Sedimentology, Diagenesis and Reservoir Quality of the Upper Palaeozoic Ørn Formation, Southwestern Barents Sea. - Comparison of the Finnmark Platform and the Loppa High

Abstract

The purpose of this thesis was to study the Upper Palaeozoic Ørn Formation of the Barents Sea, with emphasis on sedimentology, diagenesis and reservoir quality. A comparison of characteristics of the Ørn Formation between the Finnmark Platform and the Loppa High were carried out, and are based on core logging and optical microscopy of the shallow, stratigraphical core 7029/03-U-02 and microscopy of thin sections from well 7220/6-1.

The Ørn Formation consists of Palaeoaplysina buildups and rhythmically alternating limestones and dolomites, with minor amounts of evaporites and siliciclastics. The sediments are divided into facies and facies associations based on distinct characteristics and neighboring facies, which allows the construction of a depositional model. The sediments were interpreted to be deposited in peritidal environments on an arid to semi-arid, warm shallow carbonate platform with an overall regressive trend. The reservoir quality of the carbonates has been studied in relation to facies and diagenesis. The high porosity intervals are controlled by diagenetic processes including aragonite dissolution and dolomitization. Porosity destructive processes involve compaction and extensive cementation during burial with precipitation of coarse calcite spar, coarse dolomite and anhydrite cement. The dolomites generally have higher average porosity values than the limestone deposits. The effect of dolomitization was found to be crucial for high porosity zones in mud-rich facies, whereas porosity in the grain-dominated facies were mostly dependent on the amount of coarse calcite spar. Extensive dissolution and dolomitization coincides with periods of prolonged subaerial exposure, consequently the high porosity intervals are found in regressive intervals of sea level fluctuations. The extensively dolomitized facies are formed early in restricted and hypersaline buildup-, open marine- and sabkha related depositional environments. The high porosity intervals were observed in the grain-dominated facies, the top of the shallowing upward peritidal units and the extensive dolomitized mud-supported facies. The Upper Palaeozoic carbonates are highly prospective and regarded as low risk in the Barents Sea.