High Resolution Sequence Stratigraphy of the Fahud Natih C & D Reservoir Units: Constraints on Production

By

 Younis Altobi¹, Volker Vahrenkamp², Peter Homewood³

(1) University of Texas, Austin, TX (2) Petroleum Development Oman, Muscat, Oman (3) Sultan Qaboos University, Al Khod, Oman

 Vertical and lateral heterogeneities due to depositional and diagenetic variations are common in the Lower Cretaceous Carbonates of North Oman. This exerts a major constraint on oil-field development schemes such as GOGD versus water-flood development in Natih reservoir units of the Fahud Field.

Facies distributions were studied in six cored wells in Fahud Field. Core descriptions, a regional overview based on outcrop analogues, and a literature survey were used to build a core-log depositional model indicating that sediments were laid down over a low topography deeper shelf.

High Resolution Sequence Stratigraphy established the stratigraphic architecture, dividing the Natih C & D into 3 orders of cyclicity. The 4th order cycles are the highest level of cyclicity that can be correlated easily across the field. The 3rd order regressive-transgressive cycles form the larger units and sequences, roughly equivalent to the existing reservoir subdivisions (Natih A-G). These results were used to build a 3D static reservoir model for the Natih C & D Reservoir Units.

In Rudist Sands, Wackstone/Packstones and Packstone/Grainstones, where fractures occur in 4th order regressive cycles within 3rd order regressive cycles, it seems doubtful as to whether a connected fracture network exists. The reservoir rock is interpreted as too weak to maintain large-scale fractures, so water-flood development could be successful. More competent and highly permeable Rud/Floatstones, with dissolution of the shell fragments leading to a well-connected vuggy-pore system (transgressive part of a 3rd order sequence), is interpreted as suitable for GOGD development.