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AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain

Understanding Reservoir Quality in Ara Stringers (South Oman Salt Basin): Diagenetic Relationships in Space and Time

Stephan Becker1; Lars Reuning1; Peter A. Kukla1; Steffen Abe2; Shiyuan Li2; Janos L. Urai2; Suleiman Farqani3; Zuwena Rawahi3

(1) Geological Institute, RWTH Aachen University, Aachen, Germany.

(2) Structural Geology, Tectonics and Geomechanics, RWTH Aachen University, Aachen, Germany.

(3) 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 several 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 twofold. Firstly, to detect trends in the spatial distribution of diagenetic phases within the stringers and their effect on reservoir properties. Secondly, to unravel the relative timing of diagenetic phases and to link them to the burial history of the salt basin. Mineralogy, rock fabrics and geochemistry of ~ 200 samples from several petroleum wells from the late Neoproterozoic A2C interval were analyzed and combined with pre-existing data.

Our analysis demonstrates that permeability is to a large extend governed by dolomite crystal size. For a given porosity rock fabrics with larger crystal sizes show higher permeabilities. Crystal size is strongly controlled by depositional facies. Grainstone and boundstone facies show larger crystal sizes than mudstone to packstone facies. The crystal size distribution was determined for cored wells by thin section analysis and estimated for uncored wells from borehole-image-log-derived lithofacies distribution. The combination of porosity and crystal size information from logging and core data allows calculation of field-scale permeability maps with high vertical and lateral resolution. These maps comprise crucial information for better prediction of reservoir quality in the analyzed fields, planning of new exploration wells and better volumetric calculations.

An integration of the paragenetic sequence derived from thin-section analysis with results from finite element and discrete element models further helps to constrain the effect of salt tectonics on fracture formation and fluid evolution within the stringers.