--> Diagenetic Characterization of the Riachuelo Formation, Cretaceous of Sergipe Basin - Brazil, Using Isotopes Data (δ18O and δ13C), Raigosa Diaz, Mary L.; Pereira, Egberto; Rodrigues, René, #90100 (2009)

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Diagenetic Characterization of the Riachuelo Formation, Cretaceous of Sergipe Basin - Brazil, Using Isotopes Data (δ18O and δ13C)

Raigosa Diaz, Mary L.1
 Pereira, Egberto1
 Rodrigues, René1

1Departamento de Estratigrafia e Paleontologia, Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil.

Due to the importance that present the Albian carbonates as reservoir in the
Offshore Brazilian Basin, was carried out an isotopic analysis of carbonates in the Riachuelo Formation, to identify the diagenetic environment that enhanced or inhibited the porosity of these carbonates. The isotopic analysis was performed on whole-rock samples collected in the Inhumas and Carapeba quarries. Most of the section described in Inhumas Quarry is composed of rudstone with oncolites, pellets and intraclast. The carbon and oxygen isotopes signature shows a very homogeneous behavior in this facies. The δ18O values range from -5 ‰ to -4 ‰, which indicates that the cementation took place in hotter water. The dolomite’s levels have δ18O more positive signatures, pointing out to a dolomitization linked to marine environment with higher salinity. In the dolomite microfacies, δ13C and δ18O present positive signatures ranging from -1 ‰ to +1, which confirm that the dolomitization took place in a marine environment with high salinity. In the Carapeba Quarry, when the matrix is composed of sparry calcite, the δ18O values are more negative, ranging from -5 ‰ to -3.5 ‰, showing the same range of the Inhumas Quarry. The δ13C values in the whole Carapeba section presents range from -2 ‰ to +2 ‰, which suggest an initial cementation in the marine environment. In some microfacies, the matrix dolomitization is the most important diagenetic products and it shows a negative δ18O signature (-3 ‰), indicating that the dolomitization process was associated to meteoric environment. The δ13C shows an average of +1 ‰ and +3 ‰, in the previous described samples. This signature indicates that the precursor cement of the dolomitized matrix, was deposited on the marine environment. The isotopic and diagenetic study carried out in the Cretaceous carbonates of Sergipe-Alagoas Basin indicates the dolomite facies of Inhumas Quarry as the interval with the best characteristics porosity. Its origin is considered to be linked to the enrichment of Mg in a higher saline marine environment. In Carapeba Quarry, the absence of permeability and porosity is due to the strong process of compaction that the sediments were submitted after deposition.

AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil