Quantitative Evaluation of Oil‑bearing Properties in Volcanic Reservoirs Using Density‑neutron Logging: a Case Study from the Carboniferous C4 Reservoir, Block 1

This study presents a practical workflow for quantitatively evaluating the oil‑bearing potential of heterogeneous volcanic reservoirs by integrating density and neutron logging responses. Traditional petrophysical models, such as the Archie equation, often fail in volcanic formations due to complex pore structures, conductive minerals, and dual‑porosity systems. To address this, we introduce a multi‑factor empirical model that combines the density‑neutron porosity difference—a well‑established indicator of light hydrocarbons—with a Fracture‑Porosity Composite Index (FPCI). The FPCI is a composite index that integrates sonic-derived porosity with the resistivity invasion ratio and serves as an empirical proxy for matrix storage capacity and fracture connectivity. Based on core‑log‑production data from the Carboniferous C4 reservoir in Block 1 (10 wells, 263 core samples), reservoirs are classified into three types: Type 1 (best), Type 2 (intermediate), and Type 3 (poor). The classification is based on independent petrophysical parameters (core porosity, MICP data, fracture density), with production data used only for post‑classification validation. The model shows good agreement with actual production history (R = 0.73, n = 22, 95