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It is only very recently that petroleum came to be used as a raw material for chemical products.
Literal
Petroleum [subject-が] chemical-products [genitive-の] material [as-として] be-used come-to-be-past [thing-の][topic-は] very recent [genitive-の] thing is.
A textbook cleft construction: ~のは~である ('the fact that... is X'). The subject is nominalized with の (creating a 'thing-clause'), marked as the topic with は, and then identified with である as 'a very recent matter.' This structure focuses attention on the when rather than the what. Inside the nominalized clause are stacked grammar points: として ('as X'), the passive 用いられる ('be used'), and ~ようになる ('come to be X-ed'). Each layer adds a step of remove — the sentence is saying 'petroleum only recently became something that people started using as a chemical raw material.'