Seismic stratigraphic framework and seismic facies of the Aptian Pre-salt Barra Velha Formation in the Tupi Field, Santos Basin, Brazil

Abstract

The discovery of giant oil accumulations in the Pre-salt (sag) section of the Santos Basin has attracted huge interest in the lacustrine carbonate deposits of the Aptian Barra Velha Formation. Although for the significant volume of publications on the Pre-salt section, there is still limited understanding of its tectono-stratigraphic evolution, and lack of a coherent published stratigraphic framework for the Tupi Field, the largest and most productive Pre-salt accumulation. This work aims to define a seismic-stratigraphic framework in the Aptian Pre-salt interval of the Tupi Field, using 19 seismic lines, and photomicrographs of thin sections from 13 wells. Four seismic units (SU1, SU2, SU3, and SU4) and two seismic facies (SFA and SFB) were identified. The depositional pattern of the seismic units is of a continuous expansion of the accommodation space, although with significant erosion at the top of SU4. SFA is composed mostly of calcite spherulites and Mg-clays mud, which constitutes the background of lacustrine sedimentation preferentially formed and/or preserved in relatively deeper areas. SFB corresponds to crusts of fascicular calcite shrubs preferentially precipitated in relatively higher areas, and to intraclastic deposits of reworked spherulites and shrubs. The Pre-Alagoas unconformity represents the base of the study interval. The top is characterized by a regional unconformity which represents a temporal gap between the youngest of the units (SU4) and the start of the evaporitic deposits of the Ariri Formation. No additional unconformities were identified within the Barra Velha Formation in the Tupi Field, characterizing a continuously expansive deposition on a flat, tabular and onlapping pattern upon the Outer High, as a product of continuous accommodation creation, under low tectonic activity. The predominant post-depositional configuration, together with the continuity and onlap patterns of seismic units onto a remanent high, support a shallow evaporitic lake model, rather than a deep lake with microbial platforms for the Tupi Field area.