Seismic interpretation of the Mero Field igneous rocks and its implications for pre- and post-salt CO2 generation – Santos Basin, offshore Brazil

Abstract

Understanding igneous rock emplacement is crucial for deciphering any sedimentary basin's evolution. Its occurrence can be a determining factor for elements of the petroleum system, such as hydrocarbon generation, migration, trap formation, and reservoir quality. The high-impedance nature of igneous rocks in relation to their surrounding host rocks renders amplitude anomaly-based attributes sufficient for the identification of igneous bodies within siliciclastic sequences. However, this is not necessarily valid in pre-salt settings because of the high-impedance carbonates that make the impedance contrast subtle. Furthermore, if the emplacement characterization of the various magmatic pulses that occur in a given area over geological time were better evaluated, it would be possible to assess the gases (such as CO2) they produce with higher reliability. In this paper, the principal geometric and petroelastic characteristics of the igneous rocks of the Mero Field are described in detail in order to estimate the thermogenic production of CO2. Both manual and semi-automatic seismic interpretation were carried out to quantitatively and properly reduce the uncertainties inherent to the seismic mapping of these rocks. Thus, a 3D probabilistic igneous facies classification was performed based on a Bayesian inference using the P- and S-impedance well log data available and seismic elastic inversion volumes. These volumes of occurrence probability were useful for semi-automatically extracting maps and geobodies, which, along with conventional manual seismic interpretation, provided hypothetical volumetric scenarios for Mero igneous rocks and also for CO2 production. Five main intrusive features were recognized based on mainly in seismic geometries found in the Mero Field, two of them being Lower Cretaceous rift-related magmatism synchronous with the pre-salt reservoir deposition and the other three being related to the Santonian/Campanian magmatism. The analyzed data suggests that deep intrusions were responsible for the installation of a multiple hydrothermal vent system in the Santos Basin. During pre-salt sedimentation in the Lower Cretaceous, the emplacement of tholeiitic sills, primarily in low-energy facies at structural depressions, may have played a significant role in the thermal alteration of the sedimentary framework and would have released a significant amount of CO2 into the atmosphere. It's possible that the reactivation and reuse of these older structures (i.e., sill-related hydrothermal vent complexes generated by tholeiitic intrusions) during alkaline Santonian/Campanian magmatism might have contributed further to the production of additional carbon dioxide, which would be trapped in pre-salt reservoirs in this instance.