Abstract

Determination of the nature and distribution of fractures and stylolites in reservoir rocks requires careful analysis of core, and correlation of fracture and stylolite frequencies with geological drivers. Traditional geological skills are required to interpret the data and to enable a field to be populated with fractures and stylolites in a reservoir model. The relative timing of events plays a crucial role in determining the nature of fractures. For example, open fractures tend to be clustered around faults if the open fractures and faults formed at the same time, but clustering does not occur if the open fractures pre-date or post-date the faults.

Introduction

Fractures and stylolites are being increasingly recognised as influencing or controlling production in the carbonate reservoirs of the Middle East. Open fractures enhance fluid flow but can also cause water breakthrough problems. Zones of increased cementation around stylolites can reduce porosity and act as barriers to fluid flow, although stylolite-related open fractures can increase permeability. Core provides the only opportunity to directly analyse reservoir rocks, so full use should be made of this valuable resource to provide inputs into reservoir simulation models.

The aim of this paper is to show how traditional geological skills can be applied to determine the geometry, history and distribution of fractures and stylolites. Of particular importance is the need to correlate fracture frequencies and stylolite intensities with other geological factors. Core data from Middle East carbonate reservoirs are compared with exposed analogues in the Oman Mountains and southern England.

Controls on the distribution of fractures

Fracture is a general term for a crack across which there is some displacement. Geological fractures include faults, dykes, veins and joints. Veins are mineral-filled [e.g. 1], are commonly mode I fractures, and usually form as en echelon segments in a vein array. Joints are mode I fractures, usually with apertures of less than a few millimetres, across which there has been no measurable shear displacement [e.g. 2], and are not mineral-filled.

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