An oil (I) is claimed for use in oil-based drilling fluids in drilling operations where the fluid is used to carry debris out of a subsea bore hole and the fluid-contaminated debris are dumped in the sea. (I) is characterised by giving less than 5% mortality of brown shrimps when tested in aerated sea water at 15 deg.C for 24 hr. at a concn. of 0.1 (pref. 0.333) ml/l, and having a viscosity ofless than 6 cS at 40 deg.C. (I) is pref. a naphthenic or paraffinic oil with an aromatics content of less than 5%, a 20 deg.C viscosity of less than 10 (esp. 1-7) cS, a 40 deg.C viscosity of 1-5.5 (esp. 1-5) cS and a 100 deg.C viscosity of 0.7-2.5 cS. It is pref. less viscous than diesel oil.
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