Presalt reservoirs of the Santos Basin: Cyclicity, electrofacies, and tectonic-sedimentary evolution

Abstract

Presalt reservoirs of the Santos Basin accounted for more than 50% of Brazilian hydrocarbon production in the first two months of 2019. Its most important reservoirs are found in the Barra Velha Formation; thus understanding its genesis and geologic history is essential. This formation is composed of carbonates deposited in an alkaline lacustrine environment with a multiplicity of facies from boundstones and grainstones to mudstones. We have performed a sedimentary analysis based on the integration of sidewall core (SWC) samples and well logs from two wells coupled with seismic patterns discrimination to characterize the tectonic and depositional evolution of the Barra Velha Formation in a sector of the Outer High of the Santos Basin in this study. Our method initially consisted of the evaluation of well logs, aiming to identify shallowing and flooding upward cycles of the second and third orders for the paleoenvironmental conceptualization upper rift and sag phases. Then, we defined electrofacies through the integration of SWCs, gamma ray, and acoustic impedance logs using the crossplot approach. Finally, we described seismic patterns throughout the study area and correlated with results from well data analysis. Therefore, the Barra Velha Formation was subdivided into three zones: the lower, intermediate, and upper zones. Well 1 has a facies association characteristic of a proximal and stable environment during deposition, whereas well 2 initially presents in a distal environment sedimentation that evolves to a more proximal setting. This fact evidenced differences with respect to base-level variations indicating compartmentalization within the lacustrine environment of the upper rift and sag phase that corroborated with the detailed seismic pattern interpretation of the study area.

Application of Seismic Architecture Interpretation in Pre-salt Carbonate Reservoirs of the Buzios Field, Santos Basin, Offshore Brazil

“…We recognized the transition between lake bottom patterns of the upper rift phase passing upward to aggradational / progradational carbonate platform during the sag phase, indicating distal facies at the base of the sequence, which can be attributed to the fault development in the region during the Barra Velha Formation deposition. This fact helps to corroborate that the tectonic quiescence expected for the sag phase is not that much of a truth indicating that the upper section of the Barra Velha could be subdivided into upper rift and sag phases, which was proposed by Wright & Barnett (2015), Neves et al (2019), andSilva (2021).…”

“…The Barra Velha Formation is also subdivided into upper and lower Barra Velha, where the lower one is affected by a characteristic rifting tectonic regime and is described by reworked facies, with rudstones and grainstones being the prevailing facies (Arienti et al, 2018). The upper one, beneath the salt, shows a predominance of flat arrangement in situ facies, portraying a sag phase (tectonic quiescence) (Wright & Barnett, 2015;Neves et al, 2019;Silva, 2021). Because of this aspect, the layers are conformable.…”

Geochemical Characteristics and Distribution of the Subsalt Source Rocks in the Santos Basin, Brazil

“…The deposition time of the Itapema Formation is earlier than that of the Barra Velha Formation, and there is no excessive tectonic movement during this period. 13 , 27 , 38 Therefore, it can be inferred that the R o of the Itapema Formation source rock should be higher than that of the Barra Velha Formation. The average value of R o of the Barra Velha Formation is 0.60%, and the TAI is between 2.6 and 2.7, indicating that it is in the low-mature stage ( Table 5 ).…”

Electrofacies Definition and Zonation of the Lower Cretaceous Barra Velha Formation Carbonate Reservoir in the Pre‐salt Sequence of the Santos Basin, Se Brazil

“…The pre-salt carbonates are highly heterogeneous which hinders production forecasting (Mohriak et al, 2015). The carbonates exhibit vertical and lateral variations of petrophysical properties that result in a complexity which is reflected in their electrofacies characteristics, as has also been observed in reservoir rocks in other basins worldwide (Jesus et al, 2019;Oliveira et al, 2019;Puskarczyk, 2019;Solemani et al, 2020;Zheng et al, 2021).…”

“…These authors defined ten electrofacies zones in the reservoir: they showed that there was a predominance of stromatolites and carbonate spherulites in the upper zones, and an increase of grainstones and stromatolites and a decrease in spherulite content in the lower zones. Also in the Santos Basin, Neves et al (2019) defined electrofacies in the Barra Velha Formation reservoir in the Outer High area by integrating sidewall cores and gamma-ray and acoustic logs from two wells. Based on these electrofacies, the formation was divided into three zones: a lower zone whose top was at about 5500-5600 m depth and whose thickness varied from 150 to 210 m; an intermediate zone (top at ~5450 m; thickness varying from 100 to 200 m); and an upper zone (top at ~5300 m; thickness varying from 100 to 150 m).…”