Abstract
Lacustrine carbonate rocks form important hydrocarbon accumulations along the Brazilian continental margin, some of which are contained in oil fields in which coquinas are one of the main reservoirs (viz. Campos Basin). The complexity and heterogeneity of these deposits make them a challenge in terms of reservoir description. For the necessary classification and paleoenvironmental interpretation of the coquinas, it is essential to evaluate many aspects including biological (such as carbonate productivity), sedimentological (energy regime in the depositional environment, transport of bioclasts, terrigenous supply), taphonomic (fragmentation of shells, abrasion) and diagenetic processes. The facies analysis applied in this study is considered a more appropriate classification approach to understand these coquinas, since it is more flexible and comprehensive than the existing classifications for carbonate rocks. The material investigated here consists of rock samples of the coquinas from the Atol Quarry of the Morro do Chaves Formation (Barremian/Aptian), Sergipe-Alagoas Basin. These rocks that crop out in the Atol quarry complex can be considered as a case study for similar coquinas reservoirs found in the Brazilian continental margin basins. Six sedimentary facies were described, using the main taphonomic (fragmentation of shells) and compositional (presence of micrite and siliciclastic matrix) features as a diagnostic criteria. Two carbonate facies, two mixed carbonate-siliciclastic facies and two siliciclastic facies (mudstones) were identified. From the facies succession, combined with a review of the literature on the subject, the following depositional paleoenvironments were defined: high-energy lake platform, lacustrine delta in a high-energy lake platform and lake-centre. In this paper, a new facies model for the studied coquinas succession is proposed.
Highlights
Lacustrine carbonate rocks are important hydrocarbon reservoirs in Brazil, present in oil fields such as in the Pampo, Badejo, Linguado and Trilha fields, in the Campos Basin
The material for this study consists of the coquinas of Morro do Chaves Formation, Sergipe-Alagoas Basin
Six facies were defined according to the main attributes: taphonomic and compositional as shown in the summary table below (Tab. 1)
Summary
Lacustrine carbonate rocks are important hydrocarbon reservoirs in Brazil, present in oil fields such as in the Pampo, Badejo, Linguado and Trilha fields, in the Campos Basin. These reservoirs, known since the 1970s, were previously studied by Carvalho et al (2000), Castro (2006), Muniz (2013), among others. The material for this study consists of the coquinas of Morro do Chaves Formation, Sergipe-Alagoas Basin These rocks, which crop out at the Atol quarry have been considered as analogues for similar reservoirs in Campos and Santos basins (Kinoshita 2007; Corbett et al 2013; Câmara 2013). The coquinas of the Morro do Chaves Formation at the Atol Quarry can contain high amounts of siliciclastic material, reaching values up to 50% of siliciclastic matrix
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