Oil

 Canada produces more oil than it needs and most of the excess is sold to the US. It turns out that Canada sits on gold mine. It's estimated that Canada has one of the biggest oil reserves in the world with most of it in the form of oil sands.

Are there any mistakes?
Thanks.

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Canada produces more oil than it needs, and most of the excess is sold to the US. It turns out that Canada is sitting on a gold mine (?). It's estimated that Canada has one of the biggest oil reserves in the world, with most of it in the form of oil sands.

'Gold mine' is a poor choice here, since we are already talking about another mineral, oil.  Metaphors when too close to their source context, become lame (much as did 'bottom facing'-- wasn't that your phrase also, New2 ?).  If you were very adept, you could turn the lame metaphor into an appropriate and much more effective pun by writing that ' Canada is sitting on a black gold mine'-- since 'black gold' is an epithet for 'oil'.
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New2grammar

Canada has one of the biggest oil reserves

Thanks, Optilang. Is the above OK? It means Canada only has one big oil reserve. I usually hear the pural even though the news talks about one country.

It sounds OK to me

It has one of the biggest reserves in the world.

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The metaphoric content is too near the literal content.  'Bottoms' (as in human bottoms) do not 'face'-- the face is at the opposite end of the item.
Similarly, oil is not gold, but both are mined minerals in a real sense.  'Sitting on a gold mine' loses its effectiveness as a figure of speech when it can be confused in the context with an actual gold mine.
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Mister Micawber

'Sitting on a gold mine' loses its effectiveness as a figure of speech

I have to disagree MM. "gold mine" is a well understood idiom and doesn't engender any confusion in the context of N2G's original post.