A stream including oxygen is provided to a rock sample from a subterranean zone. The rock sample is subjected to a first heating process to thermally oxidize at least a portion of the contaminant present in the rock sample. A first level of total organic content (TOC) of the rock sample is determined based on the first heating process. The rock sample is subjected to a second heating process to thermally oxidize at least a portion of the organic material present in the rock sample. A second level of TOC of the rock sample is determined based on the second heating process.