Green Clays: Uses and Pitfalls
Date: Thursday, 18th April 2013
Venue: Royal Cambrian Academy headquarters in Crown Lane, Conwy @ 6:30pm. Refreshments @ 6:00pm.
There was a time when field geologists were content to call green granules in sedimentary rocks glauconite and have done with it, in fact the practice continues up to a point and two of the localities I shall discuss have been so described in the past, somewhat erroneously.
The seminal work of Odin and others in the 1980s showed that, in recent sediments, green granules of the chlorite family of clays (odinite-rich verdine) were only found in recent, shallow marine, low latitude sediments, while glauconitic granules occur on the outer shelf. I shall show that, in this instance the present is not necessarily a key to the past. In ancient sediments both chloritic clays and glauconite can be demonstrated to have formed in non-marine sediments, while glauconite in situ has been recorded in shallow (<50m?) marine sediments. The picture is further “muddied” by the fact that glauconite may be reworked into fluvial/estuarine sediments.
As glauconite is more clearly associated with very low rates of sediment deposition than are the chloritic clays, it may be that glauconitised faecal pellets are more frequently at an advanced stage of maturation, and hence induration, when buried, than are pellets replaced by chlorite. In terms of reservoir quality this means that glauconite is less likely to result in poor reservoir quality than is odinite (or chlorite as it becomes with diagenesis). The fact that odinite-rich verdine deposits are found nearshore and seaward of deltas and estuaries suggests higher rates of sedimentation, and hence less chance of the pellets becoming sufficiently mineralised to be firm enough to resist compaction. If, at some stage in the transformation, clay is dissolved, this may provide the solutes necessary for chlorite rim precipitation, but at present this remains speculative.
Dr Jenny Huggett is the proprietor of Petroclays, a consultancy specialising in clay mineral, and general clastic sediment analysis and petrography.
Petroclays was formed in 1990, and now in addition to working for the petroleum industry, works part time as a researcher and supervisor at the Natural History Museum, and the University of Oxford, Earth Sciences Department. From 1982 until 1990 Jenny was employed as a sedimentologist by BP. She obtained both her BSc and PhD from Imperial College, London.