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Structure and Seal Analysis of Hydrocarbon Fields |
1 Statoil, PO Box 300, 4001 Stavanger, Norway
2 Rock Deformation Research Group, Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
The Sleipner Vest field, located in blocks 15/6 and 15/9 on the Norwegian Continental Shelf, contains hydrocarbons (mainly gas condensates) within the marginal marine units of the Middle Jurassic Hugin Formation. The field is segmented into fault-bounded compartments, which exhibit differences in gas-water contacts of between 10 and 100m. Microstructural analysis of core samples has identified three principal fault types; cataclasites, developed from clean sandstones, framework phyllosilicate fault rocks created from impure sandstones and clay smears developed from phyllosilicate-rich units. The distribution of these has been linked to the phyllosilicate content of the undeformed Hugin reservoir at the time of deformation. Petrophysical property analysis has been used to quantify the representative permeabilities and threshold capillary pressures of the fault rocks and their undeformed equivalents. Juxtaposition/seal diagrams and detailed fault plane maps were also constructed and provide a basis for mapping sand-sand juxtapositions and the distributions of the different fault rock types with assigned permeabilities and capillary threshold pressures. The results provide an explanation of the mapped variations in gas-water contacts. An important correlation exists between the type of fault rock predicted to dominate the fault plane near the hydrocarbon-water interface and the difference in hydrocarbon depths across the faults. Compartments separated by faults with windows of juxtaposed clean sandstone cataclasites have small hydrocarbon-water level differences (7–11 m), suggesting that the low capillary threshold pressure (more permeable) cataclastic fault rocks control communication. Compartments separated by faults predicted to have extensive phyllosilicate-rich fault rocks (developed from impure sandstones and clay rich units) with high capillary threshold pressures correlate to larger hydrocarbon-water differences (>39 m), reflecting the reduced communication.
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