Presalt Carbonate Cyclicity and Depositional Environment: A Combined Bhi and Nmr Approach to Markovian Cycles in the Barra Velha Formation (Santos Basin, Brazil)

The presalt strata are idealized as a lake-dominated breakup depositional sequence recorded between the South American and African continental margins to which there is no present-day analogue. Most of the complexities of the presalt reservoir rely on addressing carbonate cyclicities at different scales using limited cored intervals where well-log and seismic correlation is still unclear. In this case, the application of borehole image logs (BHI) and nuclear magnetic resonance (NMR) techniques allows not only the scale correlation and extensive data acquisition, but also to establish a more robust relationship between depositional facies and their petrophysical significance. For that, this work combines BHI and NMR data to propose a feasible scheme of acoustic facies for the Barra Velha Formation in the Santos Basin. To verify the statistical significance of the acoustic facies’ vertical organization, this work applied the Embedded Markov Chain method along a c. 500 m interval, deriving four depositional cycles associated with shallowing and reworking lacustrine cyclicities. Abrupt changes in cyclicity patterns were adopted as stratigraphic markers to subdivide the succession into lowermost, intermediate, and uppermost intervals that record the depositional history of deep, shallow, and emergent lake environments, respectively. By comparing the thickness variation of the Markovian cycles (i.e., accommodation space) with the seismic onlap curve variation (i.e., lake level) throughout the presalt interval, a larger-scale superimposed cycle was unravelled, suggesting that accommodation space of the studied presalt lake was largely controlled by fluctuations in the lake level. Finally, this work aimed to create a set of diagnostic criteria for the description and interpretation of acoustic facies and cyclicities in the Barra Velha Formation that demonstrate the petrophysical and statistical significance and its seismic scale observation.