Research on the Effectiveness Classification and Fluid Identification Methods of Complex Paleoreef Reservoirs Based on Pulse Neutron Logging

With the increasing exploration of the Bohai Bay area, deeply buried metamorphic rock paleoreef reservoirs have become a key focus of exploration. These reservoirs typically consist of fractures and dissolution cavities, exhibiting strong heterogeneity and characteristics of low porosity and permeability. Their pore structures are extremely complex, which brings considerable uncertainty to the classification of reservoir effectiveness. At the same time, the contribution of the rock skeleton to the resistivity logging values in metamorphic rock paleoreef reservoirs is much higher than that of formation fluids. Consequently, conventional fluid identification methods based on resistivity logging are significantly limited in these types of reservoirs. Thus, the effectiveness classification and fluid identification of metamorphic rock paleoreef reservoirs pose a considerable challenge to logging personnel.

This paper conducts an in-depth analysis of the logging response characteristics of pulse neutron logging in metamorphic rock paleoreef reservoirs, specifically the counting rates of inelastic gamma rays, the decay rates of gamma rays over time, and the counting rates of thermal neutron capture gamma rays. It proposes a novel method for fluid property identification in metamorphic rock paleoreef reservoirs using pulse neutron logging.

The analysis reveals that the gamma counting rate of the far-detector of pulse neutron logging is significantly influenced by high-density minerals such as biotite and pyroxene. A method for correcting the gamma counting rate based on lithology calibration for high-density rocks is thus introduced. When the reservoir contains gas or is a gas layer, the gamma long-short source distance counting rate for inelastic collisions shows a distinct “intersection” characteristic. This feature can effectively identify gas layers. When the reservoir is a liquid-bearing layer, a novel fluid identification method based on the reconstruction of the rock skeleton's relative atomic weight curve is proposed. This method first optimizes the multi-mineral model based on X-ray diffraction analysis to calibrate mineral content, then calculates the relative atomic weight of minerals based on the mineral element content, and finally derives the theoretical relative atomic weight curve for the rock skeleton along the entire well section. The effective reservoir is identified using the intersection feature between this theoretical curve and the macroscopic thermal neutron capture cross-section curve obtained from pulse neutron logging. Additionally, oil-water layers are discriminated based on oil and gas shows recorded in the field.

This innovative method for correcting the gamma counting rate based on lithology and reconstructing the relative atomic weight curve for fluid identification has been successfully applied to the effectiveness classification and fluid identification of the Paleoproterozoic paleoreef reservoirs in the BZ26 and BZ27 oilfields in the Bohai Bay. The logging interpretation accuracy exceeds 85%, which partially addresses the limitations of conventional resistivity logging in evaluating Paleoproterozoic paleoreef reservoirs.