The challenging carbonates from the Pre-Salt reservoirs offshore Brazil: facies, palaeoenvironment and diagenesis

The Santos Basin in offshore Brazil belongs to the major carbonate reservoir province known informally as the Pre-Salt. A detailed analysis of cores, plugs, sidewall samples and thin-sections from two wells in a sector of this basin was undertaken, on an interval corresponding to the upper part of the Lower Cretaceous Barra Velha Formation. Several facies and subfacies types have been classified independently from existing terminologies for these carbonate rocks. The specific features of the studied material are discussed and comparison to other nomenclatures is made. The facies include, namely: laminated argillaceous limestones; in-situ grown carbonates with a range of singular morphologies (of debatable origin), from laminar crust, stromatolitic-like fabric to robust or delicate shrubby structures; intraclastic grainstones/packstones, whose clasts may be mostly remnants of shrubs or differ; spherulitic grainstones/packstones; locally, oncoidal grainstones/rudstones and packstones/ floatstones. These rocks are interpreted as depositional and diagenetic or essentially diagenetic facies. The latter are regarded as originally similar to some of the former ones but now disguised, because they were differently impacted by diagenesis processes over different time periods. Recognizing these effects is relevant, because they produce different reservoir facies characteristics and quality. The best potential reservoirs are some of the depositional and the mostly diagenetic facies. The palaeoenvironmental setting of the studied facies fits into a shallow, low-gradient lake. Charophyte and other possible, unidentified algae, albeit extremely rare, are documented for the first time. The facies pattern shows some cyclicity, suggestive of both water depth and chemistry changes. If the interpretations in this study are correct, they (i) reinforce existing datasets to improve understanding of these facies; (ii) raise caution concerning some facies previously considered as separate depositional products, which in fact may instead result from diagenetic modifications; (iii) stress the relevance of microfacies analysis and diagenetic petrography to help deciphering how diagenetic modifications impact on reservoir facies properties and quality; and (iv) add new issues to the naming and textural classification problems of the Brazil PreSalt carbonates.

Automated summary

The Santos Basin in offshore Brazil is part of the major carbonate reservoir province called the Pre-Salt. The analysis of cores, plugs, sidewall samples and thin-sections from two wells revealed various facies and subfacies types of carbonate rocks, which have been classified independently from existing terminologies. These facies include different depositional and diagenetic characteristics, impacting the reservoir facies properties and quality.