Evaluation of the total organic carbon of source rocks in lacustrine basins using the variable-coefficient ΔLgR technique — A case study of the Xujiaweizi Fault Depression in the Songliao Basin

Abstract

To lay the foundation for the exploration of shale gas, we calculated the total organic carbon (TOC) of Shahezi Formation source rocks in the Xujiaweizi Fault Depression in the Songliao Basin by using the measuring data and the logging curves. Because the source rocks in the study area were formed in a lacustrine basin, they are characterized by strong organic heterogeneity, making it difficult to objectively characterize any changes in the TOC of the underground source rocks based on discrete and limited sampling. In addition, because the source rock is relatively rich in shale and poor in organic matter, the logging response features of high natural gamma, medium-high interval transit time, and medium-low resistivity, making it easily identified. However, because the logging parameters to predict the TOC of source rocks are not universal, it is impossible to establish a prediction equation that is universally applicable. To solve the above problems, we used the variable-coefficient [Formula: see text] technique to predict the TOC of the source rocks in the study area. We defined the two key parameters that affect the TOC prediction in the classic [Formula: see text] technique as the undetermined coefficients, and the coefficients were determined according to the logging and geologic data of the individual wells. The application results indicate that the variable-coefficient [Formula: see text] technique has an average relative error of 17.5% in the prediction of the TOC, which is 16.1% lower than that of the classic [Formula: see text] technique. Thus, the prediction results can effectively reflect the vertical variation in the TOC of source rocks. Based on the logging evaluation results for the TOC of source rocks in 35 wells throughout the study area, the thickness of the high-quality source rocks in the fourth member of the Shahezi Formation was mapped. The thickness of the high-quality source rocks in the fourth member of the Shahezi Formation is generally [Formula: see text]. There are two centers of greater thickness in the plane, with the maximum thickness of more than 70 m.