Study On Shale Gas Seismic Detection Method In Pengshui Block

The upper ordovician wufeng formation and the lower silurian longmaxi formation in Sichuan basin and its peripheral area are deposited in shelf facies viscous flow anaerobic environment and are abundant in graptolite and organic black shale.Compared with American shale gas basin,Sichuan basin and its peripheral area wufeng formation and longmaxi formation enjoy unique geological conditions,mainly reflected that Silurian shale is formed in earlier age,has long history of deposition and experiences complicated tectonic movement and thermal history.Recent geological survey shows that Sichuan basin and its peripheral area is rich in shale gas,with main controlling factors including organic shale development degree,deposition condition,gas content and fracture.The southern mountains in Pengshui Block enjoy obvious undulated landform.Carbonatite exposed area is characterized by low SNR(signal noise ratio)of seismic data,great synclinal curvature and poor imaging effect of two-dimensional seismic data.The gradual denoising method is adopted and its principle is to extract the weak and effective signal in removed “noise” and feed it back to seismic data,thus ensuring the energy of effective signal to the maximum under denoising.With adoption of optimal energy pre-stack time migration technique,imaging effect of shale gas two-dimensional seismic data obtains marked improvement.Based on the analysis of well logging curve in well data in study area,corresponding P wave impedance curve is calculated.There is a great difference of P wave impedance curve in high TOC shale bed section and other shale bed sections.Therefore,high TOC shale bed section can be clearly recognized through P wave impedance curve.Well logging constrained sparse spike inversion method is applied in the prediction of the thickness of high-quality shale.The thickness of high-quality shale in the upper ordovician wufeng formation and the lower silurian longmaxi formation in Pengshui Block ranges from 21 m to 25 m.For normal compacted formation,formation compaction effect becomes stronger with the increase of depth and the overlaying pressure,leading to the decrease in rock porosity.Seismic wave has high propagation velocity in such rock with decrescent porosity.The porosity of rock in abnormal high pressure formation is greater than that of rock in normal compacted formation.Thus,the density of rock in abnormal high pressure formation is declined and seismic wave has slow propagation velocity.On the basis of analysis of precise velocity,Fillippone method is applied in the prediction of the pressure coefficient in the upper ordovician wufeng formation and the lower silurian longmaxi formation.As a result,the pressure coefficient in overall study area varies from 0.9 to 1.15.The shale gas is in general preservation condition.Rock physics analysis shows that since the shale in PY1 well(well depth of 2136 m to 2160m)silurian longmaxi formation bottom to the ordovician wufeng formation has various beneficial features such as high TOC,large effective porosity,high brittleness and lower minimum horizontal closing pressure,the shale is the advantageous exploration target.Besides,the analysis also shows that the VPVS,the lame constant*density and the density respectively belong to the sensitive elastic parameter of TOC,the brittleness and the effective porosity,and scalar properties of the minimum horizontal closing pressure can qualitatively reflect the level(high or low)of the minimum horizontal closing pressure.Prestack inversion technique can be used for the inversion of various sensitive elastic parameters and for the quantitative interpretation.The threshold level of VPVS which is used to judge the level(high or low)of TOC content is 1.67;the threshold level of lame constant*density which is used to judge the level(high or low)of brittle mineral content is 33Gpa*g/cc or the threshold level of VPVS which is used to judge the level(high or low)of brittle mineral content is 1.67.To sum up,the shale in the upper ordovician wufeng formation and the lower silurian longmaxi formation in Sichuan basin and its peripheral area enjoys favorable condition in thickness,preservation,gas content and brittle mineral content.In Sangtuoping syncline in Pengshui Block,the practicable class I favorable area and class II favorable area respectively spread out 263.01Km~2 of land and 122.77Km~2 of land.