Complex sentence: When such kerogens are present in high concentration in rocks such as shale, and have not been heated to a sufficient temperature to release their hydrocarbons, they may form oil shale deposits.
Split into simpler sentences: When such kerogens are present in high concentration in rocks such as shale they form possible source rocks. Shales rich in kerogens that have not been heated to a sufficient temperature to release their hydrocarbons may form oil shale deposits.