Petroleum seepage and post-accumulation migration

The processes governing the loss of petroleum from traps and its migration to the surface are poorly understood. Leakage from traps does not necessarily result in surface seeps, nor are all occurrences of surface petroleum the result of present-day trap leakage. Three types of seep are recognized: unaltered gassy seeps, seeps affected by surface dispersal processes, and seeps altered by subsurface processes. Unaltered seeps form about 30% of the recorded population and are characterized by the presence of a free gas component. The buoyancy of the gas phase appears to drive most seeps to the surface. Non-associated oil seeps, often sharing their point of efflux with spring water, are altered by near-surface processes, especially the segregation of the oil and gas phases at the water table. Seeps altered by subsurface processes are frequently sour, containing free hydrogen sulphide, and are often associated with leakage pathway mineralisation. The geological context and characteristics of seepage deserve attention, leading to more realistic models for oilfield growth and decay, and to a better understanding of the environmental and mineralising effects of seepage.