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Título

Unveiling the Tectonic and Diagenetic Events of the Campos Basin's Presalt: A Multidisciplinary Approach

Texto do resumo

The Campos Basin (SE Brazil) hosts significant hydrocarbon accumulations in reservoirs above and below a layer of evaporites. The rift to post-rift lacustrine carbonates that constitutes the Pre-Salt reservoirs to the north of this Basin have been the object of several studies focused on the depositional and diagenetic characteristics of these Barremian to Aptian carbonates in the last 10 years. More recently, studies based on detailed 2D and 3D seismic interpretation and wellbore data described the geology of its crystalline basement and tectono-stratigraphic evolution of the presalt succession. The basement is constituted by anisotropic, high-grade supracrustal rocks in the East and relatively isotropic granitic rocks in the West. Lithologies, structural style and age suggest a correlation with the onshore Cabo Frio Tectonic Domain. The overall rift evolution probably started during the Berriasian with a peak of rift-related faulting from the Hauterivian to Barremian, followed by a rift transition and post-rift stages (Barremian to Aptian). The pre-existing metamorphic fabric was selectively reactivated whenever its orientation was favourable. In addition, selective reactivation of rift normal faults by compression occurred towards the end of the post-rift stage and during the evaporite deposition. Nevertheless, two knowledge gaps have yet to be filled: 1) a detailed study of diagenesis in the crystalline basement and rift phases, and 2) the timing of diagenetic events that affected the presalt succession. In this study, samples from these geologic units were analysed for mineral composition and paragenetic evolution, fluid temperature and salinity, stable isotope compositions, and LA-ICP-MS derived U-Pb ages of carbonate phases.
Petrography and geochemistry show diagenetic phases composed predominantly of quartz, calcite and dolomite to ankerite series, and secondarily pyrite, Sr-barite and svanbergite. Additional studies of stable isotopes, fluid inclusion microthermometry and LA-ICP-MS U-Pb geochronology in the diagenetic minerals provide evidence for three tectono-diagenetic events that occurred under distinct diagenetic settings, named Barremian-Aptian (BADE, 125-117 Ma), Albo-Cenomanian (ACDE, 103-98 Ma), and Campanian-Maastrichtian (CMDE, 83-70 Ma). Each phase is characterised by distinct minerals, precipitation temperatures, and burial conditions. The hydrothermal qualifier, identified by the temperature contrast between fluid and host rock, was initially high during BADE, then diminished over time (through ACDE) until it achieved equilibrium with the host rocks during CMDE. Therefore, the diagenetic conditions were initially hydrothermal and progressively attained geothermal equilibrium under deep-burial conditions towards the Late Cretaceous. These diagenetic episodes were not necessarily coeval with magmatic events were are always coeval with basin-scale tectonic events described previously using balanced cross-sections in the post salt section. Since these events were time-related with tectonic events at basin scale, they are interpreted as episodes of “structural diagenesis” (c.f. Laubach et al., 2010). Multidisciplinary studies that include diagenetic events constrained by geochronological data will certainly lead to more robust conceptual geologic models, and therefore, to a more reliable management of resources

Palavras Chave

Campos Basin; pre-salt; Geochemistry; U-Pb geochronology; diagenesis