Determination of Total Organic Carbon Content in Shale Formations With Regression Analysis

Abstract

By correcting both the positive and negative ΔlogR separation resulting from the resistivity in organic-deficient shales, the traditional ΔlogR correlation is modified, validated, and applied to determine the total organic carbon (TOC) content in shale formations. The TOC content is determined once the Fisher distribution, which represents the significance of each model, and Student's t-distribution, which denotes the significance of every variable in the models, have achieved values equal to or higher than their respective threshold values at a confidence level of 95%. Using a total of 45 sets of logging measurements, the newly proposed correlation is found to be able to reproduce the measured TOC values with a root mean-squared absolute difference (RMSAD) of 0.30 wt % and root mean-squared relative difference (RMSRD) of 23.8%, respectively. Uranium concentration, apart from interval transit time and resistivity, is found to be key in determining the TOC content in organic-rich shale without other radioactive minerals. By combining the reading of DGR (i.e., the difference between the spectral gamma ray with the radioactivity and the computed gamma ray without uranium), the traditional ΔlogR technique has now been improved and extended to the negative ΔlogR separation resulting from the resistivity in organic-deficient shale higher than that in organic-rich shale.

Geochemical Analysis for Determining Total Organic Carbon Content Based on ∆LogR Technique in the South Pars Field

“…In this regard, the common ∆LogR correlation was reshaped and confirmed by modifying the positive and also the negative isolation of ∆LogR as a result of the resistivity from shales with low values of TOC. As revealed by Wang et al [14], the resistivity in the shales rich in OM was lower than organic-deficient shales. The benefit of this approach was that the value of TOC was reproduced with two aspects of relative and absolute differences within half and whole weight percent, respectively.…”

“…On the other hand, the response of resistivity log in the source rock presented a meaningful relationship between the maturity level of OM and types of fluid. While hydrocarbons generate in source rocks, the resistivity log would increase in value [14]. However, low-values LLD (<10 ohm.m) record for the samples could indicate the immature state of the Gadvan Formation (Figure 4f).…”

“…Building on the work of Khoshnoodkia et al [16], Wang et al [14] pointed out the architecture of well-log data and its preparation as followings. Before the data were analyzed, the depth of the samples was matched.…”

“…In this regard, the SGR approach does not attempt to give enough information and should be combined with other log data to represent the accurate content of the U. Therefore, a combination of SGR and computed gamma-ray (CGR) provides further support for the evaluation of the conceptual premise of source rocks [14].…”

Evaluation of the Total Organic Carbon (TOC) Using Different Artificial Intelligence Techniques

“…The most recent and current studies focus on estimating the TOC by improving the accuracy of ∆logR model [11][12][13] or by applying machine learning techniques [14][15][16].…”