The Puffin Field: the appraisal of a complex HP-HT gas-condensate accumulation

Abstract

Abstract The Puffin Field was discovered by well 29/5a-1, drilled in 1981 to test a Base Cretaceous closure. The well encountered over-pressured gas-condensate-bearing Upper Jurassic shallow marine sandstones below 14,000 ft. The well flowed over 20 × 10 6 SCFGD with 2650 barrels per day of condensate. Appraisal of the discovery, which fell into five phases, involved the acquisition of two 3D seismic surveys and drilling of eight wells. This paper describes the appraisal of Puffin from an historical perspective. The initial geological model involved an intra-graben horst draped by a relatively uniform blanket of Fulmar Formation sandstones. The first two apprisal wells were disappointing and hinted at previously unsuspected complexities. The second pair of appraisal wells, although proving up further hydrocarbons, penetrated thick Triassic sequences, with the Pentland and Fulmar Formations being absent. These results were incompatible with the existing geological model and a new model was developed. This envisaged the presence of several large Triassic-cored pods, formed in response to salt withdrawal and local tectonism, separated by narrow elongate zones (inter-pods) in which Middle and Upper Jurassic sections were present. Four further appraisal wells were drilled and proved the model to be robust with respect to stratigraphy and structure, although it did not always accurately predict hydrocarbon distribution. The reservoir geology of the shallow marine Fulmar Formation and fluvial Skagerrak Formation reservoirs will be outlined. The wells indicated considerable differences in hydrocarbon contacts between wells and differences in fluid gradients and pressures between the pods. The geological evolution of the field will be described.