Origins and Habitats of Super-Giant Fields
Richard S. Bishop, Ph.D.
International Oil and Gas Consultant
Geologic events that produce super-giant 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 trap limits field size and, because of this, the focus of this talk is on the origin of large traps with large seal area. 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 percent of super-giants.
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 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 super-giants have Jurassic or Cretaceous reservoirs. This concentration results from the combined effects of post-breakup sag of Pangea coincident with Mesozoic rising sea level. The deposition of areally extensive sediments increases seal area and deposition of source and reservoir.
The Zagros foldbelt is a significant exception to most super-giants. The Zagros is a geologically young, detached compressional foldbelt without a sag event and has exceptionally large folds. The age is important because the typical Zagros ‘sledrunner’ faults have not yet penetrated the leading folds that contain the largest fields.