Using Outcrop Analogs To Improve 3D Heterogeneity Modeling Of Brazilian Sand-Rich Turbidite Reservoirs

Moraes,M.A.S., M.R. Becker, M.C. Monteiro Petrobras Research Center (CENPES), Rio de Janeiro, Brazil, and S.L. Almeida Netto, Petrobras E&P Campos Basin, Macaé, Brazil

Abstract

The Eocene Annot sandstone, of southern France, is a sand-rich, deep-water system, the properties of which can be used to model similar subsurface reservoirs. Such reservoirs are common in the eastern offshore Brazilian basins.

Typically, the modeling process involves (1) recognition of main intra-reservoir stratigraphic units, (2) understanding controls on shale continuity, and (3) determining facies continuity and connectivity. The occurrence of 10-20 m thick intra-reservoir stratigraphic units, which represent stages or substages of turbidite system evolution, and are correlatable at interwell distances, has been independently observed at outcrops and in the subsurface. Such observation represents an important reduction of high-resolution stratigraphy uncertainty, and gives confidence in defining the basic stratigraphic framework.

At a smaller scale, in these high-energy, sand-rich systems, shale continuity, a major factor controlling fluid movement in the reservoir, is mainly resultant of the frequency and intensity of erosional processes. A relationship between shale thickness and continuity is clear, but with significant scatter. Most shales show continuity of several hundred meters or more; erosive "windows" appear in certain areas, locally allowing vertical communication. Nevertheless, shale overlapping causes a general tendency of low effective vertical permeability.

Facies continuity is estimated using outcrop data calibrated with well and seismic information. As the reservoirs are finer grained than the outcrops, it is inferred that the system should be less anisotropic as a result of greater channel sinuosity. The final subsurface 3D models showing low effective vertical permeability and moderate horizontal anisotropy are quite different from what is usually expected for sand-rich systems. As the models commonly fit well reservoir flow behavior, this case illustrates how outcrop and subsurface data should be fine tunned if reliable reservoir evaluation and production forecasts are to be obtained.