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Structural Trapping of Buoyant Fluids


Structural traps comprise assemblages of surfaces having high capillary threshold pressure for hydrocarbons. Individual traps may include top-, bottom- and side-seal surfaces, the latter commonly being a fault. Faults may form sealing surfaces in 2 ways: (i) juxtaposition, so that intervals of high capillary threshold pressure (e.g. shales) are juxtaposed against the reservoir unit in the trap; (ii) fault-rock, where new material (generated during slip) forms a barrier to lateral flow even when reservoirs are juxtaposed. In order to risk prospect volumes in exploration, the first step in fault characterisation is to map the distribution of offset layers at the fault. This pattern (‘Allan diagram’) is captured by the set of separation polygons, i.e. the horizon/fault intersections in 3D. These same lines define the displacement pattern on the fault surface, and so can also be used as a QC of the interpretation, since it is well established that displacement patterns of fault networks show a high degree of kinematic coherence. The separation polygons are also a key part of horizon maps, since that is how faults are represented on structure maps. However, construction of fault polygons remains a weak step in the workflow in many E&P companies. Fault rocks are generated from the wall-rock layers slipping past each other and so their composition is determined by the wall-rock lithologies and amount of displacement. The detailed internal structure of the fault rock is impossible to predict and hence it is usual to consider an upscaled proxy such as Shale Gouge Ratio, with the assumption that high SGR represents clay smears and low SGR represents clay-poor fault rock such as disaggregation zone or cataclasite. Suitable estimates of hydraulic properties can then be applied to different parts of the fault surface. Even when the juxtaposition pattern and fault-rock distribution both indicate that a fault is likely to be sealing to cross-fault flow, there is a remaining risk that buoyant fluid may escape out of the trap up the fault. Such up-fault leakage can occur simultaneously with across-fault seal. It is dependent on the stress state of the fault, in particular how close the surface is to being critically stressed. The stress state is itself dependent on the reservoir pore-pressure, and so can be changed during trap filling or production. In the latter case, pore-pressure changes may be sufficient to induce slip on previously inactive faults.