Caroline Gill of the University of Edinburgh described footwall release faults, a new fault classification, at the American Association of Petroleum Geologists convention in Calgary, Canada. John Underhill, University of Edinburgh, presented the findings based on 3D seismic work done on the Murchison field, UK North Sea.
Footwall release faults are strike perpendicular syn-sedimentary faults that decrease in displacement as they move away from the main controlling fault. The investigators found that secondary faults on the upthrown (footwall) side of the Murchison fault have similarities to an earlier described fault type on the downthrown (hanging wall) side of a primary fault. Brittle failure of the rocks on the footwall side of the primary fault increases prospectivity by compartmentalizing the upthrown structure to form multiple hydrocarbon traps.
Charles Barnes of Stone Energy presented a different role for secondary faults in the Gulf of Mexico. He found that compact structures, formed on the downthrown side at the juncture of the primary fault and secondary fault, create a zone of weakness for the upward movement of hydrocarbon-bearing fluids.
Interpreters can see the compact structures on 3D seismic near the top of geopressure on the downthrown sides of deeply rooted faults. Fluid expulsion from geopressured shales on the upthrown side of the main fault form collapse features around the zones.
The features can be identified by bright spots on 3D seismic and may be the link between producing fields and source rocks at depth. These zones of weakness develop upward along the primary fault plane and have a seabed expression as mud volcanoes or smaller gas vents.