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Título

DEPOSITIONAL, DIAGENETIC, AND TECTONIC SETTING OF THE BARRA VELHA FORMATION IN THE NORTHERN SECTION OF THE SANTOS BASIN

Texto do resumo

The pre-salt carbonate deposits of the Santos Basin constitute one of the most important petroleum fields in the world. Since their discovery, studies over the past twenty years have focused on the depositional and tectonic events that have shaped the heterogeneous reservoir quality. The Aptian interval of PreSalt is recognized by Barra Velha Formation that composed by biotic and abiotic carbonates, associated by Mg-Silicates precipited from alkaline lacustrine water. This study concentrated on a cored well in the Santos Basin, drilled in a structural high within the Outer High of the Basin. Sedimentary, petrological, and geochemical analyses were used to identify the carbonate lithologies, syngenetic and diagenetic constituents, porosity types, and tectonic features. Moreover, seismic and borehole image logs served to identify sedimentary boundaries between the lower and upper Barra Velha, describe fault zones, and characterize porosities along the non-recovered intervals. The studied well core rocks consist of three lithofacies types: 70% shrubstones, 25% spherulestones, and 5% calcilutite in the first recovered interval, while 65% spherulestones, 14% shrubstones, and 21% calcilutite are present at depths in the second recovered interval. The early and deep-burial diagenetic constituents are calcite, magnesian calcite, microcrystalline, lamellar, and blocky dolomites, and microcrystalline silica. On the other hand, hydrothermal minerals recognized are saddle dolomite, macrocrystalline quartz, spherical and fibro-radial chalcedony, pyrite, and barite. The tectonic features interpreted suggest a predominant strike of the normal fault, which formed the structural high, trending NE-SW. The fractures observed in the well log and borehole image (BHI) consist of two main groups, with NW-SE and N-S strikes, and a fracture density that decreases from the top towards the log base. Associated with the fracturing, the brecciation in the first interval includes concentrations of chaotic and mosaic breccias, forming a cataclastic fine matrix, while in the second interval, there is a trend toward the accumulation of crackle breccia and rocks showing increasing preservation. Six pore types were defined using the cast well core, thin section, and borehole image log, classified as inter- and intraparticle pores, fractures, channels, vugs, and caverns. High-porosity zones are concentrated in the upper interval of the BVE, characterized by features with a thickness equal to or greater than 1 meter. The distribution of lithofacies in this study, compared with previous Barra Velha Formation environment models, reveals a significant influence from fluctuations in lake levels, controlled by alternating humid and arid periods. These climate-driven variations directly impact water supply and the geochemical composition of the lake, leading to the precipitation of syngenetic and eodiagenetic minerals. During deep burial, high temperature and pressure conditions alter the texture of blocky dolomite. The fracture density and classification of breccia suggest variations in rock deformation within the fault zone, which serve as conduits for acidic hydrothermal fluids that facilitate hypogenic karstification.

Palavras Chave

Carbonate deposits; Fault zone; Hydrothermal minerals