290 PETROLEUM GEOLOGY/The Petroleum System Figure 29 Phase separation of upward migrating petroleum depending on the initial gas/oil ratio (GOR) and reductions in temperature and pressure In terms of a seal, efficiencies range from near perfect in the case of intact salt (Halite) to ‘highly leaky’ in the case of thin and poorly compacted mudstone containing a significant portion silt and fine sand Diagenetic cement can greatly enhance the sealing efficiency of more porous strata, even when relatively shallow In essence the efficiency of a seal must be measured against the charge rate if generation continues, or the time since charge ceased if in the past A more specific and dramatic way of breaching the seal of the trap is faulting For structural traps, the prediction of whether a fault is a seal or a migration conduit is complex, the controlling factors being only poorly understood at present Some unusual seals comprise a hydrodynamic trap, where a counterflow of formation water opposes the buoyancy of the oil, and shallow seals in the Arctic involving the permafrost layer or gas hydrate (see Petroleum Geology: Gas Hydrates) The concept of the ‘half-life’ of an oil or gas field has been proposed, this being defined as the time (millions of years) required for half of the charge volume to escape through the seal Gas sealed by mudstone has a half-life in the range 30–60 My, and oil about an order of magnitude greater A continuous and stable halite seal can be considered near perfect with an infinite half-life A number of intrareservoir processes are listed towards the base of Figure 30, and of these the most common are bacterial degradation and thermal cracking These occur when the reservoir is unusually cool (165 C), respectively Of the listed intrareservoir processes, bacterial degradation is arguably the most destructive In essence it occurs when petroleum accumulates in a reservoir that is at a temperature allowing bacterial degradation of the oil or gas As reported, the maximum