Integrating Geochemistry, Charge Rate and Timing, Trap Timing and Reservoir Temperature History to Model Fluid Properties in the Frade and Roncador Fields, Campos Basin, Offshore Brazil

John Guthrie¹, Christian Nino¹, Hassan Hassan², and Pam Morelos¹
¹Hess Corporation, Houston, Texas
²University of South Carolina, Columbia, South Carolina

Understanding the distribution of oil quality and its impact on the development of deepwater reservoirs has been a major challenge in many offshore basins of Brazil. Traditional geochemical approaches have used bulk properties (API gravity, viscosity, and sulfur content) and the biomarker compositions of oils to resolve the effects of source rock facies, thermal maturity, and biodegradation on oil quality in the present-day reservoir. These techniques, however, cannot fully resolve the effects of hydrocarbon charge timing, charge rate, timing of trap formation and reservoir temperature history on the quality of the oil. This study tests an interactive biodegradation tool in Trinity to model these controls in the Frade and Roncador fields of the Campos Basin, offshore Brazil. The modeled results are constrained by the geology and the composition of the present-day fluids in the reservoir.

The Roncador and Frade fields contain lacustrine-derived oils in both Upper Cretaceous (Maastrichtian) and Lower Tertiary (Eocene) reservoirs that range in API gravity from 14 to 33°. In the eastern portion of the Roncador field better quality light oil (average API 28°) occurs in Upper Cretaceous (Maastrichtian) reservoirs. The southwestern portion of the field contains mostly heavy oil (average API 17°). The Frade field to the west of Roncador also contains heavy oil (API ranging from 16 to 18°) in shallower, Lower Tertiary (Eocene) reservoirs. Geochemical analyses have identified the depletion of n-alkanes and the presence of demethylated hopanes in varying proportions in the oils indicating a complex history of biodegradation and mixing from at least two hydrocarbons charges in the reservoir. Both 1D and multi-surface models for the area were constructed to help resolve the history of biodegradation in the reservoir and mixing from the dual hydrocarbon charge.

Figure 1A illustrates the modeled expelled volumes from the lacustrine Lagoa Feia source rock in the fetch area associated with the eastern portion of the Roncador Field. Hydrocarbon expulsion begins in the Late Cretaceous (about 90 my) and the expulsion curve is dominated by two distinct pulses (Pulse 1 and Pulse 2). Pulse 1 represents an early charge from the Late Paleocene to Early Oligocene (60 to 35 my) and Pulse 2 a more recent charge from the Middle Miocene to present day (15 to 0 my). The thermal model suggests that the Lagoa Feia source rock within the fetch area for the trap has expelled approximately 9BBO to present-day. However, geologic information indicates that the trap formed sometime in the Late Eocene, resulting in a loss of the early portion of the charge. As a result of this loss, the expelled volumes available since trap formation are only 5.3 BBO. At the time of trap formation, the modeled API gravity that arrives in the reservoir was about 38° but rapidly decreased in the latter part of Pulse 1 to about 14° (Figure 1B). During Pulse 1, reservoir temperatures ranged from 30 to 45°C over a period of about 20 my, enhancing the effects of biodegradation and lowering API gravity of the fluids. At the onset of Pulse 2, the expelled volumes began to increase dramatically suggesting that higher maturity, light oil was beginning to mix in the reservoir. Concurrently, the modeled API gravity increased to present-day values of about 30°, which is very similar to the average measured API gravity of 28° observed in the eastern Roncador field. Due to present-day reservoir temperatures of about 70°C, only minor amounts of biodegradation occur and the oil remains relatively unaltered as illustrated by the gas chromatogram on Figure 1B.

Additional examples of charge rate and timing, trap timing, and temperature history will be presented for other parts of the Roncador and Frade fields to illustrate the importance of these factors on controlling the quality of oil in the present-day reservoir.

Figure 1A. Charge timing and volumes from the lacustrine Lagoa Feia source rocks.