Abstract
The object of this study is magnesian clay minerals present in carbonate rocks of the post-rift phase of the pre-salt in the Santos Basin. These rocks developed in an Aptian-age alkaline lacustrine environment. This study summarizes the formation of clay minerals associated with different lithotypes in a range of 19 m and a depth of more than 5100 m. They were characterized from petrographic analysis by optical microscopy, X-ray diffraction (total sample and clay fraction), and modeling by Newmod®; and examined and analyzed by scanning electron microscopy. An approach based on identifying lithotypes and characterization of microsites allowed us to understand the occurrence of different clay minerals. Kerolite was the most abundant mineral in the sampled range. It occurs in lamellar aggregates under greater preservation of the original rock lamination and in association with spherulites and shrubs. The Stv/Ker mixed layers occurs in the same association, and formed finer unlaminated aggregates associated with the more intense dolomitization and silicification processes. Saponite occurs associated with detrital minerals forming clayey levels intercalated with microcrystalline carbonates. Fluids with a high Mg/Si and pH < 9 favor the precipitation of kerolite. The increase in pH during diagenesis may be responsible for the formation of Stv/Ker mixed layers.
Highlights
The sampling was performed by focusing on more clayey levels, and the term used here is lithotype rather than facies, since the approach here referred to the main carbonate constituent of the level, and did not have a direct correspondence with the sedimentological and/or stratigraphic processes for this rock sequence
The key control on the precipitated magnesian silicates is the composition of the water from which these minerals formed, as is the case of saponite that occurs in episodes of greater sedimentary input to the lake, and kerolite and Ker/Stv mixed layers in periods without this influence
Kerolite is the most abundant clay mineral in the sampled interval, and occurs as lamellar aggregates of high birefringence that constitute the lamination of the rock, free of transformation processes
Summary
Magnesian clay minerals are a diverse group that can occur in sedimentary and non-sedimentary contexts [1]. The knowledge about magnesian clay minerals from the lacustrine environment comes mainly from studies of large deposits such as Amargosa (USA) [2], Jbel Rhassoul (Morocco) [3], and the Madrid Basin (Spain) [4], where they are markers of specific physicochemical conditions. The magnesian clays belong to two families: the 2:1 minerals (kerolite, stevensite, and saponite); and the fibrous minerals (sepiolite and palygorskite). Saponite and palygorskite contain more aluminum than stevensite, kerolite, and sepiolite. For this reason, they are generally considered as resulting from the reaction between detrital minerals and Si- and Mgrich solutions [5], while stevensite, kerolite, and sepiolite are formed by direct precipitation from solutions
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
