Royal Dutch/Shell said yesterday that a significant oil spill had occurred in Ogoniland, Nigeria, where the company has volatile relations with local residents. The company said it did not yet know the volume of the spill, but suggested the cause was arson near one of its facilities. Ledum Mittee, president of the Movement for the Survival of Ogoni Peoples, said the spill was a serious one and the group will “insist that there should be a proper cleanup and not a cover-up.” Meanwhile, residents in the Nigeria Delta have to deal on a daily basis with more than 100 natural-gas flares, a waste byproduct from the oil-drilling process. Enviros and human rights groups say the flares, 300 feet in height, threaten air quality and are a big source of greenhouse gases.