The Controlling Factors And Seismic Prediction Methods Of Tight Gas Reservoir "Sweet Spot" In Sulige Area

The Sulige gas field in the northern Ordos Basin is the largest gas field ever discovered in China.The main gas-bearing reservoirs in the Upper Paleozoic Shan 1-He 8 members are characterized by low porosity,low permeability,low abundance,strong heterogeneity and “wide and thin” distribution.The gas and water distribution pattern in the gas field is so complicated that the productivity in adjacent areas could be significantly different.Therefore,it has important significance to summarize the distribution characteristics and controlling factors of the “sweet spot” and look for effective seismic prediction methods for the gas exploration and development.Considering the exploration difficulty in the Upper Paleozoic tight gas reservoir,the reservoir sedimentology,natural gas geology,logging geology,sedimentary petrology,geophysics and some other theory and technical methods,together with analysis of core,thin section,drilling,logging,gas testing data and the pre-stack,post-stack 3D seismic data were applied in this paper for analyzing the “sweet spot” distribution characteristics and controlling factors from aspects of structure,sedimentary facies,source-reservoir-cap rock,and fracture.Then the S well area in southeastern Sulige which has typical exploration issues are detailly studied to analysis the control effect of different geological factors on gas reservoirs distribution and the productivity variation.In order to predict the “sweet spot” spatial distribution,the seismic methods of predicting the gas reservoir distribution and productivity variations are studied.The main innovative knowledge and conclusions are as follows:(1)Based on the comprehensive and thorough analysis of Sulige gas field and S well area,we confirmed the effect of structure,reservoir,sedimentary facies,source rock,cap rock and fracture on high permeability “sweet spot” under low permeability conditions.The sedimentary microfacies is the most important factor for controlling the sweet plot distribution.This factor made the spatial diversity of reservoir characteristics through influencing the sandstone reservoirs structure,sandstone reservoirs distribution and the diagenetic reformation process.In order to predict the distribution of “sweet spot”,this paper combined the pre-stack inversion,frequency division attenuation and some other methods with sedimentary microfacies,and verified the seismic prediction results through the geological characteristics.(2)To solve the prediction difficulty in gas-bearing sandstone reservoir,the average amplitude attribute,absorption attribute and post-stack inversion were combined to accurately depict the distribution range of channel sand body.On that basis,we carried out pre-stack Vp/Vs inversion to identify gas-bearing sand body in channels and higher productivity gas-bearing sand body in high energy channel.(3)Since the production capacity is so complex,the paper using the multi-parameter tight gas reservoir productivity prediction method that based on the 3D seismic data and gas testing data.By comparing the intercept,gradient characteristics of the gas-bearing sandstone with the productivity data,we discussed the AVO anomaly characteristic with different gas-layers.Based on the sensitivity of Poisson's ratio to the variation of gas saturation,the inversion of pre-stack gas saturation could be used to accurately characterize the variation of sand body thickness with high gas saturation.The pre-stack P-wave anisotropic fracture prediction method is combined with post-stack attribute prediction method to identified the location of fractures.(4)The productivity prediction on the basis of high frequency gas-bearing attenuation theory were done within the frequency domain.The higher the main frequency attenuation intensity is,the better the productivity capacity is.In general,the area with high productivity capacity could get energy tuning 15 Hz earlier than that with low productivity capacity.The comparison between gas-bearing anomaly area and sedimentary microfacies characteristics suggested that the high production anomaly area usually located in the diara of the high energy channel,the middle-low production anomaly area located in the flank of the high energy channel.The geophysical prediction results are in good agreement with the geological controlling factors.(5)In this paper,we combined the sedimentary microfacies,sand body structure,gas generating strength of source rocks,reservoir physical property,cap rock thickness and other geological controlling factors together with sand body thickness,gas-bearing sandstone thickness,dominant frequency attenuation degree,fracture density and other seismic prediction results for forecasting the distribution of sweet spot.Four favorable wells locations were suggested from the result of prediction.