Abstract: Assessment of Hydrocarbon Systems in Selected Basins on the Atlantic Margin of Brazil

Schenk, Christopher J. - U.S. Geological Survey

The USGS World Energy Project is currently assessing the undiscovered oil and gas potential of hydrocarbon systems in priority basins throughout the world. This study focused on the assessment of hydrocarbon systems in selected basins along the Atlantic margin of Brazil, including the Campos, Santos, and Espirito Santo basins. The approach was to first define and document the petroleum systems in a basin, define assessment units, and then assess these units using a geologic-based historic field size analysis approach to arrive at sizes and numbers of undiscovered fields. Undiscovered resources were then calculated using both a Monte Carlo method and an Adaptive Fractile method. We obtained field sizes for existing oil and gas fields in the Campos Basin from a commercially available database. Field sizes were grown prior to our analysis to account for field growth using a function developed for offshore fields in the Gulf of Mexico.

This study focused on the Campos Basin of Brazil, which has been the center of intense exploration activity for 25 years. Many geologic studies of the petroleum geology of the Campos Basin are publicly available, and all available information was utilized in this study. The most prolific source rock identified in the Campos Basin is lacustrine mudstones of the Neocomian Lagoa Feia Formation, which is considered the source for most of the petroleum in the Campos Basin. Although other workers have interpreted several petroleum systems within the Lagoa Feia Formation in the Campos, we simplified this to one petroleum system that was sourced by mudstones of the Lagoa Feia Formation.

The Lagoa Feia petroleum system was divided into three units (or plays) for the purpose of assessment: Cretaceous carbonates, Late Cretaceous and Tertiary turbidites, and Salt Dome Tertiary turbidites. The Cretaceous Carbonates Assessment Unit was the first to be explored in the Campos Basin, and includes Neocomian Lagoa Feia and Macae Formation carbonate reservoirs and potential Lagoa Feia Formation and Macae Formation proximal conglomerates and minor Neocomian basalt reservoirs. Typical fields in this play include Garoupa, Bonito, Bajedo, and Linguado. The Late Cretaceous-Tertiary Turbidites Assessment Unit includes siliciclastic reservoirs of Late Albian, Cenomanian, Turonian, Santonian-Maastrictian, Eocene, Oligocene, and Miocene age. Most of the reservoirs in this unit are turbidites, and are typified by fields such as Namorado, Marlim, Barracuda, and Roncador. This is the most significant unit in the basin based on discovered petroleum. The hypothetical Salt Dome Tertiary Turbidite Assessment Unit includes potential turbidite reservoirs in the Eocene, Oligocene, and Miocene-Pliocene that may exist in ultra-deep water in an area characterized by salt domes. There has been no drilling to date in this assessment unit. This hypothetical unit was heavily risked on the presence of adequate source rocks. Similar assessment units to these three exist in the adjacent Santos and Espirito Santo basins.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil