng-Longmaxi Formation In Dingshan Area

This paper studied the structural characteristics of field and target formation,periods of structural movement and structural evolution history by field geological survey,core observation,imaging logging(FMI),seismic data,acoustic emission experiment of rock and other technical means of shale in Wufeng formation and Longmaxi formation in Dingshan area,southeast Sichuan,combining with the regional structural background.Theoretical guidance of its was structural geology,reservoir geology,petroleum geology,unconventional oil and gas geology and so on.The structural genetic model of the study area was discussed.Further research on the preservation conditions of shale gas was carried out.Finally,the evaluation system of the preservation conditions of shale gas was put forward,and the favorable areas for preservation were selected.The main conclusions were as follows:The structure of the study area was mainly composed of NE fold and NNE,NW direction faults,and its structural morphology was controlled by NNE direction Qieyushan fault.Field geological survey shows that the main fractures were conjugate shear fractures of high angle plane "X" type and the "X" type shear fractures in low angle profile in the field.A small number of regional shear fractures and tensional fractures were developed.The core fractures were mainly shear fractures,and there were also tensile fractures,shear tension fractures,interlayer fractures,and so on.The types of core fractures were various.The attitude of fractures in field were consistent with fractures produced by the core observation and imaging logging interpretation.The periods of structural movement of the study area were studied by field outcrop,core,imaging logging,fracture filling homogenization temperature test,burial thermal evolution history and acoustic emission experiment of rock and regional structural characteristics,combined with regional structural characteristics.The result showed that there were 3 periods of structural movement in area.The first period of structural movement was dominated by the compression of the Jiangnan Xuefeng uplift from the southeast to northwest in the middle-last Yanshan movement,developed the NE anticline and the NE direction fault system.The second period of structural movement was dominated by the compression of northeast to southwest as a combined effect of the rise of Xuefeng uplift,the central Guizhou uplift and Chuanzhong uplift in the end of Yanshan movement and the middle stage of the Himalayan movement,developed the a series of northwest direction thrust structures.The third period of structural movement fonned in the late Xishan movement,at which time the study area was still in environment of the structural compression,but dominated by the slow uplifting.The structural stress of third period inherited the previous structural compressional stress from northeast to southwest,but the structural stress was relatively low intensity,short duration and unstable.The main structural parts had a weaker influence,mainly due to the reformation of the previous structural system.The shale gas preservation conditions were controlled by various factors in the study area.This study focused on structural factors.On the basis of fully considering the complex geological conditions in the study area.It refered to the research results of the existing evaluation methods and indexes of shale gas preservation conditions.Combined with the exploration and practice expeience in the study area,the distance from the target formation to the outcrop denudation area,the distance to the Qiyueshan fault,buried depth,the structural position,internal structural conditions,fracture development degree of the target formation,pressure coefficient were optimal selected as the main factors of affecting the shale gas preservation.A comprehensive evaluation system of the conservation was established,and a multi-factor superposition and comprehensive geological analysis method was used to predict favorable areas for shale gas conservation.The comprehensive analysis showed that the I favorable area was located in a deep-buried and low-slope area with a buried depth between 3000m and 4500m,which was far from the Qiyueshan fault zone and the outcrop denudation area of the target formation,the fracture development degree of the area was ? or ? grade,the secondary fault angle of that area was less than 50°,the angle between the fault azimuth and the maximum principal stress orientation in horizontal direction of that area was greater than 60°,and the area had a large predicted pressure coefficient.