Abstract: Studies of Hydrocarbon Migration: an Important Discipline in Hydrocarbon Exploration
HERMANRUD, CHRISTIAN, HEGE MARl NORDGARD BOLAS, CHRISTINE FICHLER and ASTRI RORNES, and ROAR HEGGLAND
All hydrocarbon-bearing reservoir rocks comprise parts of subsurface pressure compartments, within which fluids can move freely over geological time. The fluid pressures within each of them reflect the density of the fluids as well as the velocity of groundwater movement in hydrodynamic areas. Such pressure compartments are sealed by relatively impermeable barriers (e.g. sealing faults and caprocks), and pressure differences between neighboring compartments reflect the resistance to fluid flow cross their boundaries.
As the porosity of reservoir rocks in most pressure compartments (both overpressured and almost normally pressured) is reduced during burial, it is inferred that compartments leak as they subside. The important factors to be addressed for risk evaluation are thus: (a) how do they leak; and (b) where do they leak?
Most overpressured, hydrocarbon-bearing structures in the Norwegian North Sea are currently leaking, as evidenced by seismic chimneys and hydrocarbon shows in caprocks (e.g. the Hild, Huldra, Gullfaks, Oseberg and Tommeliten fields). However, the leakage rates are apparently insufficient to empty the reservoirs. In contrast to this, all of the overpressured structures in the Haltenbanken area of the Norwegian Sea (some 300 km to the north) are empty, although the presence of residual oil suggests that the reservoirs have been charged. A possible explanation of this phenomenon is that different stress histories in the two areas have led to differences in the leakage rates, and thus to their prospectivity.
Pressure compartments do not necessarily leak at their shallowest point, as leakage may well occur in downflank positions (e.g. through faults or fault intersections). Studies of hydrocarbon indicators from seismic data (e.g. seismic chimneys, bright spots, pockmarks) and aeromagnetic data can be used to help identify both migration routes and the locations of leakage points.
AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria