ICE 2022


The success of the Pre-salt carbonate reservoirs of Brazil is attributed to the high-quality reservoirs composed of coquina lithofacies of the Itapema Formation (ITA), Rift Sequence, and the ‘Post-rift Sequence’ of the Barra Velha Formation (BVE) formed from in-situ carbonate lithofacies (e.g., spherulestones and shrubstones) and reworked lithofacies (calcilutites, calcarenites and calcirudites). In the Mero Field, both formations display moderate-to-high porosity-permeability character. In addition to the lithofacies complexity and the high-degree of porosity-permeability heterogeneity, the effect of diagenesis (especially silicification) on the reservoir properties is not yet fully understood. The objective of this study is to characterize the occurrence and distribution of silicified zones in a well from the Mero Field using whole core, core plugs, sidewall core plugs and over 120 petrographic thin-sections and integrating these results with the petrophysical analysis of well-logs and seismic data. Petrophysical analysis using wireline logs (including BHI, NMR and ECS) together with XRF data, focused on characterizing porosity distribution in relation to the silicification. The silicified zones observed in the studied well are mainly concentrated in the bioclastic calcarenite and calcirudite lithofacies (coquinas) of the upper ITA and the calcilutites and calcarenites which occur in basal part of the lower BVE. Well-logs responding to a silica zone in the bioclastic ITA, show a high-porosity 10m thick interval, where the silica partially or completely fills original moldic porosity, but not the interparticle (mainly remaining) porosity. In the BVE, quartz, chalcedony and chert occur in centimetric tabular bands associated with spherulestones and spherulitic calcarenites also in a 10m thick interval. Unlike in the ITA the silicified zone of the lower BVE displays low porosity; the lateral continuity of the silica bands may strongly affect vertical permeability and form baffles/barriers. In addition, dolomite fills residual pores in calcarenites after silicification. The presence of these silicified zones has a clear effect on the reservoir elastic properties. Low silica content yields a significant reduction in the bulk density and P-wave sonic velocity in logs that is being investigated in the 3D seismic volume, assisted by a neural network acoustic impedance inversion.