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The Depositional and Diagenetic Effects on Microbial Carbonates Pore System Expressed on Well Logs

Abstract

The fabric and pore system of microbial carbonates are controlled by depositional settings. However, these primary rock characteristics are diagenetically modified in different ways and degrees during subsequent burial. Consequently, units with the same initial primary characteristics may end up with very different pore systems and petrophysical properties. A proper evaluation of microbial carbonate reservoirs needs to distinguish between depositional or diagenetic pore systems, and this could be very important in well logs. This paper differentiates intervals with different pore systems based on petrographic and well log analysis in Lower Cretaceous microbial carbonate units, in some eastern Brazilian Atlantic Basins. Microbial carbonate units in six wells were sub-divided into intervals defined by different signatures on porosity, resistivity and lithological logs. These intervals also had their pore systems determined by petrography in terms of depositional and diagenetic history. The log signatures and petrographic data were integrated to understand how the microbial and diagenetic fabrics and pore systems are expressed on well logs. Predominantly depositional controls on the pore system are marked by a negative correlation between gamma ray logs and porosity logs. The petrographic analysis shows that the pore system is better connected on large homogeneous microbial fabrics (e.g. thrombolites and large digitate stromatolites) compared to laminated or smaller heterogeneous fabrics. Diagenetic modifications that reduced the porosity by dolomite and silica cements define intervals with large separation between porosity logs and the photoelectric factor log. Intervals with diagenetically enhanced pore systems commonly are associated with dissolution processes showing a positive correlation between the porosity logs and photoelectric factor. In the diagenetically enhanced pore systems the depositional trend between gamma ray logs and porosity logs is obscured. The differentiation between depositional and diagenetically controlled pore systems in microbial carbonates units by petrographic and well log analysis may contribute to a better understanding of these reservoirs. The units detailed in this work indicate that intervals dominated by depositional or diagenetic characteristics have distinct petrographic and petrophysical characteristics, which can lead to different log signatures.