Clay Smear (SGR) Along Faults and the Implications from Water Flow and Fluid Overpressures
Ane E. Lothe1, Are Tommeras2, and Hans Martin Helset1
1Basin Modelling, SINTEF Petroleum Research, Trondheim, Norway
2Migris AS, Trondheim, Norway
Lateral fluid flow along and across faults are one of several factors that control pore pressure distribution in a sedimentary basin.The sealing properties of faults are defined by the throw, burial depth, lithology and diagenesis. Clay smearing is widely recognized as one of the processes controlling clastic fault seal properties.
Here we describe a technique to model the influence of clay smear along faults on water flow. A similar approach has earlier been applied to hydrocarbon migration modelling. The method employed is based on the recognition of the fault seal capacity related to the percentage of sand/clay in a sequence that has moved past a point on a fault, termed the Shale Gouge Ratio (SGR). SGR values on the modeled fault surfaces are calculated from the fault throw and shale volume (Vshale) distribution on the fault surface. Vshale-logs from nearby wells are interpolated using a fast interpolation scheme constrained by a 3D stratigraphical model. The effect on the fault transmissibility is implemented using a fault permeability model outlined from well data (Sperrevik et al. 2002). The pressure build up in the basin is simulated over geological time using a 3D pressure simulator.
The clay smear model has been tested on well data from the northern North Sea where the lateral pressure distribution within Cook Fm and Statfjord Fm reservoir units have been modelled. Very good match with measured data from wells are obtained, giving realistic pressure distribution throughout the basin. Cut-off values for fault permeabilities are evaluated in order to remove artificially high and low values. Ref: Sperrevik, S. et al. (2002). Empirical estimation of fault rock properties. Hydrocarbon Seal Quantification. A. G. Koestler & R. Hunsdale. Amsterdam, Elsevier Science B.V. NPF Special Publications 11, 109-125.
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