Leonardo Borghi

The Aptian pre‐salt lacustrine carbonates represent the most significant oil exploration target along the South Atlantic margin. Dolomitization of the Santos Basin Barra Velha Formation (BVF) is critical for reservoir quality, yet its origin, patterns, and evolution remain poorly understood. This study characterizes the petrographic features of core samples from the Sururu field, identifying four distinct dolomite types—blocky, lamellar, spheroidal, and saddle—within the diagenetic framework of the BVF and evaluating their impact on reservoir quality. Dolomite precipitation occurred predominantly during eodiagenesis through multiple pathways, mainly involving the replacement of Mg‐rich clays or cementing secondary pores. Blocky dolomite formed mostly through consumption of Mg‐clay precursors, evolving from scattered floating rhombs to coalescent mosaic textures. Spheroidal dolomite is associated with the early replacement of peloidal Mg‐clays, whereas lamellar dolomite filled shrinkage pores within the laminated clay matrix. Saddle dolomite represents a later‐stage cement, commonly linked to hydrothermal fluids. Scanning electron microscopy (SEM)–energy‐dispersive spectrometer (EDS) data indicate Ca‐enriched, nonstoichiometric compositions, with Mg mol fractions ranging from ∼0.37 to 0.45, as well as the presence of Si–Al‐rich clay remnants within crystals, supporting a close association between dolomite formation and Mg‐clay precursors. These different dolomite fabrics exert contrasting effects on reservoir properties. Blocky crystals occurring in early, partially coalescent stages, as well as lamellar dolomite associated with shrinkage pores, tend to coexist with matrix‐dissolution porosity and display a wide range of permeability values. In contrast, the development of mosaic blocky dolomite and late saddle cement significantly reduce both porosity and permeability. This study enhances the understanding of dolomite formation in alkaline lacustrine systems and provides valuable insights into reservoir quality and heterogeneity in the pre‐salt succession.

Silicification is a major diagenetic process in carbonate reservoirs, profoundly influencing mineralogy, pore-system architecture, fluid-flow pathways, and reservoir quality. Although chert is widespread in the Aptian Barra Velha Formation of the Santos Basin pre-salt province, its origin and relationship with fluid circulation, karstification, and reservoir evolution remain poorly constrained. This study investigates cherts within a 145.65 m cored interval distributed across five cores using an integrated multiscale workflow combining core description, petrography, UV epifluorescence, SEM-EDS, portable and micro-XRF, computed tomography (CT), micro-CT, fluid inclusion petrography, and coupled silicon and sulfur stable isotope analyses. Four chert geometries were identified: tabular, nodular, domal, and irregular. Tabular and nodular cherts preserve primary carbonate fabrics and record replacement-dominated silicification followed by progressive recrystallization from opal and chalcedony to microcrystalline and macrocrystalline quartz. Domal cherts occur exclusively within microbialite-bearing facies and preserve filamentous and reticulated fabrics within microcrystalline silica, suggesting localized early silica precipitation associated with microbial substrates. In contrast, irregular cherts are concentrated within fractured, brecciated, and karstified intervals and are characterized by diffuse silicification, macrocrystalline quartz cementation, sulfate mineralization, and strong structural control. Five karstified intervals, representing approximately 37% of the studied succession, contain the most interconnected pore systems identified by CT and micro-CT, with porosity increasing from approximately 3.1% in continuous cherts to 26.7% in brecciated intervals. These zones coincide with irregular cherts, sulfate-rich mineral assemblages, and fault-controlled permeability pathways, locally producing bulk-density values up to 3.37 g/cm³. Silicon isotope compositions range from δ³⁰Si = −0.116‰ to +1.476‰, with tabular cherts consistently exhibiting the heaviest values (up to +1.476‰) and irregular cherts the lightest (down to −0.116‰), whereas sulfur isotope compositions (δ³⁴S = 8.0–33.4‰) reveal isotopically heterogeneous sulfate-bearing fluids. Associated mineral assemblages—including barite, celestine, fluorite, titanite, goyazite, dawsonite, and pyrite—record the circulation of chemically evolved mixed fluids through structurally enhanced pathways. The integrated dataset supports a multistage fluid-flow model involving early silica precipitation, replacement silicification, karstification, structurally focused fluid migration, sulfate mineralization, and late hydrocarbon charging. Chert formation is interpreted as the product of a long-lived diagenetic system in which fluid mixing, structural permeability, progressive silica recrystallization, and fault-controlled fluid flow collectively governed silicification and the development of reservoir heterogeneity in the Barra Velha Formation.

