A portmanteau word ... is a blend of words in which parts of multiple
words are combined into a new word, as in smog, coined by blending
smoke and fog, or motel, from motor and hotel.
Oilscape (like seascape and mindscape) is formed by joining a descriptive noun to the ending "scape" by analogy with landscape. Originally "scape" didn't have any connection with paintings. It acquired the meaning from being part of landscape.
This phenomenon happens sometimes in English. A notable example is the suffix "-gate". Originally it was part of the name "Watergate", however journalists later took and used the "gate" part of the name to indicate any kind of scandal. There is no semantic justification for this other than by association. Before Watergate, no-one would have associated the word gate with scandal.
The suffix -gate is a word ending that has its roots in an American scandal that occurred in the 1970s
English-speaking journalists love brevity, especially in headlines and squashing words together in this way is one of the tools of their trade.