ABSTRACT: Snap, Crackle & Pop: Risking Top Seal Integrity

SKERLEC, GRANT M., PetroQuest International Inc., Franklin, PA

A new approach to top seal risk is based upon quantitative estimates of both the mechanical properties and the strain history of the seal. Destruction of seal integrity by fracturing occurs when a brittle top seal is subjected to sufficient strain. Seal integrity is preserved if the strain is less than a critical threshold value or if the top seal is sufficiently ductile. Seal integrity determines not only the presence or absence of hydrocarbons but also the vertical distribution of hydrocarbons in complex traps with stacked reservoirs/seals.

Rock mechanics data show that the ductility of a shale top seal can be inferred from shale density or seismic velocity. Dense, compacted shales are brittle, and low-density, less compacted shales are ductile. The implications for top seal analysis are profound since ductility and consequently seal risk must change with progressive subsidence and time.

Quantitative strain analysis of seismic profiles provides estimates of the magnitude of strain and its variation with time. Estimates of strain, ductility, and confining pressure during trap evolution have been successfully used to predict top seal integrity of North Sea salt structures. The approach is equally applicable to traps in other structural styles including foreland fold and thrust belts, Gulf Coast growth faults, and convergent wrench faults.

The woeful state of ignorance concerning both top seal and fault seal is due to the lack of an empirical database of seal behavior in producing basins as well as the lack of critical data necessary for seal prediction.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)