Unusual origin almost includes large source of fuel (6)

'source of fuel' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's a link between them I don't understand?

'unusual origin almost includes large' is the wordplay.
'unusual' indicates anagramming the letters.
'almost' means to remove the last letter (almost the whole word is kept).
'includes' means one lot of letters goes inside another.
'large' becomes 'L' (eg in clothes sizes).
'origin' with its last letter taken away is 'origi'.
'origi' is an anagram of 'oirig'.
'oirig' placed around 'l' is 'OILRIG'.

Can you help me to learn more?