Geological And Reservoir Modelling Of The Thistle Field

Abstract

Abstract A reservoir model of the Thistle Field was derived from seismic mapping, geological modelling of five appraisal wells, log interpretation and reservoir performance simulation, under various development options. This paper describes the contribution of all these activities to the selection of development well locations and how the results of the first seven wells fitted with the original model confirming the correctness of the decision to proceed with field development based on data from five appraisal wells. INTRODUCTION The Thistle Field is located sane 200 km northeast of the Shetland Islands, lying, as seen in Figure 1, in the very prolific northern part of the Viking Graben. The field is mainly in UK Offshore Block 211/18, but extends into Block 211/19. The licence holders in the Block 211/18 part are BNOC, Deminex UK Exploration and Production Limited, Deminex Oil & Gas (UK) Limited, Santa Fe Minerals (UK) Ltd., Tricentrol Thistle Development Ltd., Burmah Oil Exploration Ltd., Ashland Oil (GB) Ltd. and Charterhouse Petroleum Development Ltd. Block 211/19 is licensed to Cenoco, Gulf and BNOC. Block 211/19 and 211/18 were awarded respectively in the Third and Fourth Rounds of UK Offshore Licensing. Operator for the unitised. field is BNOC (Development) Ltd. The Thistle Field discovery well 211/18-2 was completed in July 1973 and confirmed as a commercial discovery by wells 211/18-3 and 211/18-4 later in 1973. Based on these three wells the decision to proceed with field development was made early in 1974 but two further appraisal wells were planned. The extension into Block 211/19 was confirmed by well 211/19-1 in 1974 whicg encountered the field's oil-water contact at 9322 feet subsea, and the northern extent of the field defined by well 211/188 in 1975. The field lies in about 530 feet of water and is currently the deepest water development in the North Sea. The stratigraphy of the area is fairly typical of the North Sea Viking Graben with a predominantly claystone sequence from the quaternary to the base of the Upper Cretaceous. The very attentuated Cretaceous consists of limestones and claystones and these overlie the highly organic shales of the Upper Jurassic with a considerable hiatus between the two. The organic shales give way to more normal claystones which overlie the Middle Jurassic Brent Formation, the sandstones of which constitute the Thistle Field reservoir. These rest on Lower Jurassic shales, siltstones and sandstones, which in turn overlie the interbedded sandstones and red claystones of the Triassic. The overall area is characterised by the development of numerous tilted fault blocks. In much of the basin these blocks have a westwards tilt, but in the Thistle Field the tilt is eastwards. The field is bounded to the west and north by faulting and to the east and south by an oil-water contact. This paper describes how a model of the Thistle reservoir was based on an interpretation of all available geophysical, geological, petrophysical and reservoir engineering data. A development scheme is outlined. The results of seven development oil wells drilled to date substantiate the reservoir model by comparing predicted with actual reservoir properties.