Dunlin Field - A Review of Development and Reservoir Performance to Date
Authors: Van Rijswijk J.J. , Robottom D.J. , Sprakes C.W. , James D.G. 
Published: December 2007

This paper discusses the reservoir geology, fluid properties, and reservoir performance of the Dunlin properties, and reservoir performance of the Dunlin field in the U.K. sector of the northern North Sea. Because of the heavily faulted nature of the field, the development plan has needed continuous updating in the light of new geological and reservoir information. This has necessitated the use of a flexible plan beginning early in the history of the field.

Introduction

The Dunlin field is situated in the northern North Sea, close to the Norwegian boundary between latitudes 61 deg. 13'N and 61 deg. 19'N. It straddles the border between the Shell/Esso Block 211/23 and the Gulf/Conoco/British Natl. Oil Corp. Block 211/24. Both groups agreed early in the appraisal of the field to a unitized development with Shell as the operator. This approach has proved extremely beneficial to the efficient operation of the field. Of the fifteen development wells that have been drilled from a single concrete platform, 11 were producers, three were water injectors, and one was a dry hole.

Oil is produced from the Middle-Jurassic Brent sands, encountered at depths from 8,700 to 9,700 ft true vertical depth subsea (TVSS). The field oil/water contact is found at approximately 9,165 ft TVSS, although "oil down to's" (ODT's) have been found significantly deeper in several appraisal wells.

For the purpose of assessing reserves, the field has been subdivided into several "fault-bounded" blocks (see Fig. 1). The currently estimated remaining reserves of the field are 244 MMbbl, of which 212 MMbbl are within the unit area. Areas 3, 5, and 6 lie outside the Dunlin Unit area. The reservoir fluid is a highly undersaturated oil containing small quantities of solution gas. The bubble-point pressure, tank oil gravity, and solution GOR of most of the oil are 960 psi, 36 deg. API, and 240 scf/bbl, respectively (Table 1). psi, 36 deg. API, and 240 scf/bbl, respectively (Table 1). Geological Setting

The Dunlin structure has an elongated north- northwest/south-southeast trend with extensions to the northwest and southwest. Dunlin is highly faulted and the majority of the reserves are contained in a central horst block (Area 1 of Fig. 1) bounded on the cast, south, and west by faults and on the north by dip closure. The sands of this block dip to the north and west and are truncated on the southeast by erosion at the top Brent level.

The following reservoir subdivisions mapped in Dunlin are based on the stratigraphic nomenclature of Deegan and Scull:Tarbert plus Upper Ness,Mid-Ness shale,Lower Ness,Etive,Rannoch, andBroom.

The Tarbert has been included (at least initially) with the Upper Ness because its presence or absence is very difficult to predict due to the uneven crestal erosion of the Brent sands. From Shell's sedimentological studies in this and neighboring fields, the geological history of the Jurassic has been reconstructed as follows.

Toward the end of the Lower Jurassic Age, the area was dominated by marine conditions with the deposition of the shales and siltstones of the Dunlin group. The seas then shallowed and the Middle Jurassic sedimentation was represented by the deposition of the Brent group. This was deposited as a delta that prograded across the Dunlin shales from south to north.

JPT

P. 1713