Abstract

This study is part of a fully integrated petroleum system assessment of new frontiers in the deep and ultra-deep waters of the Santos Basin. It includes geochemistry from more than 50 selected wells containing pre-salt oils and potential source rock systems representing the entire pre-salt sequence drilled to date.

The pre-salt hydrocarbon province located in the offshore Santos Basin, southern Brazil, has attracted the attention of exploration companies with the discovery of the Tupi oil field in 2006. Since then, many discoveries, including Mero, Buzios, Sapinhoá, Atapu, Itapu, and others, have made this basin one of the most prolific oil provinces in the world. Because the number of wells that have penetrated the deeper Barremian sequences in the ultra-deep water in the Santos Basin are scarce, and data publications are virtually nonexistent, the exploration of the pre-salt hydrocarbon province has been dominated, by introducing new seismic acquisition technology, enhancing seismic quality through reprocessing, and deploying conceptual 3-D basin models. However, little direct effort has been devoted to understanding petroleum system elements and processes in the area.

In this study, a set of rock samples representing sediments of the mid-to-late Aptian Barra Velha, upper Barremian Itapema, and lower Barremian Piçarras formations from the pre-salt section of the Santos Basin was investigated. The organic-rich rocks were collected for extraction and correlation with oil samples gathered from pre-salt oil fields over a wide area of the pre-salt province.

Results of the integration of the geochemical with geological and geophysical data indicate that the most prolific source rock system is the upper Barremian Itapema Formation deposited in an euxinic lacustrine brackish-to-saline environment. This unit appears to have sourced almost all the hydrocarbons that accumulated in the pre-salt lacustrine shrublike texture, reworked and spherulitic limestone and coquina reservoirs in the Santos Basin. In contrast, the lower Barremian Piçarras Formation and the mid-to-late Aptian Barra Velha Formation appear to have minor importance.

Although the upper Barremian Itapema Formation has not been encountered by the drill bit in outboard areas of the basin, its presence has been suggested by the existence of partially cracked oils sourced from deep depositional pods in fields such as Jupiter, Tupi, and Mero. The source to trap migration route for these cracked oils suggests a long-distance pathway from very deep source depocenters, located at the eastern part of the Tupi Outer High trend in the distal part of the ultra-deep-water Santos Basin, through southeast to northwest transform faults to the reservoirs. The identification of these very deep depocenters using a structural map of the basement creates an entirely new way of looking at oil generation, migration, fluid heterogeneities, and accumulations within the Santos Basin. The identification of the location of depo-pods of generation of the Barremian mega-sequence outside the already explored regions of the Santos Basin drastically lowers the exploration risk in the outboard unexplored areas of the basin.