Jan-Henk van Konijnenburg1,
Jose Luis Massaferro1,
(1) Shell Exploration and Production Technical Aplications and Research (SEPTAR), Rijswijk, Netherlands
(2) Petroleum Development Oman (PDO), Muscat, Oman
Abstract: Explaining early water breakthrough in a tough carbonate reservoir: the Natih E formation, Al Ghubar Field, Oman
The Al Ghubar Field is located in the Ghaba Salt basin of Central Oman. The structure consists of a low relief dip closure developed on the northern side of a major normal fault. The field consists of 3 stacked carbonate reservoirs (Shuaiba, Natih E and Natih A/B). The middle Cretaceous Natih E reservoir is the topic of this integrated study.
Current gross production is about 680 m3/d, with a water-cut of up to 80%. Well production history is characterized by early and massive water break-through, which has been attributed to both the presence of open faults or fractures, and, for some wells, to proximity to the OWC. Production from the field will go into steady decline without further development.
The Al Ghubar Natih E reservoir was deposited as part of a 3rd order sequence on the interior of the regional Natih platform. The reservoir comprises a 4th order cyclic alternation of wackestones, packstones and grainstones, with the occasional presence of algal boundstones. Porosities in pack- and grainstones are between 30 and 40%. Pore connectivity is best in the grainstones with permeabilities as high as 1D. The location of these grainstones is determined by the 3rd and 4th order depositional cyclicity and can be predicted by integrating well log correlation and seismic stratigraphy.
Throughout the oil-saturated part of the Natih E cemented streaks are encountered with varying degrees of fracturing on a cm to a dm-scale. These cemented streaks are thin (20-70cm) and probably aerially extensive although they can rarely be correlated between wells. The fracture network in these streaks considerably enhances the permeability of the matrix. The combination of high permeable grainstones and cemented streaks can explain the observed water-cut behavior.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana