Dados da Submissão

Título

Geochemical analysis of lacustrine carbonates and structural implications at the pre-salt, Santos Basin

Texto do resumo

Currently, most of Brazil's oil production comes from pre-salt reservoirs in the offshore basins of the Southeast, occurring in continental deposits of lacustrine carbonates. Although it has aroused great interest in the industrial and academic sectors in recent decades, a limited number of studies detail the paleoenvironmental evolution or integrate a quantitative approach associated with spatial/stratigraphic distribution and related to depositional or structural context. Our main objective, therefore, is to evaluate the correspondence of structures generated and/or reactivated during the tectonic evolution and the deposition of the pre-salt layers in the Santos Basin, focusing on the Barra Velha Formation and its isotopic signal, since it is the main hydrocarbon reservoir. In this way, the tectono-stratigraphic context is evaluated from the interpretation of 2D seismic sections. This makes it possible to analyze the dynamics of tectonic evolution, quantify and describe the distribution of faults in the pre-salt layers throughout the study area in the basin. Isotopic analyses are essential to provide critical constraints on the formation environment of the reservoir rocks in the Iracema region of the Tupi field. Stable isotope analyses of C and O and radiogenic Sr were carried out on carbonate samples from sidewall borehole facies in a 101 m column. Major elements (Ca, Mg, Fe, Mn, Sr and Rb) also had their concentrations measured. The sample values of δ13C vary between + 0.79 and + 3.16‰, with average values of +2.22‰. The values of δ18O vary between - 1.22 and + 3.86‰, with average values of +1.18‰. The values of 87Sr/86Sr vary between 0.7130 at the top of the section and 0.7144 at the bottom, with average values of 0.7135. The high δ13C and δ18O values mainly record the climatic effects responsible for controlling the deposition of sediments from the Barra Velha Formation. Environments with high temperatures and high salinity tend to favor the precipitation of dolomite and/or calcite rich in Mg. While the inflow waters are enriched in δ13C due to evaporation in a restricted closed environment, the δ18O isotopic signals may have been conserved to some extent, since the carbonates did not undergo intense diagenetic processes that could mobilize or reprecipitate Sr and Mg in large quantities. Other observations that corroborate the lower degree of alteration, preserving characteristics closer to the original carbonate, are high Ca/Mg and δ¹⁸O, in addition to the positive correlation between Ca and Sr. The same source or process introduced Rb and Sr, so during diagenesis, elements such as Sr can be replaced by Ca in the carbonates, while Rb can be incorporated into secondary mineral phases or associated clay materials. High 87Sr/86Sr values indicate a significant contribution of Sr from continental erosion or terrestrial sources, although there may be another source. Results indicate that the mapped structures may have participated in weak diagenetic processes in the well sampled, since the values obtained for isotopes are in line with those found in the Santos Basin. When compared to other basins, it is seen that they did not function as conduits that would allow the transportation of hydrothermal fluids, which would considerably alter the results obtained. Therefore, it is understood that the samples used preserved the characteristics close to the original carbonate and the structures served as conduits during diagenetic processes.

Palavras Chave

Isotope Geochemistry; diagenesis; faults; Tupi field