1) Like for any industry, your best guess is generally a good one. I.e. whichever countries/cities are known for x industry, that's likely where consulting firms will be.
2) This is an awesome excuse to network! Ask HR/Recruiters which offices have strong oil&gas/chemicals/energy practices and ask if they can put you in touch with anyone from there
That being said, off the top of my head:
Oil & Gas
- Middle East
- Houston, Texas
- Australia (Perth)