Surface seepage and sub-surface destructive processes as controls on the distribution of giant oilfields
Study of a database of 350 giant oilfields show these to be dynamic short lived phenomena, with a median age of 35 Ma. A third show evidence of post-entrapment destructive processes, particularly erosion, fault leakage and gas flushing. Biodegradation is a destructive process most active during oil entrapment. Re-entrapment of oil released from spilling or breached traps is common. These processes are illustrated with case examples from SE Asia and throughout the world. The main controls on oilfield preservation are post-entrapment tectonism and seal type, with temperature and hydrodynamic regimes being secondary factors. Destructive processes are concentrated in shallow and deep zones and in seepage-prone traps such as compressional anticlines. Such factors strongly influence the distribution of preserved light oilfields, with preservation potential maximised in tectonically quiescent basins with evaporite or thick mudstone seals e.g. the Middle East and the Permian Basin, or in basin centres distant from inverted or uplifted zones e.g. Central Sumatra. More attention is required in prospect and regional evaluations to models involving post-entrapment leakage and re-migration.