Continental carbonate research has become a major interest during the last decade due to the giant hydrocarbon reservoirs in the South Atlantic, located offshore Brazil (Campos, Santos and Espírito Santo Basins), but also... [ view full abstract ]
Continental carbonate research has become a major interest during the last decade due to the giant hydrocarbon reservoirs in the South Atlantic, located offshore Brazil (Campos, Santos and Espírito Santo Basins), but also offshore Angola. They comprise an Aptian continental succession formed in a rift-sag setting. One of the most common features observed in the so-called Pre-Salt succession are shrub-like fabrics, considered one of the most promising reservoir lithologies in the South Atlantic prospects.
The shrub-like morphologies are very common in Tivoli continental carbonate deposits. An integrated methodology, including Nuclear Magnetic Resonance (NMR), Mercury Intrusion Porosity (MIP), Micro-Computer Tomography (μCT), porosity and permeability measurements, petrography and SEM analyses, was introduced to gain insight into the complex variations of the pore network within the shrub facies in Tivoli.
The shrubs present different morphologies characterized by intergrowth and interdigit growth framework porosity. The shrubs were affected by minor diagenetic alteration, evidenced by thin spary calcite cement covering the structures, and limited dissolution. Vertical and horizontal plugs yield porosities from 0.8 to 20.9% and permeabilities from 0.001 to 5255mD. The limited connectivity of some samples relates to facies heterogeneities, partly inherited from the sedimentological context, rather than to diagenetic processes.
The MIP dataset showed two main pore-size classes, interpreted as microporosity and macroporosity. Integrating MIP with NMR allowed to split the samples in four groups: 1) unimodal response in both NMR and MIP, 2) unimodal response in MIP and bimodal NMR curves, 3) bimodal response for both NMR and MIP with a “minor” microporosity signal, 4) bimodal curves for both NMR and MIP, with well developed peaks for both macro- and microporosities.
Independent of the shrub morphologies, the four groups were plotted in a poro-perm chart and showed a coherent correlation. The analysis of the Reservoir Quality Index (RQI), calculated as 0.0314√k/ϕ, showed that unimodal samples had the best reservoir characteristics, which decreased towards bimodal samples. The results evidenced that the amount of microporosity in the pore network decreases the quality of the reservoir. Under SEM, the microporosity (less than 10μm) is observed in the core of the shrub stems, but not in all.
Understanding complex variation in pore network is the key to improve reservoir description and exploitation. So, NMR, MIP and RQI are important tools to perform shrub pore-typing, valuable to predict the distribution of petrophysical properties in reservoirs.
