Abstract - One of the main mechanisms for generating fault seal is by shale smearing or shaley gouge formation in fault zones. Fault sealing potential in clastic sequences is related to the percentage of shale within the part of the sequence that has moved past a point on the fault surface; termed the Shale Gouge Ratio (SGR). Here a method is described for incorporating the capillary effects of fault seal due to the presence of shaley fault rock into migration modelling based on ray tracing methods.
Ray tracing methods of hydrocarbon migration modelling assume that hydrocarbons migrate along the top of a permeable carrier interval and upwards along the steepest dip governed by buoyancy, using grids that represent the topography of the top carrier interval. We describe a new method in which the effects of faults on migration are accounted for by depressing the top carrier surface by an amount equal to the hydrocarbon column height that can be supported at the fault. The first step is to calculate the distribution of SGR for a representative sequence containing a single carrier interval for the entire range of fault offsets in a study area. From a knowledge of the throw variations along the length of a given fault trace, the distribution of SGR values over the fault surface can be derived. The SGR values are then converted to supportable hydrocarbon column heights, based either on a direct calibration between SGR and column height or indirectly via a calibration between SGR and fault rock capillary threshold pressure.
The method accounts for fault throws both less than and greater than the carrier interval thickness. In the first case the minimum capillary threshold pressures of the fault rock determines the onset of hydrocarbon leakage. In the second case hydrocarbon leakage will occur along the fault surface with the migration pathway governed by the spatial distribution of fault rock capillary threshold pressures over the fault surface.
The method identifies those points on faults that are likely to determine the paths of major migration arteries and should therefore help identify those areas where detailed structural mapping and fault seal analysis is required. Varying the relationship between SGR and fault seal capacity, by comparing modelling results with actual hydrocarbon distributions, allows examination of the sensitivity of modelling results to fault seal calibration parameters. The method is demonstrated for an area from the North Viking Graben. Of the many uncertainties attached to this type of migration modelling, the paucity of property data for within plane flow is identified as the most significant.
In: Hydrocarbon Seal Quantification, (edited by Koestler, A.G. & Hunsdale, R.). Norwegian Petroleum Society (NPF), Special Publication, 11, 187-201, 2002.