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Application of Stratigraphic Slice Technique Based on Seismic Inversion in Predicting Subsalt Carbonate Reservoir

Abstract

In recent years, with the discovery of a series of extra-large subsalt carbonate reservoirs along Brazil's South Atlantic coast, subsalt carbonate reservoirs, especially their geophysical prediction techniques, have attracted wide attention. Although the subsalt formation imaging is clear, there are still some shortcomings in the existing 3D PSDM data in the study area. First, the main frequency of seismic data in the target layer is low, less than 20hz, and the frequency band is narrow. Secondly, the shielding effect of the salt is resulting in the weak reflection energy of the subsalt carbonate beach that is not conducive to internal characterization. Therefore, we have adopted a series of measures to ensure carbonate reservoir’s clear distribution characterization .The wavelet reconstruction is the seismic inversion preprocessing technique. By comparing the seismic data before and after wavelet decomposition and reconstruction, we can find that the energy of high frequency component of seismic data has been enhanced. When calibrating a single well, well B drilled a set of high impedance igneous rocks 60 meters thick at bottom of salt. The distribution area of igneous rocks is smaller and less continuous after wavelet reconstruction, which is consistent with geological reasons for igneous rock formation in the study area. Also the tuning effect of anhydrite and bottom of salt can be distinguished after wavelet reconstruction.In view of the fact that there are few wells drilled in subsalt carbonate reservoir in the study area, and the conventional CSSI is not obvious for revealing the heterogeneity of carbonate reservoir, we fuse velocity model and well calibration constrained model to establish the initial impedance model in the inversion process. By inversion, we can clearly reveal the lithofacies distribution and heterogeneity of presalt’s carbonate reservoirs. According to the characteristics that the low impedance distribution reflects the better physical properties of carbonate reservoirs, we can see that the reservoir physical properties of the western structure are generally better than those of the middle structure, and the eastern structure is not well judged because of the seismic data quality. In order to better reveal the heterogeneity of carbonate reservoir under salt, the inversion impedance body is sliced along the bottom of salt. The subsalt strata are divided into several slices from top to bottom, and each slice is analyzed one by one. The subsalt carbonate reservoirs with medium and low impedance characteristics mainly distributed in the western and central structures, and the remaining low impedance distribution areas may be the sedimentary distribution area of clastic rocks in depression period. On the vertical section,the carbonate reservoirs with excellent petrophysical characteristics mainly concentrated in the presalt sag strata.