The indices reflecting the origin of parent materials in organic matter types can not completely represent hydrocarbon generation ability.However

many geochemical indices indicate that the hydrocarbon potential of the epicontinental marine carbonate rocks is rather low.The ratioes of H/C and O/C of kerogens in these carbonate rocks are plotted in the area of type Ⅲ kerogens in the van Krevelen chart.The H/C ratio of kerogen in carbonate rocks is much lower than that in the neighboring mudstone.Maceral observation also indicates the poor hydrocarbon generation potential of OM in epicontinental carbonate rocks

including the converting of amorphous liptinite to micrinite

the occurrence of blakc structures in the center of algainite and large amount of maceral alike to vitrinite.The distribution of organic facies can explain the transformation of the OM in these carbonate rocks by oxidizing condition.The hydrocarbon generation potential of the sapropelic OM in the epicontinental carbonates is only equivalent to that of type Ⅲ kerogen .When studying the hydrocarbon generating amount of this type of carbonate rocks in basin simulation

they should not be represented by the low mature marls with high TOC and high hydrocarbon index.