Coquinas from itapema formation, mero field (Pre-salt of santos basin): Microfacies analysis and depositional paleoenvironment

Abstract

In Brazil, the study of carbonate rocks has become very important due to the discovery of oil reservoirs in the Pre-salt interval on the east margin basins (Campos and Santos).The coquinas from Itapema Formation (Barremian/Aptian, Santos Basin) are carbonate rocks interpreted as of lacustrine origin in the literature, and generally represent heterogeneous deposits from the sedimentological and diagenetic points of view. Besides that, these rocks are characterized by great faciological diversity and variability of their porous system. The purpose of this work is to propose a paleodepositional model for the coquinas of the Itapema Formation (Santos Basin) through facies analysis (litho and microfacies), using the identification of the main primary and diagenetic constituents, as well as taphonomy of the shells. For this purpose, 18.2 m of well cores and 56 thin sections were used, collected from cores n.1 and n.2 of well 2-ANP-2A-RJS. As a result, three lithofacies were identified, which were detailed by petrographic characterization, allowing the identification of 12 microfacies. Facies analysis shows that coquinas are essentially composed of disarticulated bivalve shells and their fragments. In smaller proportions there are peloids, ooids, oncoids, enveloped grains and intraclasts of coquinas (sometimes silicified), oolite, and micritic; calcite, silica and dolomite cements; and sometimes some micritic matrix. Four facies associations (microfacies) were established, considering the nature of the bivalve shells found, such as the degree of reworking (fragmentation and rounding); granulometry of the bioclasts; content of ooids, oncoids, peloids and intraclasts; absence or presence of micritic matrix; types of pores and diagenetic features (types of cements and dissolution). Such facies associations belong a tract of facies depicted by four lacustrine scenarios, namely: Emerged uplifted, shallow subaqueous, intermediate subaqueous and deep subaqueous. The high depositional energy processes involved (waves and currents), either tractive or gravitational, resulted in facies practically free of matrix, particularly micrite. Coquina intraclasts, abundantly found, suggest an active tectonics, which promotes the uplift of deposits of coquinas precociously lithified in the margin of an acient lake that later are eroded and resedimented into the basin.