The Influence of Dolomite Crystal-Size and Sorting on the Reservoir Quality within Carbonate Stringers of the South Oman Salt Basin

Morwinski, Jana ¹; Becker, Stephan ¹; Reuning, Lars ¹; Kukla, Peter ¹; Lopes Cardozo, Gideon G.²; Farqani, Suleiman ²; Rawahi, Zuwena ²
(1)Geological Institute, RWTH Aachen University, Aachen, Germany. (2) Petroleum Development Oman, Muscat, Oman.

The Ediacaran-Early Cambrian Ara Group of the South Oman Salt Basin consists of six carbonate to evaporite (rock salt, gypsum) sequences. These Ara Group carbonates are termed A0C to A6C from the bottom towards the top of the basin. Differential loading of locally 5 km thick Cambrian to Ordovician clastics onto the mobile rock salt of the Ara Group caused growth of isolated salt diapirs, which resulted in strong fragmentation and faulting of the carbonate intervals into several isolated so-called ‘stringers’. These carbonate stringers represent a unique intra-salt petroleum system, which has been successfully explored in recent years.

The goal of this study is to examine the influence of dolomite crystal-size and sorting on reservoir quality. In total ~600 thin-sections from several petroleum wells of two producing fields have been analyzed and combined with pre-existing PoroPerm data. Although the carbonate stringers experienced a complex history of diagenetic processes, the dolomite crystal-size distribution depends largely on the primary sorting and hence sedimentary facies. Facies belonging to the same Dunham class are characterized by similar crystal-size distributions, with the largest crystal sizes typically found in grainstones and boundstones. Generally, the highest permeabilities are observed in zones that are dominated by these large crystal-sizes.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.