Abstract - One of the main mechanisms for generating fault seal is by shale smearing or shaley gouge formation in the fault zone and fault sealing potential is therefore generally considered to be related to the amount of shale within the faulted sequence (Yielding et al. 1997). This relationship is quantified by calibrating some function of the proportion of shale within a faulted sequence against observed fault sealing behaviour. The simplest of these functions, the percentage shale within that part of the sequence which has moved past a point on the fault surface, is termed the Shale Gouge Ratio (SGR). Calibrations of SGR against fault sealing potential can be used to risk and evaluate fault seal prospects (Gibson 1994). While this approach is widely applied to fault trap evaluation, to our knowledge it has not previously been implemented in hydrocarbon migration models. Here a method is described for incorporating the effects of fault seal due to the presence of shaley fault rock into migration modelling based on ray tracing methods. The method is implemented within SEMI migration modelling software (Sylta 1991).
Ray tracing methods of hydrocarbon migration modelling assume that hydrocarbons migrate along the top of a permeable carrier bed and upwards along the steepest dip according to buoyancy. Ray tracing studies are performed on grids representing the topography of the top carrier bed. Structural traps are culminations in the carrier bed topography which are filled with hydrocarbon until either a spill point is reached or until the seal capacity of the top seal is exceeded by the buoyancy exerted by the trapped column. The method adopted here is to modify the elevations of the carrier bed at faults, so that the grid cells are depressed by an amount equal to the hydrocarbon column height which can be supported by the fault. A fault which offsets the carrier bed is in effect a curtain which hangs beneath the carrier by an amount equal to the seal potential at each point along the fault trace. Represented in this way the ray tracing method employed in SEMI software automatically locates the highest across-fault spill point.
In a migration/exploration setting, it is not feasible to perform a detailed fault seal analysis on all of the faults which may affect the loci of focussed migration pathways. The method adopted here is to first evaluate SGR for a representative sequence containing the carrier interval, offset by a notional fault which has throws between zero and the sequence thickness. The distribution of SGR over such a notional fault is displayed on a sequence/throw juxtaposition diagram (Fig. 1). From this diagram and a knowledge of the throw distribution along the length of a fault trace, the distribution of SGR values over the fault surface can be derived. The SGR values at each point in Fig. 1 can then be converted to supportable hydrocarbon column heights. This conversion is based either on a direct calibration between SGR and column height or indirectly via a calibration between SGR and fault rock breakthrough pressure (Gibson 1998).
The incorporation of fault properties into ray tracing migration models requires consideration of two cases. In the first case the carrier interval is self juxtaposed and hydrocarbon leakage may occur across the fault. In this case the hydrocarbon column height which can be supported by the fault is determined by the breakthrough pressures of the fault rock separating the upthrown and downthrown carrier interval. The column height is therefore determined by the minimum buoyancy pressure required to exceed the fault zone breakthrough pressure. This case is represented in the carrier bed topography as a grid cell which is depressed by an amount equal to this minimum column height. The second case is where the fault throw exceeds the thickness of the carrier interval and hydrocarbon leakage will occur along the fault surface. In this case the column height is governed by fault zone breakthrough pressures along that portion of the fault which connects the downthrown and upthrown carrier interval (the self-separated region in Fig. 1). Within the carrier bed grid, a fault surface separating the upthrown and downthrown carrier is represented as a grid of transformed elevations accounting for the supportable hydrocarbon column heights over the fault surface. The result of SEMI ray tracing across this elevation grid is equivalent to deriving the path of minimum resistance to flow across a grid of fault surface breakthrough pressures and is equivalent to modelling migration within the fault zone.
Sample output from SEMI modelling in Figure 2 shows a gas accumulation, the height of which is controlled by the fault seal potential at the spill point labelled 1. Hydrocarbon leakage from the weakest point on this and other faults results in very focussed hydrocarbon migration with focussing of gas promoting the bypassing of existing oil accumulations. Hydrocarbon migration in areas where shale/clay smearing occurs is strongly focussed. In such areas examination of the mapped fault distribution is critical to understanding the migration history and 2D cross-sectional modelling is inadequate. The method presented here identifies those points on faults which are likely to determine the paths of major migration arteries and assist the explorationist in identifying those areas where a more detailed structural mapping and fault seal analysis is appropriate. Varying the relationship between SGR and fault seal capacity allows examination of the sensitivity of modelling results to the details of the fault seal calibration.
Figure 1: Sequence/throw juxtaposition diagram showing the variation in SGR with fault throw for the vshale curve shown on the left hand side. The inset illustrates the construction method. The two horizontal lines show the elevation of the top and base of the carrier interval on the upthrown side of the notional fault and the diagonal lines show their downthrown elevations. The horizontal lined fill shows the area where the carrier interval is self-juxtaposed. The vertical lines show the area of the plot relevant to estimation of column heights when the carrier interval is self-separated and within which fault zone migration occurs.
Figure 2: Map showing the results of SEMI modelling. The map area is shaded according to gas flow rate over a model time step where high flow rates are darker colours. Fault polygons are white and all faults dip to the north. The regional dip of the carrier bed is also to the north. Gas is shown grey and oil in black. Bold lines show the main fault related spill points. The spill point labelled 1 is discussed in the text. The map area is 4x8km.
References
Gibson, R. G. 1994. Fault-zone seals in siliciclastic strata of the Columbus Basin, offshore Trinidad. A.A.P.G. Bull. 78/9, 1372-1385.
Gibson, R. G. 1998. Physical character and fluid-flow properties of sandstone-derived fault zones. In: Structural Geology in Reservoir Characterisation (edited by Coward, M. P., Daltaban, T. S. & Johnson, H.) Spec. Publ. geol. Soc. Lond 127, 83-97.
Sylta, Ø.1991. Modelling of secondary migration and entrapment of a multicomponent hydrocarbon mixture using equation of state and ray-tracing modelling techniques. In: Petroleum Migration (edited by England, W. A. & Fleet, A. J.) Spec. Publ. geol. Soc. Lond. 59, 111-122.
Yielding, G., Freeman, B. & Needham, D. T. 1997. Quantitative fault seal prediction. A.A.P.G. Bull. 81/6, 897-917.
Abstract of talk given to:
EAGE 61st conference and technical exhibition, Helsinki, Finland, 7-11 June 99