Carbonate reservoirs of the Brazilian Pre-salt province are characterized by complex pore systems strongly controlled by diagenetic processes, particularly dolomitization and silicification, whose impact on reservoir quality remains scale dependent and poorly constrained. This study investigates porosity evolution in spherulestone- and shrubstone-hosted reservoirs of the Barra Velha Formation (Santos Basin, Brazil) through the integration of multi-scale 2D and 3D characterization techniques. A total of 130 core intervals were analyzed using petrography, Machine Learning (ML)–based image analysis, Routine Core Analysis (RCAL), and micro-computed tomography (μCT). Four reservoir petrofacies were defined based on mineralogical composition, pore types, and diagenetic overprint, and three silica-body geometries (tabular, nodular, and irregular) were identified, showing strong relationships with host-rock lithology. Results indicate that dolomitization, particularly in petrofacies 1 and 2, enhances porosity by promoting interparticle and interframework pore systems. In contrast, silicification, dominant in petrofacies 3A and 3B, generally reduces visible macroporosity through pore-filling and replacive processes. However, 3D analyses reveal that silicified intervals may retain significant volumetric porosity due to microporosity not captured by 2D methods. Consequently, porosity estimates vary significantly with analytical scale, with ML-based analyses underestimating porosity relative to RCAL and μCT data. These results demonstrate that apparent porosity loss at the microscale does not necessarily correspond to reduced reservoir quality at larger scales, highlighting the importance of multi-scale approaches. Silicified intervals may therefore represent overlooked (“hidden”) reservoirs. This study provides a framework linking diagenesis, petrofabric, and analytical scale to improve reservoir characterization in complex lacustrine carbonate systems.

Classifying lithotypes from a reservoir is crucial for assessing drilling, production, and operational risks. Traditionally, accurately identifying the rock lithologies has been a manual, time-consuming, and error-prone process. Recent advancements in deep learning-based and computer vision have shown promising results in classifying rock lithology. However, many state-of-the-art techniques struggle when working with limited, imbalanced, and fine-grained rock datasets, revealing room for improvement. To address these limitations, this paper proposes an innovative strategy that integrates the Vision Transformer and the Central Difference Convolution architectures to classify the major lithology classes in carbonate rock images. The proposed method combines an Encoder Transformer with a Convolutional Neural Network to learn invariant representations of carbonate textures, eliminating manual feature extraction and performing effectively for small and large datasets. We conducted extensive experimental evaluations on two real-world datasets, demonstrating that: (1) our approach is competitive on fine-grained rock datasets; (2) it surpasses most baseline methods in most evaluation scenarios. Specifically, our proposal outperforms the second-best method (ViT) by 8% in the whole-image classification, and exceeds the best CNN model (VGG) by at least 24% in patches classification, considering the balanced accuracy metric.

