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Ray-Tracing Approach Constrained by Capillary Pressures in a Multi-Drain System : Application to Reconcavo Basin, NE Brazil

Caroline SULZER1, François LORANT1, and Luiz Felipe COUTINHO2
1 Institut Français du Pétrole, 1-4 avenue de Bois Préau, 92852 Rueil-Malmaison, France
2 Petrobras Research Center, Cidade Universitaria, Quadra 7, Ilha do Fundão, 21949-900 Rio de Janeiro, RJ, Brazil

Map-based simulation – or ray-tracing – is a widely used approach to evaluate the occurrence and the volume of oil and gas prospects. This method assumes an instantaneous drainage migration from one or more source rocks into reservoir layers, with an average displacement path that may be assigned either upwards, downwards or horizontal. Given a series of fetch areas and geometrical traps – i.e., structural tops and leakage points – along a stratigraphic layer, hydrocarbon accumulations are then computed according to oil and gas generation rates and expelled volumes. This calculation eventually takes into account migration losses by means of a seal efficiency coefficient that needs to be calibrated from case to case.

In order to model the spilling of oil and/or gas from deeper reservoir layer into shallower layer, depending on whether each layer can trap the hydrocarbons that migrate into it or not, we developed a new algorithm that takes capillary leakage at the top of each trap into account. Capillary pressure maps derived from both the porosity and the hydrocarbon saturation are imported from a preliminary P, T, maturation 3D block simulation. Then these maps are used to successively calculate the vapour, then the liquid accumulation heights, assuming (i) that accumulations are always at pressure equilibrium, and (ii) that the geometry of a drainage area can be approximated by a vertical cylinder. Hence, step-by-step migration and oil-to-gas separation along a series of stacked reservoirs can be represented by this algorithm. Additionally, we included the possibility to constrain migration paths by taking the history of fault permeability into account, in both source rocks and reservoirs.

With this approach, we have addressed the evaluation of oil and gas accumulations in the Reconcavo Basin, located north-eastern Brazil. This aborted graben, formed in the Lower Cretaceous, is structured by 2 major NW-SE strike slip faults along which the hydrocarbon deposits occur. The petroleum system includes a pre-rift sequence, where the main reservoirs are located (Sergi and Agua Grande Fm), a rift sequence where two lacustrine source rocks occur (Gomo and Taua members of the Candeias Fm), and a post-rift sequence characterized by isolated sandy reservoirs.

Various migration paths and leakage scenarios, including the role of faults, have been considered. This work shows that the instantaneous migration is significantly improved by the use of capillary leakage in terms of oil and gas occurrence prediction. However, the approach is not always predictive for gas/oil ratios estimation in small size accumulations.


AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands