Abstract
Geologic events that produce supergiant fields as proposed here recognize that field size is limited by either the size of the trap or the volume of charge. Charge commonly exceeds trap volume so that trap limits field size, and, because of this, the focus is on the origin of large traps with large seal area. This report shows that large seal area is commonly associated with sag of various origins and eustatics. Sags, which are areas of deposition and subsidence without significant faulting, involve more than 80% of supergiants.
Sag events perform multiple functions in the formation of hydrocarbon traps; they increase both bed seal and reduce vertical fault migration and cross fault leakage, thus increasing trap volume and lateral migration. Cross fault leakage is reduced due to decreased repetition of reservoir-to-reservoir contact. Sags also increase thermal maturation, which may increase charge even as structures continue to grow.
Cretaceous reservoirs host almost one-third of all giants, and almost half of all giants have Jurassic or Cretaceous reservoirs. This concentration results from the combined effects of postbreakup sags of Pangea coincident with Mesozoic rising sea level. The deposition of areally extensive sediments increases both seal area and deposition of source.
The Zagros fold belt is a significant exception to most supergiants. The Zagros is a geologically young, detached compressional fold belt without a sag event with exceptionally large folds. The age is important because the typical Zagros “sled runner” faults have not yet penetrated the leading folds that contain the largest fields.
