Deformation Processes and Fault Seal Mechanisms: A Combined Microstructural Analysis from the Ula Field, Central Graben, North Sea

KNIPE, R. J., and S. ANDERSON, Leeds University, Leeds, U.K., and A. MITCHELL, BP Norway, Stavenger, Norway

The understanding of fault seal generation is crucial to the assessment of the migration and trapping of hydrocarbons. The effectiveness of the seal depends upon the porosity and permeability characteristics of the fault zone, which is controlled by the microfabrics present. The range of processes which affect the microfabric evolution and sealing properties of faults from the Ula field of the Central Graben, North Sea, will be reviewed. The study illustrates the use of microstructural analysis for (a) the identification of the sealing mechanisms, (b) the timing of the seal development during the diagenetic and burial history, (c) an estimation of the sealing capacity of different seals when combined with physical property measurement, (d) the distribution of seal types within a fault array, and (e) the stability of seals during later geological events.

The field contains three types of seal: (1) collapse seals, produced by fracturing, grain deformation, dissolution, and grain boundary sliding; (2) cement seals, developed by the precipitation of phases in and adjacent to the fault; and (3) juxtaposition seals, generated by the juxtaposition of lithologies with different sealing capacities.

The examples presented highlight the need for future programs to combine microstructural studies of fault seal with the analysis of the larger scale fault geometry and the diagenetic evolution in fault zones and reservoirs. Such an integration is needed to understand

and quantify the interaction between reservoir development and faulting.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)