High-resolution facies modeling of presalt lacustrine carbonates reservoir analog: Morro do Chaves Formation example, Sergipe-Alagoas Basin, Brazil

Abstract

Barremian lacustrine carbonates, mainly coquinas, are the reservoir rocks of the large presalt petroleum discoveries in deep to ultradeepwaters, at the southeast Brazilian margin, namely, the Santos, Campos, and Espirito Santo Basins. The complex geologic setting of those discoveries, with massive overburden of salt bodies, makes the seismic imaging of the carbonate reservoirs an expensive and challenging issue. In this setting, accurate facies models are a requirement to the predictability of the carbonate reservoir intervals. We have developed an analog high-resolution coquinas facies model based on an interpretation of a pseudo-3D ground penetrating radar survey. We relied not only on the trace amplitude analysis, but also on trace attributes to reduce the ambiguity of the interpretation due to possible visual biases, which may relate electromagnetic reflection amplitude to geologic relevance. We have used the dip and texture attributes to help us better understand the subsurface and to focus on the main stratigraphic features of the reservoir analog. We have applied the dip attribute to discriminate among distinct facies and to provide a dip-steered horizon-tracking tool, honoring the stratigraphic layering. We have also used a texture attribute to downgrade high-frequency noise in the amplitude cube and emphasize the main stratigraphic horizons separating reflections with a similar and consistent pattern. Then, we produced a joint interpretation, based on three trace attributes, that is, amplitude, dip, and texture, to highlight the main radar facies. That resulted in a facies model in which three southwest–northeast-striking coquina bars (reservoir facies), associated with transgressive systems tract interbedded with shales (nonreservoir facies) of highstand systems tract, are clearly marked.