Take a look at page 17 and 18 from the link provided below to find the principal business code for oil and gas royalties. You can also take a look at the FAQ below for an additional way to search for the principal business code for oil and gas royalties.
https://www.irs.gov/pub/irs-pdf/i1040sc.pdf
https://ttlc.intuit.com/replies/3300135
