Structural Evaluation of Petroleum Sealing Capacity of Faults

Abstract

Japan National Oil Corporation is conducting a five-year project on the Evaluation of Traps and Seals. As part of this Project we have designed a conceptual model for evaluating faults as petroleum conduits or barriers in sedimentary basins. This structural evaluation draws on several geological databases from the North Sea, UK, Niger Delta, Utah, Texas, Gulf of Mexico, Trinidad, Persian Gulf, and Indonesia. Our model integrates various parameters affecting the flow properties of faults into tectonic setting and fault style, fault geometry, fault damage zone, fault rock in contrast to host rock characteristics, and fault chronology and time-scale (geological, production and seismic time scales). We distinguish between "fault traps" (due to juxtaposition of shale against sandstone layers) and "sealing faults" (in which mechanical and chemical changes along the fault plane produces a barrier to petroleum flow. Allan-type juxtaposition diagrams (fault-plane section analysis) are useful tools to evaluate fault traps but not sealing faults. Major processes of sealing faults include fault compaction and cataclasis, cementation due to fluid channeling along the fault, and clay smearing. Data on fault rocks and their corresponding undeformed host rocks show that faulting processes can reduce the rock permeability by two orders of magnitude. Quantitative a approaches for assessment of clay-smearing include Clay Smear Potential (CSP), Shale Smear Factor (SSF), and Shale Gouge Ratio (SGR). All these parameters depend on the thickness of faulted shale layer and fault throw. The available data indicate that CSP of >15, SSF of <7, and SGR of >18% are threshold values for fault sealing in normal faults. While across-fault direction may be a membrane seal for petroleum flow, the fault damage zone may actually increase the permeability for the flow along and in the vicinity of the fault if the open fractures are not healed by mineralization and collapse. Analysis of fracture density, aperture, orientation, and connectivity, as well as fault-zone diagenesis are therefore important in fault-sealing evaluation. Our conceptual model is helpful in exploration because faults commonly occur in sedimentary basins and such a model provides a tool to rank a population of fault-bound prospects based on their relative and probable fault sealing capacity.