The Barra Velha Formation, representing the uppermost Aptian formation of the Santos Basin, predominantly comprises dolomudstones, intraclastic grainstones, and rudstones. Additionally, spherulitic and Mg-rich claystones suggest deposition in a shallow alkaline lacustrine environment. Despite the significance of reworked deposits within the Barra Velha Formation, limited attention has been given to understanding their origin, provenance, and their link to high well productivity. This study aims to characterize the reworked carbonate facies and establish their diverse genesis using a multi-scale approach. Through the analysis of core samples retrieved from the upper 19 m of the Upper Barra Velha Formation, we identified eight sedimentary facies, including microfacies grouped into two distinct associations: back-barrier deposits and foreshore-shoreface transition deposits. Back-barrier deposits consist of shrubstones, laminated shrubby spherulitestone, laminated mudstone, massive grainstone, and massive rudstone facies, composed of in situ facies and carbonates reworked by wave and distal hyper-concentrated flow. Foreshore-shoreface transition deposits comprise laminated grainstone, pebble grainstone, and massive rudstone. The facies analyses provide insights into swash-backwash effects, hyper-concentrated flow, and high wave reworking deposits. Detailed interpretation of seismic reflectors in the Uppermost interval of Barra Velha Formation revealed two distinct geometric patterns: high-amplitude onlapping seismic reflectors toward the basement and bidirectional high-amplitude reflectors on the platform edge. These patterns primarily consist of spherulitestones, shrubstones, and grainstones-packstones BHI facies. Gravity flow deposits of a fan-delta in the Lowermost interval of the Barra Velha Formation were interpreted as rudstones-grainstones and mixed reworked carbonates/volcanic rocks BHI facies, associated with low-amplitude sheet-like shape reflectors dipping toward the slope seismic reflectors. Based on the constructed onlap curve derived from the seismic lines in the study area and the described facies associations, it is evident that the lake margins are expanding, conducive to the formation of reworked facies mainly associated with wave action.
Tag: Barra Velha Formation
Paleokarst features in the Aptian carbonates of the Barra Velha Formation, Santos Basin, Brazil
Seismic data, borehole image logs, and conventional well logs were used to investigate the distribution and characteristics of paleokarst features in the Aptian carbonates of the Barra Velha Formation in a pilot area of the Santos Basin, Brazil. Multiple seismic attributes were used to enhance details on the seismic data and highlight key seismic parameters including strata deformation and geometry, continuity of seismic events, and fault patterns. The study found that karst structures are controlled by faults and fractures along structural highs, which served as a conduit for the flow of dynamic fluids that dissolved the carbonate materials. Several closed, circular depressions and bright spots identified in the northeastern portion of the study area represent possible sinkhole structures. Epigenic and hypogenic processes due to the action of meteoric water, hydrothermal activity, and intra-formation acidity along regional unconformities in the Barremian-Aptian may have been responsible for the dissolution. Limitations of this study are related to the difficulty of integrating multiple datasets with various scales. However, the higher confidence for the occurrence of the karst features is provided by borehole images at the sub-seismic scale. The findings of this study hold significant relevance for the strategic planning of energy development and carbon sequestration initiatives in the Brazilian continental margins, thereby aiding in informed decision-making.