The petrophysical characteristics of Macabu formation (Aptian, Campos Basin) continental carbonates: Core and special well logs (UBI and NMR) analysis integration

Abstract

To interpret carbonate heterogeneities and understand the correlation between these rock features and their petrophysical parameters, an integrated dataset formed by thin section, core sample description, image log, and nuclear magnetic resonance analysis were used. This interactive approach enables the comparison of depositional and acoustic image log facies and their permo-porous behaviors. The advantage of the proposed integration lies in the ability to expand the interpretation of depositional facies to the entire imaged well interval. The results highlight the occurrence of six depositional facies, four ultrasonic image log facies, and two intervals with different permo-porous behavior present at Macabu Formation (Aptian, Campos Basin). Based on core sample and thin section analysis, three in situ carbonates were described: F1 – shrubstones, F2 - spherulitic grainstone-wackstone and hybrid forms, F3 - stevensitic claystone with spherulites, and others like transported facies: F4 - intraclastic grainstone-rudstone, F5 - laminated Detrital packstone-grainstone, and F6 - laminated dolomudstone. According to ultrasonic signal amplitude and structural patterns, such as descriptive criteria, the ultrasonic image log facies characterization resulted in: UIF1 - bright-heterogeneous, UIF2 - dark laminated, UIF3 - bright massive, and UIF4 - dark dendritic. The NMR well log analysis helped us divide the formation into two main zones, the first with good reservoir behavior and the other with poor behavior. This approach resulted in the correlation between facies and reservoir quality, indicating those with better fluid flow potential, in which facies F1, F2, F4, and UIF4 showed the best results. Finally, the integration between the different facies analysis demonstrated that several lithofacies may have the same appearance in the acoustic image, leading them to integrate the same ultrasonic image facie, this occurs due to their similarity in roughness.