Quantitative analysis of basin-scale heterogeneities using sonic-log data in the Yanchang Basin

Abstract

A random medium is used to investigate reservoir heterogeneities in this study. Random media are characterized by autocorrelation functions that allow a construction of spatially anisotropic random structures with different correlation lengths and fluctuation standard deviations. Based on the analysis, we calculate a power spectrum using fast Fourier transform (FFT), which is observed in spatial wavelengths ranging from a few metres to a few thousand metres. Correlation distance and root mean square (RMS) height are directly obtained from the power spectrum. Numerical experiments show that the correlation length and fluctuation standard deviation can cause correlation distance and RMS height undergoing variations. Combining the characteristics of statistical parameters and sonic-log data, we quantitatively analyse the reservoir heterogeneities in the Yanchang Basin. The correlation distance and RMS height of coarse lithofacies in fluvial sandstones interpret a high-energy deposit and strong heterogeneity, affected by different lithological combinations. The correlation lengths decrease gradually from shales, tight sands to gas-bearing sands. Using the sonic-log data from 28 wells in the Yanchang Basin, we compute the isolines of correlation distances and RMS heights for both the He-8 and Shan-1 members in the studied area, which present a correlation with the distribution of gas. This offers an improved foundation for reservoir lateral prediction. © 2010 Nanjing Institute of Geophysical Prospecting.