Does the tag petrochemistry encompass natural gas?
At the moment, the tag's usage guidance only mentions crude oil.
I wasn't aware of the tag and I'm glad you brought it up. Presently, it is being misapplied.
The definition of petrochemistry (OED, Webster's, Wikipedia, my education as a geologist, etc.) is (Webster's Third New International) (1) the chemistry of rocks; and (2) the chemistry of petroleum; especially a branch of chemistry dealing with the production of petrochemicals.
Petrochemical is, as expected, a word that can be used as an adjective for (1) or (2).
What I think suits this site is a definition for petrochemistry which is defined as (I'm stealing from Webster's)
"Related to chemicals isolated from petroleum or natural gas or a derivative produced from such a substance by a chemical reaction."