Abstract
In the evaluation of source rocks, the total organic carbon (TOC) is an important indicator to evaluate the hydrocarbon generation potential of source rocks. At present, the commonly used methods for assessing TOC include △ log R and neural network method. However, practice shows that these methods have limitations in the application of unconventional intervals of sand-shale interbeds, and they cannot sufficiently reflect the variation of TOC in the vertical direction. Therefore, a total organic carbon (TOC) evaluation model suitable for shale and tight sandstone was established based on the effective medium symmetrical conduction theory. The model consists of four components: nonconductive matrix particles, clay minerals, organic components (solid organic matter and hydrocarbons), and pore water. The conductive phase in the model includes clay minerals and pore water, and other components are treated as nonconductive phases. When describing the conductivity of rock, each component in the model is completely symmetrical, and anisotropic characteristics of each component are considered. The model parameters are determined through the optimization method, and the bisection iteration method is used to solve the model equation. Compared with the classic TOC calculation method, the new model can evaluate the abundance of organic matter in shale and tight sandstone, which provides a new option to assess the TOC of rocks based on logging methods.
Highlights
In assessing the exploration potential of the shale oil system, the first task is to clarify the abundance, type, and maturity of the source rock organic matter [1,2,3]
The argillaceous content is generally less than 20%, and the pores are composed of conductive formation water and nonconductive hydrocarbons, while, in shale source rocks, clay minerals and nonconductive organic matter are the main components
The calculated results are acceptable compared with the measured data, but the total organic carbon (TOC) trends predicted by the three methods show apparent differences in the same interval (Figures 9 and 10)
Summary
In assessing the exploration potential of the shale oil system, the first task is to clarify the abundance, type, and maturity of the source rock organic matter [1,2,3]. Previous studies classified source rocks based on multivariate analysis and established the petrophysical model of source rocks They treated the organic matter as a part of the rock, and the logging curve response was a function of organic matter content [12, 13]. Based on the logging responses to organic matter and hydrocarbons, Liu et al [14, 15] proposed the variable coefficient Δ log R method by introducing the proportional coefficient K and wave impedance, thereby improving the accuracy in predicting TOC This method can evaluate the free hydrocarbon content in the pores of shale source rocks. The newly established model is applicable to different regions by adjusting the parameters
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