Sealing Attributes of Giant Fields from Around the World
Jose I. Guzman1, Rod Sloan2, Shengyu Wu1, and Shaoqing Sun2
1 C&C Reservoirs, Inc, Houston, TX
2 C&C Reservoirs, Ltd, Maidenhead, Berks, United Kingdom
A systematic analysis of sealing parameters from over 220 giant fields (>500 MMBOE EUR) in 84 basins worldwide reveals that seal capacity, i.e., the calculated hydrocarbon column height that a seal can support, is more important than the degree of trap fill. The average hydrocarbon column height retained is ~1500 ft and the maximum is ~7500 ft. However, over half of the fields occur in underfilled traps. This suggests that if structural closure is sufficiently large and hydrocarbon charge is adequate, giant accumulations can occur even if the traps are leaking.
Over 33% of all reservoirs in giant fields are sealed by top-dominated seals that involve a combination of simple top seals and lateral faults, updip faults and/or lateral stratigraphic seals. Nearly 25% of the reservoirs are capped by simple top seals alone, that is, no other sealing mechanism is involved, and 22% are sealed by an updip fault or a stratigraphic seal. Three quarters of the reservoirs are top-sealed by clastics (mostly shales), and only ~10% are sealed by evaporites. Top seal thickness varies from 10 ft to 6500 ft and averages ~700 ft, but seal thickness by itself is not a reliable indicator of hydrocarbon column height or of seal efficiency.
Giant fields sealed by updip faults usually require that fault throws be larger than reservoir thickness, and that average reservoir dips be >5°. Stratigraphic sealing is most common by onlap or by updip clastic facies changes in low-dip traps with large productive areas.