Sedimentary processes and paleoenvironment reconstruction of the Barra Velha formation, Santos Basin, Brazilian pre-salt

Abstract

The Barra Velha carbonates are widely known for their complexity regarding facies, diagenetic features, and depositional history. The absence of recent analogues representing the depositional setting of these rock formations leaves many uncertainties concerning the sedimentary process that generated them. The present work aims to propose a model for the depositional environment of the Barra Velha Formation based on sedimentary features identified through facies description and interpretation. The employed data consists of 645 m of core descriptions, 2300photomicrographs, 1201 plug samples, and 44 thin sectionsdistributed throughout 34 wells in two Santos Basin oil fields. Fourteen facies were identified and classified into four categories: in situ, microbial, reworked, and altered. The in situ facies are composed of spherulites, shrubs, and mud. Sedimentological evidence points out that spherulites and shrubs precipitated within layers of mud, as a post depositional process. Microbial facies have several characteristics that refer to stromatolites and occur laterally and vertically associated with in situ facies. The spatial association between them indicates that in situ and microbial facies formed in the lower energy portions of the lake. Reworked facies were formed by grain fragmentation and transportation, indicating an energetic lake-littoral zone, where grainstones and packstones formed in the shallowest portions, and wackestones formed in the less energetic portion of the littoral zone. The altered category comprises the weathering profiles. These profiles resemble soils and developed in response to periods of strong evaporation, when lake brines would become dense enough to infiltrate into the groundwaters, leading to the precipitation of the dolocretes in the mixing zone. The spatial distribution of the facies corroborates the sedimentological interpretations. In situ and microbial facies are frequently found in the structurally lower regions of the study area. In contrast, reworked facies and weathering profiles are more common on structural highs.