The importance of core for carbonate reservoir evaluation: a case study from the Barra Velha Formation, Santos Basin, Brazil

Abstract

Abstract 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 log and whole-core datasets converge on a model of aggrading carbonate mounds which 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- 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, centimetre-scale conduits and higher vertical permeability. This difference can only be reliably characterized via the integration of whole-core samples with other datasets and has a significant quantified impact on sweep and production performance.