Display omitted •The aged hydrocarbon contaminated site investigation is done using geochemical and geophysical methods.•The low resistivity anomalies in ERT are related to by-products of hydrocarbon degradation.•The ERT and Waxman and Smits models are used to obtain the by-product plume distribution range.•Quantify the amount of hydrocarbon degradation corresponding to the by-product plume. During the investigation of aged hydrocarbon contaminated sites using electrical resistivity tomography (ERT), the low-resistivity anomalies in the detection profile have been widely considered to be caused by the hydrocarbon degradation by-products. However, these low-resistivity anomalies are a comprehensive reflection of the formation’s electrical characteristics. In this study, we used ERT and geochemical methods to investigate a hydrocarbon-contaminated site that has been abandoned for more than 30 years. There are several obvious low resistivity anomalies in the site, which are related to hydrocarbon mineralization. Based on the law of Waxman and Smits, a petrophysical relationship formula suitable for this specific site was fitted, and the true resistivity after inversion was input into the formula to calculate the pore water resistivity. The electrolytic conduction contribution of the pore water was isolated, and the distribution of the by-product plume was determined based on the electrolytic conduction contribution of the pore water. Based on mineralogical identification and geochemical data analysis, the biogeochemical model of hydrocarbon mineralization was clarified. The degradation by-products (Fe2+ and H+) are the controlling factors that lead to changes in the groundwater resistivity. Based on electrochemical experiments and titration analysis experiments, the quantitative relationship between the low resistivity anomalies of the pore water and the degradation by-products (Fe2+ and H+) was established, and the biodegradation model was used to calculate the hydrocarbon degradation corresponding to the by-product plume. The results show that the hydrocarbon degradation corresponding to the by-product plume determined from the ERT detection results in the selected study area is approximately 2.8% of the total hydrocarbon content. Although this is a rough estimate based on a small amount of discrete geochemical data and ERT detection results, it improves our understanding of the natural attenuation state of hydrocarbon-contaminated sites and provides convincing data to support future estimations of the site's natural attenuation time.