Modeling the HC-Filling History of a Norwegian Oil Field Integrating Basin and Reservoir Scales

R. Ondrak¹, R. di Primio¹, and L. Leith^2
1GFZ-Potsdam , Potsdam Germany
²StatoilHydro, Trondheim ,Norway

The integration of basin modeling predictions on a reservoir scale has remained difficult to date mainly due to nearly an order of magnitude difference in resolution between basin and reservoir models. The advent of local grid refinement techniques in basin modeling allows now different scales and resolutions to be taken into account. Here we present our first case study where we attempt to reconstruct the HC-filling history of a Norwegian field by combining regional and reservoir scale models. The basic understanding of field compartmentalisation and reservoir filling routes is based on organic geochemical results. The main focus of the basin modeling work lies in the reconstruction of the evolution of different kitchen areas as well as that of the migration pathways, which lead to the filling of the reservoir. In addition to the spatial relations of the different kitchen areas, the temporal evolution of kitchen area maturity is of major concern for the project.

In a first step we have modeled the temperature and pressure history of a hydrocarbon province on a passive margin on a regional scale. The regional model was constructed based on seismic horizons of the main stratigraphic units and well-tops of about 100 exploration wells. The model has been calibrated with vitrinite, temperature and pressure data, which was obtained from a public data base supplemented by confidential data.

The calibrated basin scale model provides the frame work to study HC generation for different kitchen areas. Due to large differences in the burial of the source rocks in different parts of the basins HC generation is not homogenous throughout the basin. The onset of generation within the different kitchen areas results in changing drainage areas and migration pathways through time contributing to the filling of the hydrocarbon reservoir. In the studied basin first charge occurs from the north-west of the field during Late Cretaceous time. During subsequent burial of the study area the main source rock sequence enters the oil window progressively from the North West to the South in the course of the Tertiary and up to the present. Migration of hydrocarbons into the reservoir occurs likely along faults during Cretaceous charge, switching to long-range migration along a structural ridge and controlled by fill-spill in the Tertiary. During the last 3-5 Ma hydrocarbon supply from the deep kitchens has been cut off by diagenetic sealing of the main fault controlling also the occurrence of overpressure within the system. However, not only timing but also the HC composition migrating into the reservoir changes with time affecting its composition as well as the physical properties of the trapped hydrocarbons. As source rocks mature and generate larger amounts of gas the fluid composition within the reservoir is modeled to change from an undersaturated black oil to a two phase system, matching the present phase state of the field.

Different HC-migrations modeling approaches are used to model migration pathways and the filling history of the area and the comparison of the modeling approaches with respect to accuracy and efficiency of the methods is an important aspect of the project. Presently, we use either a combination of Darcy flow and ray tracing or invasion percolation modeling to simulate the HC-filling history of the reservoir. An important aspect of the project is to model downward expulsion of a source rock through a relatively impermeable layer into a carrier bed correctly. According to general consensus the reservoir is filled form distant kitchens via long migration routes and fill-spill structures. This can be modeled successfully at present using the combined Darcy flow and ray tracing approach while the correct filling of the reservoir containing stacked sands is more accurately achieved with the invasion percolation model.

For a detailed reconstruction of the filling history of the field a basin model on reservoir scale was constructed based on a very detailed reservoir production model. The approach used here is thus a that of a “model in model”, and not actually a local grid refinement. The reservoir model was simplified by significantly reducing the number of units within the reservoir focusing on the main reservoir horizons and intercalated seals. The Quaternary to Cretaceous overburden units were added to the reservoir model to obtain a model which can be combined with the regional basin model. The small-scale reservoir will obtain the HC-fluid volumes which migrate into the reservoir from the large scale regional model. The detail filling history of the reservoir compartments is then calculated within the reservoir model.

AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.