Borehole based sedimentary cyclicity and structural analysis: applications for reservoir characterization studies in the post- and presalt carbonates of Santos and Campos basins, SE Brazil

The increasing research interest in carbonate reservoirs is related to the large hydrocarbon reserves hosted by these rocks. Carbonate reservoirs are highly heterogeneous and complex due to depositional and diagenetic characteristics, which affect the porosity and permeability distribution and make their evaluation a difficult task. To integrate the different scales of heterogeneities that occur in these rocks is one of the great challenges in the characterization of carbonate reservoirs. Integrating different approaches that address datasets across different scales may be critical for better results in reservoir evaluation. The post- and pre-salt carbonate reservoirs represent prolific exploratory plays distributed along the Brazilian marginal basins. In this context, we performed sedimentary cyclicity analysis and acoustic borehole image structural characterization in post- and pre-salt carbonate reservoirs from the Campos and Santos Basins, SE Brazil. The cyclic depositional pattern is a common feature of marine or lacustrine shallow-water carbonates. However, the detection of sedimentary cycle patterns can be challenging due to the presence of superimposed events in the geological record, which may represent different orders of cyclicity. In this way, we proposed a multi scale spectral analysis method to assist in the detection of the sedimentary cyclicity using well log data of an Albian carbonate reservoir from the Campos Basin. First, we applied the variational mode decomposition (VMD) to decompose the gamma-ray logs into distinct components and, then, we created a cyclicity log by mapping the instantaneous frequency of a selected component. We recognized two hierarchical orders of cyclicity embedded in the geological record: meter-scale cycles, related to local variations, and long-term cycles, associated with depositional sequences. These long-term cycles enabled to establish a good lateral correlation of the main stratigraphic units across the area. The proposed method was also applied to assist in the sedimentary cyclicity analysis of a pre-salt carbonate reservoir from the Santos Basin. Based on spectral gamma-ray logs, we identified three orders of cyclicity in the succession: high-, medium-, and low-frequency cycles. The medium-frequency cycles enabled to distinguish three main sequences within the sag interval, indicating humid vs. dry periods. The low-frequency cycles were associated with two major units, Lower and Upper Sag, which were also recognized in the seismic data. The long-term cycles presented good lateral correlation and the stratigraphic units defined by these cycles showed reasonable control in the distribution of porosity and permeability. Thus, the cyclicity analysis is a valuable tool to assist in the characterization of the local stratigraphic evolution and to predict the distribution of reservoir properties. In addition, we developed a structural analysis based on acoustic imaging tools for this pre-salt reservoir. This analysis provides key sub-seismic structural information, including structural dip analysis, fractures evaluation, and estimation of the present-day in-situ stress direction. Among the most important features of the pre salt reservoir, we investigated the fractures and vugs distribution patterns, which indicated a stratigraphic control. From borehole breakouts and drilling-induce fractures, the in-situ stress direction was estimated, as well as its relationship with the orientation of the open fractures, due to the significant importance to fluid flow. These integrated studies for the characterization of carbonate reservoirs may contribute to improve the prediction of the reservoir properties distribution and provide valuable constraints to reservoir modeling, which may reduce uncertainties associated with exploration and production. (author)