A Method on the Application of Pulsed Neutron Logging for Gas Reservoirs Identification of Buried Hills in the Bohai Sea

In recent years, with the discovery of the huge production potential of buried hill reservoirs, oil and gas exploration targeting buried hills deserves in-depth research. Pulsed neutron logging offers significant advantages in evaluating gas reservoirs in conventional reservoirs. By combining and calculating the gamma count rate information from detectors at different source- to-detector distances, the fast neutron elastic scattering cross-section (FNXS) method can effectively identify gas reservoirs. However, the complex lithology and low effective porosity of the Bohai Sea buried-hill reservoirs—composed of quartz, feldspar, biotite, calcite, dolomite, and illite—result in significantly reduced accuracy when applying this method for gas reservoirs identification in these reservoirs. To address this, we propose a gas saturation evaluation method tailored to the Bohai Sea buried-hill complex lithology. Leveraging geological information and gamma counts from pulsed neutron tools, this method integrates theoretical analysis and numerical simulations, validated through a case study of a well in the Bohai Sea buried-hill. Given the diverse mineralogy but limited elemental variability in buried-hill formations, we replace the traditional mineral-weighted matrix FNXS calculation with a direct elemental-content-based approach, eliminating uncertainties inherent in converting elemental data to multi-mineral compositions. Numerical simulations of the buried-hill environment established a fitting formula linking matrix FNXS to the ratio of near-to-long detector gamma counts, enabling characterization of the comprehensive formation FNXS. Qualitative gas reservoirs identification is achieved by comparing the comprehensive FNXS curve with the matrix FNXS curve. A gas-saturated FNXS curve is then derived using porosity data, and reservoir gas saturation is quantified via a saturation formula incorporating these three curves. Analysis of 24 core samples from Well X revealed that matrix FNXS calculated from elemental content exhibited an average relative error of less than 1.5% compared to values derived from X-ray diffraction (XRD) determined mineralogy. The FNXS curves effectively flagged gas reservoirs, with computed gas saturation of 40–60%, consistent with gas logging and sampling results, confirming the method’s reliability.