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Abstract

Structural traps comprise surfaces having high capillary threshold pressure for fluid phases such as hydrocarbon or CO2. Traps may include top-seal, bottom-seal and side-seal surfaces, the latter commonly being faults. Faults may form seals by juxtaposition or by fault-rock. The first step in fault characterization is mapping the distribution of offset layers at the fault surface. This pattern (‘Allan diagram’) comprises the separation polygons. These same lines define the displacement distribution on the fault, and so can also be used as a quality-check of the interpretation. They are also a fundamental part of horizon maps. Despite their central role in subsurface mapping, construction of fault polygons remains a weak step in the workflow in many E&P companies. To characterize fault-rock effects, it is usual to consider an upscaled proxy such as shale gouge ratio (SGR), with the assumption that high SGR represents clay smears and low SGR represents clay-poor fault rock. Suitable estimates of hydraulic properties can then be applied to different parts of the fault surface. The property distribution on the fault side-seals allows an estimate of the maximum hydrocarbon column height that the trap might contain, by comparing the multiple potential leak-points and finding which predicts the shallowest contact.

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