Caracterização de minerais pesados do Maastrichtiano em poços da porção offshore rasa da bacia de Barreirinhas, margem equatorial brasileira

Abstract

The Brazilian Equatorial Atlantic Margin, consisting of a set of sedimentary basins located along the north-northeast coast of Brazil, represents a new exploration frontier due to recent discoveries of hydrocarbons in deep-water turbidite deposits. Heavy mineral characterization is a precise tool for interpreting source-to-basin relationships, enabling the reconstruction and understanding of sediment history from initial erosion to final burial. This study aimed to identify and characterize the heavy detrital minerals present in the Maastrichtian section of the Barreirinhas Basin, based on analyses of samples from four wells in the shallow offshore portion of the basin. The research emphasized the morphological analysis of minerals (integrity, habit, sphericity, roundness, etc.) obtained from fine and very fine sand fractions. The methodology included mineral concentration by density separation using bromofluoromethane (CHBr3), removal of magnetic minerals, and the preparation of eight thin sections from core samples. These samples were collected at intervals corresponding to the Areinhas/Ilha de Santana formations, associated with shallow marine and coastal platform environments, and the Travosas Formation, representing deposits from a deep marine environment. The analyzed samples revealed a heavy mineral assemblage, including zircon, staurolite, kyanite, tourmaline, rutile, apatite, amphibole, and epidote. These minerals were grouped into two main sets based on their depositional environments: in the shallow marine and platform environments (Areinhas/Ilha de Santana formations), the most representative minerals are zircon, classified as ultra-stable, and amphibole, considered unstable. In the deep marine environment (Travosas Formation), there is a predominance of abundant amphiboles, along with the presence of zircon. The relative abundance and stability of the heavy minerals highlight the role of paleoclimatic conditions and sediment transport in the formation of mineral assemblages. In the Areinhas/Ilha de Santana formations, the predominance of highly rounded zircon suggests long-distance transport, while the presence of more common and subangular amphiboles indicates proximity to source areas. In the Travosas Formation, the abundance of subangular amphiboles suggests deposition close to source areas, while the less frequent subrounded zircons indicate prolonged transport. Factors such as physical weathering, prevalent in arid climates, favored the preservation of unstable minerals like amphiboles, while the high stability of zircon ensured its concentration in both depositional settings. The analysis correlated the stability of the minerals with their potential source rocks, indicating that both zircon and amphiboles predominantly derive from acidic igneous or metamorphic rocks. Rounded zircons in both sedimentary environments may, in turn, originate from reworked sediments. The physical and chemical characteristics of the minerals provide valuable information about paleogeographic and paleoclimatic variations. Thus, the integration of mineralogical, sedimentological, and depositional data proved essential to understanding the dynamics of sedimentary systems and their evolution over geological time.