The Equatorial Atlantic Magmatic Province (EQUAMP) comprises mafic dike swarms and sill complexes that intrude into the Precambrian Borborema Province and the Paleozoic Parnaíba Basin in northeastern Brazil. These magmatic events are associated with the early stages of the West Gondwana fragmentation during the Early Cretaceous. Tholeiitic sills of Sardinha Formation are found in the central-eastern Parnaíba Basin. Two distinct geochemical groups (Colinas I and II) were identified through analyses of petrography, lithogeochemistry, geochronology, and Sr-Nd isotope data from the high-TiO2 Colinas Sills. Colinas I group consists of sub-alkaline basalts with a TiO2 content greater than 3.0 wt.%, MgO ~4.38 wt.% and P2O5 ~0.69 wt.%. Colinas II, in turn, includes most basaltic andesite rocks with lower TiO2 (2.3-3.0 wt.%), MgO ~2.73 wt.% and higher P2O5 ~0.96 wt.% and REE contents. Negative Nb and Ti anomalies and K enrichment as well as geochemical modeling using incompatible element ratios suggest that the trace element characteristics of both groups are primarily a result of a metasomatized mantle source with minor (10-20%) crustal contamination. The isotopic data indicates that Colinas I has an EM2-like signature with 86Sr/87Sr(130 Ma) at 0.705862 and ɛNd(130 Ma) of +2.8, while Colinas II has an EM1 mantle source showing 86Sr/87Sr(130 Ma) at 0.703500 and a less radiogenic ɛNd(130 Ma) of – 2.6. Partial melting modeling suggests that both groups may originate from the mixing of deep asthenosphere and enriched lithospheric mantle sources at the garnet-spinel lherzolite transition zone. We investigate an active rifting geodynamic scenario (plume-lithosphere interaction) to explain the origin of the high-TiO2 Colinas magmas. Binary mixing modeling suggests that the primary source of the Colinas I likely originated from an EM2-type (Samoan) mantle, with some extent of crustal assimilation. In contrast, Colinas II may be explained by the mixing of FOZO-type (Ascension) mantle plume and enriched SCLM-derived melts. The EM1-Gough mantle plume may also have played a role in the genesis of Colinas magmas. In this case, a small percentage of assimilated material (< 10%) from the upper continental crust is necessary for Colinas I, while Colinas II would require assimilation from the lower continental crust.

Santos Basin, a passive margin basin formed during the Neocomian, associated with the breakup of the supercontinent Gondwana and the opening of the Atlantic Ocean, is considered one of the main basins for hydrocarbon exploration and production. However, studies on organic matter in lacustrine carbonate reservoirs of pre-salt are scarce, and its characterization in sedimentary rocks can significantly contribute to a more robust understanding of these depositional systems. Here, organic geochemistry (total organic carbon and biomarkers), palynofacies, and organic petrology analyses were performed to investigate the formation processes and to characterize this sedimentary sequence (Barra Velha Formation), Mero Field, in the Santos Basin. The study of organic facies aimed to elucidate the main factors controlling sedimentation, as well as the quality, preservation, and stratigraphic distribution of organic matter within these important sedimentary intervals. Low organic carbon contents were recorded, and the predominance of organic matter derived from zoomorphs belonging to the Phylum Cnidaria, Class Hydrozoa, explain this finding. Moreover, foraminiferal test-linings were observed, which may suggest a possible marine influence in pre-salt sequences, thereby reshaping current understanding of these sedimentary rocks. Organic petrology revealed the presence of coke and solid bitumen with high reflectance values, highlighting the influence of igneous intrusions. The evidence presented here advances the current state of the art regarding the paleoenvironmental processes that controlled the evolution of these deposits, providing important insights into pre-salt lacustrine systems.

This study seeks to reduce gaps in the sedimentological and stratigraphic understanding of the onshore portion of the Campos Basin (Brazil) through lithofacies analysis and paleoenvironmental interpretation of the Emborê and Barreiras formations based on the 2-JU-1-RJ drill core, the second stratigraphic well drilled in the Paraíba do Sul River Deltaic Complex (CDRPS). These units remain the focus of ongoing debate within the geoscientific community regarding their stratigraphic framework and associated depositional paleoenvironments. Eight lithofacies and three lithofacies successions (LS1–LS3) were identified and described, representing alluvial-fan and fluvial depositional systems. The Emborê Formation (São Tomé Member) comprises distal alluvial-fan deposits (LS1), overlain by sandy braided-river deposits with contributions from debris flows and mudflows (LS2). The Barreiras Formation is represented by sandy braided fluvial deposits characterized by episodic channel drowning and postdepositional sediment modification (LS3). From a sedimentological perspective, the Emborê Formation exhibits strong affinities with deposits of the Resende Formation (Resende and Volta Redonda basins) and the Rio Doce Formation (Espírito Santo Basin), both of which are attributed to the Eocene–Oligocene period. In contrast, the Barreiras Formation exhibits a sedimentary record comparable to that described in outcrops from the onshore sectors of the Campos and Espírito Santo basins.