Abstract

Since few publications exist regarding the Middle Jurassic Uwainat carbonate sedimentology in Qatar, this paper contributes significantly to the understanding of Uwainat sedimentology and depositional process in Qatar and in the southern area of the Arabian Gulf. This paper also demonstrates how a detailed sequence stratigraphic framework for Uwainat, established for the first time in Maydan Mahzam and Bul Hanine Fields, was integrated with sedimentology, to build proper stratigraphic and facies models. In addition, the results of this study confirmed that a facies based model is an excellent solution for capturing and predicting the high permeable facies/dynamic rock types in their proper time/depth location within a high resolution stratigraphic cycle. Compared to previous work on Uwainat in Maydan Mahzam field, this study allowed a good and promising early history matching of the field performance. Integrating various disciplines to analyze data and build a 3D geological model of a heterogeneous carbonate reservoir is key to a proper workflow. The work flow developed for this study could be used to describe other carbonate reservoirs in the region.

Introduction

The Mid-Jurassic reservoirs are part of Qatar Petroleum's Maydan Mahzam and Bul Hanine offshore oil fields. These reservoirs, Araej Upper, Uwainat, Araej Lower and Izhara have been on production for 20 years.

Bul Hanine (BH) and Maydan Mahzam (MM) are mature fields located Offshore Qatar some 120 kms east of Doha. In both the United Arab Emirates (UAE) and Qatar, the Middle Jurassic comprises the Izhara and Araej Formations. The Middle Jurassic is known as Dhruma Formation in Saudi Arabia and other places in the Arabian Gulf. The Araej Formation is divided into three members: Araej Upper, Uwainat and Araej Lower.

The Uwainat reservoir is the middle member of the Araej Formation (Bathonian-Callovian age) and is considered as the most significant horizon of the Middle Jurassic Formations. The Uwainat thickness, approximately 180 feet, is relatively uniform throughout the southern area of the Arabian Gulf and consists of 5 to 20 ft thick sedimentary cycles of alternating low permeability lime wacke-/packstones, and 0.5 to 5ft thin relatively permeable pack/grainstone layers.

The grainstone layers, which impact reservoir performance, cannot be reliably detected directly from logs. Diagenesis plays a crucial role in reservoir development. The main diagenetic processes are early compaction, early leaching, dolomitization, cementation, stylolitisation and late leaching. In the main porous wack/packstones porosity is mainly of the leached secondary fine inter-particle and leached secondary intra-particle type. In the permeable grainstone beds, primary and secondary inter-particle/granular porosity are the dominant pore types.

The dominant grain types in the Uwainat sediments are pellets/peloids, benthonic foraminifera, echinoderm, shell fragments and mud. Corals are especially common in the upper part of the Uwainat reservoir and oncoids appear only in the lower part. Most of the grains are largely micritized. Throughout the Uwainat, sediments have been affected by bioturbation.

The Uwainat sediments were deposited in a shallow marine shelf environment, the nature of which is slightly restricted in the lower Uwainat, and normal marine in the upper Uwainat. The overall energy of deposition is moderate to high.

The grainstone layers within the Uwainat, which impact reservoir performance, cannot be reliably detected directly from logs because of their limited thickness. The better porosities and permeabilities occur mainly in the upper half of the reservoir.

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