G. A. Blackbourn1* and J. D. Collinson2

1 Blackbourn Geoconsulting, 26 East Pier Street, Bo’ness, West Lothian, EH51 9AB, United Kingdom.

2 29 Geneva Drive, Newcastle-under-Lyme, Staffordshire, ST5 2QQ, United Kingdom.

* Corresponding author: graham@blackbourn.co.uk

Most of the potential sandstone reservoirs within the Namurian and lower Westphalian succession of the Southern North Sea Basin are originally feldspathic sands in which the feldspar has mainly been altered to microporous kaolinite clays. The sandstones provide a moderate porosity (typically 8-15%, depending mainly on grain size), but permeability is severely limited owing to the microporous nature of much of the porosity. Permeability is typically 1 mD or less, rising to a few tens of millidarcies in occasional coarse- and very coarse-grained sands. Predicting the presence of higher-permeability reservoir zones is therefore a critical exploration problem in these successions.

Quartzitic sands have been discovered in places, especially for example in the Trent field (block 43/24), where physical reworking of sands during the transgressions that preceded the deposition of marine bands removed much of the feldspar, so that less clay was formed during burial diagenesis. Although these sandstones display moderately elevated permeabilities, commonly several to several tens of millidarcies, they are usually fine-grained, which limits their reservoir potential.

A particular type of quartzitic sandstone reservoir has been identified quite widely within the Namurian and lower Westphalian succession of the Southern North Sea. This type is indistinguishable in terms of sedimentology and inferred detrital composition from the originally feldspathic facies which now form the widespread kaolinite-rich, low-permeability sandstones. However, it has a very low kaolinite content (commonly 1-2%, compared with a more usual 5-20%) so that these sandstones display permeabilities of the order of several hundred millidarcies.

Wireline log data from nineteen wells within UK Quadrants 43 and 44 have been examined, and all relevant core logged in order to compare the depositional settings of the quartzitic and non-quartzitic sandstones. Existing data from over 500 petrographic thin sections from the area have been reviewed and 78 new thin sections analysed to determine the petrographic controls on reservoir quality in both sandstone types. The diagenetic histories of each type have been interpreted. The dissolution of feldspars during diagenesis of the quartzitic sandstones, without a substantial residue of kaolinite or other aluminium-rich mineral, is attributed to the mobilisation of aluminium within organic complexes. This is thought to require the presence of certain organic acids.

Geochemical modelling suggests that such acids would have been produced in large volumes from the underlying Viséan and lower Namurian basinal mudstones during the late Namurian and Westphalian, prior to Variscan uplift. It is proposed that these fluids cut up-section into the Namurian along synsedimentary or tectonic faults and then moved along sandstone beds below thick mudstone intervals. They were thus responsible for “cleaning up” the feldspathic sands to create high-permeability reservoirs. The probable lateral and vertical distribution of these “cleaned-up” reservoirs within the succession has been examined, and an attempt made to correlate them between wells. Their recognition provides a more focussed exploration target within the Carboniferous of the southern North Sea, and emphasises the potential for diagenesis to enhance reservoir quality in hydrocarbon basins worldwide.

Key words: Southern North Sea, Carboniferous, reservoir geology, petrography, diagenesis, basin modelling, quartzitic sandstones.

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