Abstract
Abstract This paper presents the results of a petrographic and diagenetic study of the laminated limestones of the upper part of the Aptian to Albian Crato Formation, northeast of Brazil. The applied techniques were optical microscopy, cathodoluminescence and scanning electron microscopy (SEM) coupled to a wavelength-dispersive spectrometer (WDS). Petrographic analysis has revealed that most of the laminated limestones are calcilutites with a dominance of a micritic matrix, indicating a low-energy depositional environment. Microstructures such as microfaults, microfractures, microslumps, and loop bedding were observed. Based on textural, structural and paleontological features, seven microfacies were recognized: massive limestone, limestone with parallel laminations, limestone with undulated laminations, limestone with slumps, limestone with loop bedding, limestone with ostracods and limestone with peloids. In addition, the processes of cementation, dissolution, replacement, recrystallization and compaction, which are related to different diagenetic stages, were also recognized. The diagenetic constituents found in the sections include calcite, pyrite, silica and sulfates. We can conclude that a large part of the microstructures (microfaults, microfractures, microslumps and loop bedding) can be related to local seismicity, probably due to the reactivation of the Patos Shear Zone. The diagenetic constituents indicate an early to late diagenesis (eogenetic, mesogenetic and telogenetic stage).
Highlights
Since the discovery of hydrocarbons in carbonate rocks of the so-called pre-salt layer in the marginal basins of Brazil, the interest increased in the search for rocky exposures as possible analogues of these reservoirs
The laboratory analysis is comprised of petrographic descriptions of 44 thin sections using a model BX-41 Olympus petrographic microscope at the Department of Geology, at Universidade Federal de Pernambuco (UFPE)
The studied outcrops are located in the northern part of the Araripe basin, in the upper part of the Crato Formation
Summary
Since the discovery of hydrocarbons in carbonate rocks of the so-called pre-salt layer in the marginal basins of Brazil, the interest increased in the search for rocky exposures as possible analogues of these reservoirs. The upper part of the Aptian to Albian Crato Formation, a well-exposed and preserved carbonate succession in the Araripe Basin (NE Brazil), is probably a good analogue. These carbonate rocks present features interpreted as algalic growth (Neumann 1999, Catto et al 2016), similar to those occurring in the reservoir rocks of the pre-salt layer, as well as structures that can be interpreted only as event products of diagenesis (Neumann 1999). Its origin and evolution are related to the tectonic events of the Late Cretaceous, leading to the rupture of the Supercontinent Gondwana and the subsequent formation of the South Atlantic Ocean
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