Electrofacies Characterization in Lacustrine Coquinas and Hybrid Deposits from Rift Phase: Pre-Salt, lower Cretaceous, Campos Basin, Brazil

Abstract

This study presents electrofacies models for two wells with cores and sidewall cores of Coqueiros Formation, lower Cretaceous of Campos Basin. The WELL-1 is located in deep water of the south of the basin and is composed by bioclastic carbonate, siliciclastic and hybrid rocks. The electrofacies modelling distinguished four electrofacies, which are correlated to the lithofacies coquina, hybrid rock, siliciclastic mudstone, and sandstone. The WELL-2 is situated in ultra-deep water of the north of the basin and is essentially constituted of bioclastic carbonates. The electrofacies modelling for this well differentiated four electrofacies, which are correlated to the lithofacies mudstone, coquina with carbonate mud, coquina with moderate to high degree of shell fragmentation and/or oriented shells, and poorly sorted massive coquina with variable degree of shell fragmentation. This study showed that it is possible to distinguish, with high level of accuracy, bioclastic carbonate rocks from siliciclastic and hybrid rocks based on the well logs. The electrofacies analysis distinguished particle-supported carbonate rocks from mud-supported ones. It was also possible to separate bioclastic carbonate rock with different taphonomic criteria, such as organization and degree of shell fragmentation. The electrofacies model showed to be a useful tool to interpret geological intervals in wells without rock samples. Nevertheless, it will never represent all the numerous depositional facies, and thus it can not completely replace the rock ampling in detailed sedimentological and reservoir studies.