Processos petrogenéticos e geodinâmicos relacionados às seções magmáticas de poços na Bacia de Santos

Abstract

The Santos Basin is a passive margin basin located on the southeastern margin of Brazil and detain the largest reserves of oil and gas in the country. Volcanic and plutonic magmatic events are recorded in the rift, post-rift and drift phases of the basin. This work presents petrophysical and petrographic data from magmatic section of wells 6-BG-6P-SPS, 3-BRSA-1267A-RJS and 1-BRSA-886-RJS in the pre-salt sedimentary sequence of the Santos Basin, as well as new geochemical, isotopic and geochronological data from magmatic rock samples from well 6-BG-6P-SPS. The correlation between the petrographic and petrophysical data allowed the identification of explosive and effusive felsic and effusive mafic events in well 6-BG-6P-SPS. This is the first record of felsic magmatism in the Santos Basin. In addition, subaqueous volcanism was identify in well 3-BRSA-1267A-RJS and pre-Albian intrusive magmatism with cumulatic features in well 1-BRSA-886-RJS. Well 6-BG-6P-SPS records volcanic events with ages between 135 and 121 Ma. The mafic and felsic rocks of this well are subalkaline, with tholeiitic affinity. An analysis of evolutionary processes indicated that the acid and basic volcanic rocks of well 6-BG-6P-SPS are not cogenetic by fractional crystallization or AFC. Evolutionary process modelling showed that at least one basic suite evolved by fractional crystallization with upper crustal assimilation to intermediate compositions. The origin of the acid rocks was attributed to partial melting processes of the felsic upper crust. The partial melting modelling of the mantle allowed to conclude that the genesis of the least evolved and uncontaminated basic rocks is related to lithospheric and sublithospheric mantle sources of different compositions. The isotopic compositions (Sr-Nd-Pb) of the basic volcanic rocks, intermediate and acid rocks are consistent with the participation of an enriched EMII-type component of the subcontinental lithospheric mantle. The origin of this enriched component was attributed to the subduction processes of the Wilson Cycles that culminated in the amalgamation of Gondwana in the Proterozoic and early Phanerozoic. The binary mixing modelling corroborates the participation of Tristan da Cunha plume-type and EMII-type mantle reservoirs. Episodes of two mafic and felsic volcanic events recorded in well 6-BG-6P-SPS imposed different thermal regimes from the Hauterivian to the Aptian that may explain some characteristics of the pre-salt petroleum systems of the Santos Basin.