Assessing the Role of Local Versus Regional Fluid Circulation in the Pre-Salt Sag Section of Santos Basin (Brazil) : Insights from Reactive Transport Modeling
Résumé
In lacustrine carbonate deposits, the superimposed sedimentary and diagenetic heterogeneities are highly dependent on environmental conditions, water chemistry, and hydrological properties. In this context, the lacustrine carbonate rocks of the Santos Basin represent a very unusual sedimentary system in terms of dimensions composition and diagenetic features, which raise questions about the controlling factors driving the vertical and lateral heterogeneities observed. In this study, a joint approach, integrating sedimentological-diagenetic descriptions and reactive transport modeling (RTM), is performed on the pre-salt sag section of the Santos Basin, aiming to explore diagenetic scenarios associated with different fluid circulation patterns. RTM is an effective tool to characterize the evolution of diagenesis qualitatively and quantitatively. Special attention is given to the dolomitization, silicification, and dissolution processes, which comprehend crucial diagenetic processes occurring in the pre-salt section. Homogeneous 1D and heterogeneous 2D RTM simulations are used to model four diagenetic scenarios representing the interaction between initial facies and (1) concentrated evaporative water, (2) meteoric surface water, (3) volcanic groundwater, and (4) CO2-rich fluids along a fault. The simulations showed that the evaporative water has a high diagenetic potential. However, diagenetic alterations associated with this scenario are flow rate-limited, given the low flow velocity related to diffusive transport. Near-neutral groundwater and meteoric water also promoted high modifications of the initial mineralogy, mainly Mg-clay dissolution, dominant in all scenarios. Similarities between petrographic observations and modeling results suggest that on the structural high, more prone to lake-level fluctuations, meteoric fluids enhanced dissolution features observed at the top of the studied section. The facies stacking pattern and the associated contrasts in rock properties (porosity-permeability) are crucial factors for the diagenetic alteration pattern. It influenced Mg-clay dissolution in the transitional region and structural high, providing elements for the dolomitization and silicification of these deposits and favored lateral fluid circulation in Mg-clay-rich zones. These porosity/permeability contrasts also led to some reservoir compartmentalization, thus contributing to the circulation of the CO2-rich fluids in the basin away from the fault. This modeling approach highlights the importance of deciphering the interplay between regional and local controlling factors acting in lacustrine carbonate systems.