The impact of heterogeneous salt velocity models on the gross rock volume estimation: an example from the Santos Basin pre-salt, Brazil

Abstract

The thick and heterogeneous salt section in the Santos Basin, offshore Brazil, imposes great challenges to access the pre-salt hydrocarbon reservoirs, especially in relation to seismic imaging, signal quality, and depth positioning. Some problems arise from the current velocity models for the salt section, which for the majority, assume the salt is a homogeneous halite layer. In the Santos Basin, the commonly assumed salt - halite - only makes up to 80% of the mineral in this section. The inclusion of other salts as stratification in the velocity models, based on seismic attributes, has achieved good results in the last decade, especially for depth resolution. In this work, we analyze the benefits of different velocity models, considering presence/absence of salt stratification, and comparing the gross rock volume above the oil-water contact. The results show a significant effect on depth resolution of the events, as well as on volume estimation, indicating that more reliability captured by the complex velocity models will ensure the more confident the resulting volumetric information is.

Seismic Characterization of Internal Salt Cycles: A Case Study in the Santos Basin, Brazil

“…Finally, when we compare the Inversion Model, we observe a reduction of 5%. Maul et al (2021), applying the same methodology for a larger area, which includes ours in Santos Basin, show a variation of about 14% among the models. In another study, Meneguim et al (2015) observe 3% GRV variation between two models: (i) inversion tomographic salt model and (ii) salt standard model-based seismic inversion.…”

“…Figure 9 presents the calculated modulus differences for the depth surfaces using the four salt velocity models in comparison to each well marker depth. The modulus values were chosen to represent the real magnitudes of the mismatch, since positive and negative values may cancel each other (Maul et al, 2021).…”

“…The velocity models we created have increasing complexity, such as presented by Maul et al (2021). The high-frequency cycles illustrated in Figure 4, especially those inside Unit 2 and Unit 3, were considered when building the Salt Cycles Seismic Inversion Model.…”

“…Recently, several approaches are considering details in the evaporitic section using previous information, such as drilled wells and seismic data, to create more accurate velocity models Maul, 2020;Maul et al, 2021). To complement these studies, it is crucial to understand the salt basin depositional dynamics as well as their structural behavior.…”

“…To complement these studies, it is crucial to understand the salt basin depositional dynamics as well as their structural behavior. After recognizing and correlating the major salt cycles (Gamboa et al, 2008;Fiduk and Rowan, 2012;Jackson et al, 2015, Rodriguez et al, 2018, the small salt cycles (Freitas, 2006) can be inferred using well data and modeled using any seismic inversion strategy or any geostatistical approach (Pontes et al, 2018Teixeira et al, 2020;Maul, 2020;Maul et al, 2021).…”