Abstract
The Upper Devonian Buchan Formation reservoirs in the UK Central North Sea are litharenite/sublitharenite and were deposited in fluvial–aeolian settings. The grain-coating clays in the aeolian sandstones have effectively inhibited quartz overgrowth. Hence, the reduction of reservoir quality is mainly due to mechanical compaction and early dolomite precipitation in both fluvial and aeolian sandstones; quartz overgrowth and kaolinite illitization in fluvial sandstones; and limited smectite illitization in aeolian sandstones. The carbon/oxygen stable isotopes of dolomite cements suggest a predominantly marine carbon source and precipitation temperatures between 25 and 58 °C indicating a shallow burial depth during dolomite precipitation. The temperatures and the dolomite distribution indicate that the cements originated from the overlying Upper Permian Zechstein carbonates. Extensive quartz overgrowths formed at 80 and 120 °C in the late and deep diagenetic burial history. The most probable silica source was from feldspar kaolinitization and pressure dissolution of quartz grains. Through detailed petrography and geochemical analyses, the burial–paragenesis–thermal history of the Buchan Formation has been constructed. Similar diagenetic processes are likely to have occurred in the Buchan Formation in other parts of the Central and Northern North Sea. This study may allow new petroleum plays to be considered in areas previously thought to have poor hydrocarbon potential.
Highlights
The Upper Devonian Buchan Formation reservoirs in the UK Central North Sea are litharenite/sublitharenite and were deposited in fluvial–aeolian settings
Similar diagenetic processes are likely to have occurred in the Buchan Formation in other parts of the Central and Northern North Sea
The Upper Devonian Buchan Formation has a wide distribution in the Central and Northern North Sea (Ziegler 1990) but is usually perceived to have little hydrocarbon potential (Downie 2009)
Summary
The Upper Devonian Buchan Formation has a wide distribution in the Central and Northern North Sea (Ziegler 1990) but is usually perceived to have little hydrocarbon potential (Downie 2009). A number of discoveries in both the UK and Norwegian North Sea have confirmed considerable, but highly variable hydrocarbon reserves in the Buchan Formation (Edwards 1991; Trewin and Bramwell 1991; Knight et al 1993; Gambaro and Currie 2003; Gluyas et al 2005). Previous studies of the Upper Devonian reservoirs have concentrated on the sedimentology, describing the strata as deposits of fluvialbraided and aeolian systems (Marshall and Hewett 2003; Downie 2009). No detailed report has explained the sources of these cements, when and under what conditions they were formed and how the cements control the reservoir quality
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