Ostracod colour change by thermal alteration, offshore Ireland and Western UK

Abstract

Ostracod colour changes, from white to amber to red-brown to black, are principally interpreted to be the result of thermal alteration. The changes in colour are thought to be caused by progressive carbon fixation of hydrocarbons derived from the surrounding sediments which invaded the ostracod shell. Exceptions to this scheme include faunas from organically poor limestones which remain white irrespective of the amount of heating, and grey-brown or yellow-brown specimens coloured by the oxidation of diffuse pyrite. Thirty-four exploration wells have been examined from the Irish and Western UK sectors, and these have shown a progressive darkening of colours which is correlated with both burial depth and distance from igneous intrusions. The Fastnet Basin and southern region of the North Celtic Sea exhibit higher colour change/depth ratios to the Porcupine Basin and northern region of the North Celtic Sea Basin, implying greater palaeoheat flow in the former basins.