The Importance of Core for Carbonate Reservoir Evaluation: A Case Study from the Barra Velha Formation, Santos Basin, Brazil

The lacustrine carbonates of the Barra Velha Formation are a prolific hydrocarbon reservoir in the Santos Basin, Brazil. In many fields, they comprise decimetre- to metre-scale cycles composed of laminated calcimudstones, spherulite and shrub-dominated facies. However, locally these cycles are replaced by decametre packages of re-worked shrub grainstone/rudstone/breccia and in-situ shrub framestone with significant (>30°) depositional dips. The latter could be interpreted in several ways (e.g. fault block highs, carbonate mounds) but the integration of seismic, borehole image (BHI) log and whole core datasets converge on a model of aggrading carbonate mounds that developed in or marginal to a lake setting. The core datasets in this study demonstrate a distinctive depositional fabric within the carbonate mounds. From a production geology standpoint, the crucial difference between mound-dominated and cyclothem-dominated successions is their permeability architecture. Cyclothem-dominated intervals show prominent and laterally continuous, decimetre-scale vertical matrix permeability variations. Mound-dominated intervals lack fine-scale palaeo-horizontal layering and exhibit a greater prevalence of irregular, cm-scale conduits and higher vertical permeability. This difference can only be reliably characterised via the integration of whole core samples with other datasets and has a significant quantified impact on sweep and production performance.