BENTLEY, MARK, Petroleum Development Oman, Muscat, Oman

ABSTRACT: A Critical Distinction between Fracture Types in North Oman and Its Bearing on Field Development

Natural fracturing is common to most carbonate reservoirs in North Oman, but production performance differs significantly in reservoirs dominated by joints (`extensional fractures') compared to those dominated by sub-seismic open faults (`shearfractures'). The key difference is that in jointed reservoirs the high-density, high-volume fracture network is a prime drilling target, whereas in faulted reservoirs penetration of the low-density, low- volume fracture network promotes uneconomic water breakthrough. Successful development depends on early recognition of the fracture type, particularly in fields which are marginally economic owing to thin oil column or low matrix permeability.

The importance of fault/joint distinction is illustrated by a case study from the Al Ghubar field in the Cretaceous Natih Formation of the Ghaba Salt Basin. Borehole imaging logs and core data reveal a highly heterogeneous network of small-scale normal faults with minor (<1m) displacements. Wells perforated over faulted intervals are characterised by rapid water breakthrough, and the relative importance of individual fault sets is indicated by in situ stress patterns estimated from borehole elongation.

Al Ghubar is an example of a fractured limestone in which the fractures are small-scale open faults rather than joints. In contrast to jointed reservoirs, development at Al Ghubar therefore involves the identification of small-scale faults from open hole logs prior to the selection of perforation intervals, and the use of a fractured reservoir simulator capable of describing a heterogeneous, non-orthogonal, low-volume fracture network in finely- layered carbonates.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.