The present disclosure describes methods and systems for determining source rock potential in a subterranean region of a hydrocarbon reservoir. One method includes receiving, an electron spin resonance (ESR) image from an in-situ ESR scanner that is attached to a wellbore at a first subterranean location, wherein the wellbore extends into the subterranean region of the hydrocarbon reservoir; determining, a spin concentration level of a source rock in the first subterranean location based on the ESR image; and determining, the source rock potential at the first subterranean location based on the determined spin concentration level.