Study On Seismic Identification Of Tight Gas Sandstone

In this paper,the research target is the main gas-bearing formation HE8 in X block of S gas field.Based on the geological and seismic characteristics of HE8,the velocity and density of tight gas sandstone are estimated by DEM-Gassmann-Brie rock physical model,and the geological model is established.Then,AVO?Amplitude versus offset?forward modeling and analysis are carried out.The influence of porosity,gas saturation,gas-bearing sandstone thickness and surrounding rock on the AVO anomaly type of tight gas sandstone are discussed.And also,the AVO response of tight gas sandstone is compared with that of conventional gas sandstone.AVO forward modeling provides certain guidance for seismic identification of tight gas sandstone in this area.In order to compensate for the low resolution of post-stack seismic data in this area,in this paper,pre-stack pure P-wave inversion is used to obtain the vertical P wave reflection coefficient R_p,which has higher resolution and is good for the commonly seismic inversion technology.P/S wave velocity and density of logging data are calculated or reconstructed to get elastic parameters v_p/v_s,???11???14?v_s and gas-bearing indicator M₁.The cross-plot analysis of these parameters and indicator shows they are sensitive to tight gas sandstone of HE8 in this area.After that,pre-stack hybrid iterative inversion is carried out for the real seismic data,from which the inversion elastic parameters and reconstructed gas-bearing indicator mentioned above proves to be sensitive in the seismic identification of tight gas sandstone of HE8 in this area.In addition,based on the results of pre-stack hybrid iteration inversion,the fluid factor?35?F is constructed and used for seismic identification of tight gas sandstone.In order to obtain the relatively sensitive seismic attribute--density gradient,the pre-stack noise-attenuation density inversion of the real seismic data is also carried out.Through these inversions,a variety of seismic attributes are constructed.The identification of the tight gas sandstone by these attributes indicates that the attribute v_p/v_s has the highest coincidence with the wells in the identification of tight gas sandstone of HE8 in the study area.It is slightly better than the other attributes in gas-bearing sensitivity.By setting the threshold of the attribute v_p/v_s,the good gas-bearing areas can be identified and delineated.In order to improve reliability of identification,the gas-sensitive attributes found are combined to make a gas-bearing time-thickness map,of which the coincidence rate with the wells in the identification of tight gas sandstone of HE8 in the study area is high,so it can also be used to identify and delineate areas with good gas content.The areas defined by attribute v_p/v_s,and time-thickness map show good consistency,indicating that there is better gas-bearing condition in the eastern and western parts than that in the middle part of HE8 of the study area.