Seismic exploration for tight gas sand: a case study in Sulige Gasfield

Abstract

Abstract Sulige Gasfield is the biggest gasfield found so far in China. The pay zone is He8 formation of lower-Permian. It is braided river sediment and the main reservoir is tight sandstone. The seismic exploration faces the challenges of thin bed, tight gassand and strong lateral heterogeneities. In this paper, the key techniques of data acquisition, processing and interpretation applied in Sulige Gasfield are discussed. On the acquisition side, considering the complex near surface structure, poor shooting and receiving conditions in the study area, the following measures are taken to acquire better raw seismic data. Detailed weathering layer data has been obtained through fine weathering survey such as uphole and LVL refraction survey. Shooting parameter is designed point by point with the help of weathering map. A geometry of long offset, small bin size is applied. Single-sensor recording improves the quality of multicomponent seismic data. In data processing, to both the conventional P-wave and PS data resolution improvement processing means much to the thin bed, the resolution of seismic data is enhanced progressively by cascaded deconvolution, anisotropy processing and the key MC seismic data processing techniques have been developed. As to reservoir characterization, it is a great challenge because of the thin beds and strong heterogeneity. Based on detailed rock physics analysis, the sensitive elastic property is selected for lithology and fluid prediction. The techniques of AVO analysis of PP wave, sensitive elastic impedance inversion and prestack simultaneous inversion with PP data are discussed in this paper. Multicomponent seismic has been acquired during the past few years in Sulige Gasfield, the analysis of full wave attributes and the joint inversion of PP & PS data are applied and have achieved satisfactory results. In the exploration area of Sulige, the ratio of commercial gas flow wells has been increased to 72%, and the success ratio is 81% in development area. Introduction Because of the complex near surface and underground geological conditions, we meet several challenges in seismic exploration in Sulige gas field. In seismic data acquisition, because the weathering layer is thick and varied roughly, the shooting and receiving condition is poor so the quality of raw seismic data is poor; Data processing meets the challenges of low frequency, strong surface nonconsistent and noisy; In reservoir prediction, the challenges are low dominant frequency and thin bed, small differences between pay zone and none pay zone. In response to these difficulties, in seismic data acquisition, the following steps are taken and better raw seismic data is achieved: fine weathering thickness survey with minilog and short refraction survey, point-to-point shooting parameter designing with satellite telemetry little trace gap and long array and single digital geophone receiving. In data processing, combine deconvolution, nonisotropy processing and supporting MC seismic data processing techniques have been developed. As to reservoir characterization, because the reservoir is thin (15–35 m) and strongly anisotropic, it is very difficult to characterize using P-waves alone. Based on the geological conditions of the main pay zone, we first conduct a detailed rock physics analysis, then choose the sensitive factors, choose the best AVO attributes. In order to predict the gas-bearing reservoir, we propose the sensitive elastic impedance inversion and simultaneous inversion. The techniques of full wave attributes analyisis and the Joint inversion of PP & PS data have been applied. With the help of the above techniques, we have reached favorable results.