Drivers are advised to take different routes while work is being undertaken to clear the road.
Resources from National Highways are in attendance to treat the spillage, which is in lane 2 and covers an area of approximately 1500 metres of the carriageway.
National Highways said: “Every effort is being taken to effectively treat the spillage to prevent any need for emergency resurfacing. Lanes have been closed to allow this work to take place safely at the scene.
“There are currently long delays of at least 90 minutes above usual journey times between M25 at J30 (Lakeside, Essex) and the scene. Customers intending on travelling from Essex to Kent via the Dartford Crossing are advised to allow plenty of extra journey time.
“If this closure impacts on your planned route, please allow extra journey time. Plan ahead, you may wish to re-route or even delay your journey.”