Abstract: The pyrolysis and hydrous pyrolysis simulating experiments of samples with differentTOC which confected by burned calcite,smectite,limestone and mudstone added into the same organic mat-ters respectively,show that the content of absorpted hydrocarbon by the marine carbonate rock is usually about 0.35 mg/g and the range is 0.2-0.85 mg/g,the content of absorpted hydrocarbon by the shale is usually about 1.25 mg/g and the range is 0.46-2.62 mg/g.The lower limit of total organic carbon(TOC) is about 0.08% for hydrocarbon expulsion of high-to-over mature marine carbonate rock and about 0.3% for mature marine carbonate rock rich in hydrocarbon.The lower limit of total organic car-bon(TOC) for hydrocarbon expulsion is related with the quantity of absorpted hydrocarbon by source rocks,the type of kerogen and the maturity of organic matters.