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Faulting Processes and Fault Seal Characterization |
Institute of Petroleum and Organic Geochemistry (ICG-4), Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany
Laboratory experiments have been performed to investigate the fluid transport properties of fine-grained (pelitic) sedimentary rocks with respect to their hydrocarbon sealing efficiency. The experiments comprised the molecular transport (diffusion) of gases and the pressure-driven volume flow (Darcy flow). Diffusion coefficients of methane and nitrogen in water-saturated rock samples were measured at 90°C from an equimolar mixture of the two gases (10 MPa gas pressure). Methane had lower effective diffusion coefficients but higher steady-state molecular transport rates than nitrogen. Permeability coefficients measured with water on selected unfaulted natural samples, under controlled effective stress up to 47 MPa, ranged between 10–22 and 10–19 m2. The permeability reduction with increasing stress mostly followed an exponential relationship. Permeabilities of faulted mudstones from compressive fault zones ranged from 2 up to 20 x 10–21 m2. No systematic relationship between permeability and microfault frequency within the samples or distance of samples from microfault planes could be identified. Permeability measurements were also performed on macroscopically homogeneous Carboniferous shales from a compressive tectonic stress field. These samples, which showed distinct sonic velocity anisotropies, had permeability coefficients in the nanodarcy (nDarcy; 10–21 m2) range. Only in one case was a significant permeability anisotropy associated with the sonic velocity anisotropy.
