I think Carl Christian might be on to something here. I can’t find a single characteristic pointing specifically towards cape (pintado) petrel anyway.
The relatively narrow and well marked dark tail band in contrast to the completely white undertail-coverts ; the dark brown , rather than blackish , colour of the dark parts ; the uniformly narrow , rather than broadening (towards the tip) , dark terminal band to the primaries ; the total lack of dark markings in the white areas bordering dark patches (for example along the dark forewing , on the undertail-coverts and on the sides of the neck) , all indicate antarctic petrel. I also think that the dark upper part of the neck , making a dark/white diagonal border going between the throat and the forewing indicates antarctic petrel , since cape petrel has a more distinct black head in contrast to the all white sides (with some dark spots) of the neck. Of course , it would be a lot easier if the upper part was visible , but based on this picture alone , I would have to agree that this is more likely an antarctic petrel.