Petrographic characterization of Emborê Formation based on data from2-CST-1-RJ and 9-BRF-1D-RJ wells (onshore area of Campos Basin, SE Brazil)
DOI:
https://doi.org/10.11606/issn.2316-9095.v18-121942Keywords:
Embore Formation, Campos Basin, Sedimentary PetrographyAbstract
This study aimed to contribute to sedimentologic and petrographic characterization of the Emborê Formation (Upper Cretaceous to Holocene, Campos Basin, Continental Margin of Southeastern Brazil) based on the analysis of sandstones from two deep wells located in the onshore portion of the basin (2-CST-1-RJ and 9-BRF-1D-RJ, both performed by PETROBRAS). The goal was to increase knowledge about textural and compositional aspects of the Emborê Formation, still poorly studied and presenting a significant groundwater potential, comprising the main aquifer of Rio de Janeiro State, focus of the research, resulting in the current article. After sedimentologic characterization of the core samples from 2-CST-1-RJ well and trough samples from 9-BRF-1D-RJ well, thin sections from selected 2-CST-1-RJ core samples and 9-BRF-1D-RJ lateral samples were described. The sedimentologic logs of 2-CST-1-RJ and 9-BRF-1D-RJ wells are very similar and show the predominance of sandstones with interbedded mudstones. Sandstones are generally fine to coarse, moderately to poorly sorted, grayish to greenish, with angular to subangular grains, and are composed essentially of quartz and feldspars, with a significant presence of granitic lithoclasts in samples from 9-BRF-1D RJ well. These rocks were assigned to São Tomé Member of Emborê Formation. Sandstones are classified as arkose, subarkose and lithic arkoses, showing low mineralogical maturity. Textural aspects also indicate low maturity. In most of the samples, it was described the presence of clay matrix (classified as epimatrix), related to feldspar dissolution and subsequent infiltration into the pores. The porosity reaches about 20% and its origin is mainly secondary, generated by matrix contraction, grain dissolution or grain fracturing. Pore connectivity is low. Low textural and mineralogical maturities of sandstones indicate proximity of
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