Using Wireline Logging and Thin Sections to Identify Igneous Contact Metamorphism and Hydrothermal Influence on Presalt Limestone Reservoirs in Libra Block, Santos Basin

Abstract

A methodology to confirm the direct contact metamorphism of intrusive rocks and related hydrothermal modification on presalt limestone reservoirs, in Libra Block of Santos Basin, is presented. The approach is based on systemic study of cores, sidewall cores, traditional wireline logging, ECS and NMR of boreholes. Type and characteristics of metamorphism in contact aureoles between intrusive bodies and limestones are confirmed. NMR was used to evaluate porosity change of limestone. Thin sections and ECS were used to identify the precipitant types and to confirm the modification of hydrothermal fluids influences on surrounding limestones. Intrusive rocks are observed in several wells of presalt layers in Libra Block and show contact metamorphic aureoles in the surrounding limestones, characterized by a significant change on the limestone mineral size, rock color, and reservoir pore after marbleization. By combining conventional logging and NMR logging, the thicknesses of contact metamorphic aureoles are determined in multiple wells. They conform to a binomial regression relation with the thicknesses intrusive bodies. The NMR data reveals that igneous contact metamorphism exerts negative influence on pores development for carbonate reservoirs. Reservoir physical properties decrease sharply after thermal baking, generally by 4 to 8 times. The study also confirms that related hydrothermal fluids pervasively develop and have multi-types of influences on limestones, mainly due to silicification, dolomitization and calcification. They mainly brought minerals precipitation in fractures and pores that made limestone reservoir tight. In Libra presalt layers, hydrothermal modification related to intrusive bodies mainly developed at the middle-upper intervals of BVE Fm. Combined with wells correlation sections and seismic data, a state-of-the-art model of igneous contact metamorphism and hydrothermal modification on limestone reservoirs has been developed in this paper. It can help to quantitatively predict the thickness of contact metamorphism aureole around intrusive rocks. The study also clarifies the understanding of the type of hydrothermal effects on the reservoir porosity and how to map the spatial distribution of hydrothermal deposition in the reservoir.

Post-salt magmatism in the Campos Basin, offshore SE Brazil: style, distribution, and relationship to salt tectonics

“…The occurrence of volcanoes, lava flows, and hydrothermal vents at various stratigraphic levels indicates that igneous activity was punctuated. Previous works describe three main magmatic events after rift initiation in Campos Basin: late Aptian (c. 112 -118 Ma), Santonian-Campanian (c. 70 -80 Ma) and Paleocene -Eocene (c. 40 -60 Ma) (Oreiro, 2006;Moreira et al, 2006;Winter et al, 2007;Thomaz Filho et al, 2008;Ren et al, 2019). Late Aptian magmatism is associated with the final breakup between the South American and African continents (Thomaz Filho et al, 2008).…”

“…Immediately after salt deposition (late Aptian, c. 116 -111 Ma), igneous extrusions and intrusions were emplaced during several magmatic pulses across southeast Brazil, from the late Upper Cretaceous to the Early Paleogene (Figure 2c), particularly during c. 60-40 Ma and c. 80-70 Ma (Figure 1b;Oreiro, 2006;Moreira et al, 2006;Thomaz Filho et al, 2008;Ren et al, 2019). By quantifying the percentage of volcanic fragments in different deep-water reservoirs of the Campos Basin, Fetter et al (2009) suggested that the climax of volcanism occurred in the Late Maastrichtian.…”

Post-salt magmatism in the Campos Basin, offshore SE Brazil: style, distribution, and relationship to salt tectonics

“…The authors described 10 hydrothermal vents, seven with dome-shaped geometries and 9 Please cite as https://doi.org/10.1016/j.jsames.2023.104314 three with eye-shaped geometries, all of which were associated with the two Early Cretaceous magmatic episodes (Figure 1b). Immediately after salt deposition (late Aptian, c. 116 -111 Ma), igneous extrusions and intrusions were emplaced during several magmatic pulses across southeast Brazil, from the late Upper Cretaceous to the Early Paleogene (Figure 2c), particularly during c. 60-40 Ma and c. 80-70 Ma (Figure 1b;Oreiro, 2006;Moreira et al, 2006;Thomaz Filho et al, 2008;Ren et al, 2019). By quantifying the percentage of volcanic fragments in different deep-water reservoirs of the Campos Basin, Fetter et al (2009) suggested that the climax of volcanism occurred in the Late Maastrichtian.…”

“…Basin: late Aptian (c. 112 -118 Ma), Santonian-Campanian (c. 70 -80 Ma) and Paleocene -Eocene (c. 40 -60 Ma) (Oreiro, 2006;Moreira et al, 2006;Winter et al, 2007;Thomaz Filho et al, 2008;Ren et al, 2019). Late Aptian magmatism is associated with the final breakup between the South American and African continents (Thomaz Filho et al, 2008).…”