The Structural Origin and Relating Geological Framework of the Upper Mishrif Formation in a Mishrif-Reservoired Oil Field, Offshore Abu Dhabi.

Abstract

As an initial part of geological reservoir modeling of the Upper Mishrif Reservoir integrated with production data, the relationship between the Top Mishrif Structure and the lithofacies distribution in the Mishrif Formation was reviewed based on the existing large scale geological framework derived from seismic impedance interpretation in a Mishrif-reservoired oil field, offshore Abu Dhabi.

The presence of NW to SE trending post-reef channels cutting the reef and shoal areas was clarified during this study. The factors controlling the structural origin, such as reef/shoal depositional morphology, differential compaction and tectonic modification as well as erosional topography, are carefully examined and summarized that differential compaction is the major factor and the others have locally affected on the structure. To check the reality of this interpretation and to delineate potential development area, a synthetic structural map of the Top Mishrif Formation was generated from the isopachs of the Mishrif Formation and the Laffan/Halul Formations and the structural map of the Top Mauddud Formation. The result indicates good matching of structural shape with conventional structural map and suggests a potential high area for possible development.

The medium scale geological framework, obtained from this review, in the Upper Mishrif Reservoir, is considered as a primary controlling system of fluid movement in connection with oil production. Different production performance particularly relating to water cut of some wells penetrated in the different lithofacies supports this geological framework.

The further integration of this medium scale geological framework and detailed production data is being done prior to the construction of fine scale geological model for future reservoir management.

Introduction

A Mishrif-reservoired oil field is located offshore Abu Dhabi, approximately 20km northwest of Abu Dhabi City (Fig. 1). The discovered well of this field was drilled in 1969 at the crest of the seismic structure at the Top Mishrif horizon. Total of 58 wells have been drilled in the field. Oil production from the Mishrif Reservoir continues since 1985.

The Mishrif Structure of this field had been understood during the early exploration/development stage as a pinnacle reef origin according to its shape. In the crestal area of the structure, lack of both the remarkable gamma ray/porosity markers and the sufficient cores in the Mishrif reefal limes tones have made geological modeling difficult. To compensate a shortage of geological information from well data, 2D seismic impedance data was integrated in the geological modeling in 1983. The results of its interpretation, which showed the four-staged reef development, the westward progradation nature of reef/shoal and the considerable erosion and deposition of reworked sediments (Post-reef Facies) after reefal stages, has denied a pinnacle reef origin of the structure.

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