The present disclosure describes methods and systems for determining source rock potential in a subterranean region of a hydrocarbon reservoir. One method includes: receiving, a terahertz (THz) scanning image from an in-situ THz scanner that is attached to a wellbore at a first subterranean location, wherein the wellbore extends into the subterranean region of the hydrocarbon reservoir; identifying, components of a source rock in the first subterranean location based on the THz scanning image; and determining, the source rock potential at the first subterranean location based on the identified components of the source rock.