Marine carbonate rocks are usually formed on neritic continental shelf where sea water evaporates fu-riously

living organism flourishes

and Eh and pH values change easily.Turbulent sea water with its high oxygen content destroys the soft parts of dead organism and forms carbonate rocks with low-abundance and low grade organic matters.Worldwide

carbonate rocks with TOC less than 0.2% and kerogen type of Ⅲ or Ⅱ

2

are common.It is therefore concluded that pure carbonates can not serve as effective source rocks.The TOC of carbonate rocks increases as the shale content becomes bigger

and only a small amount of marls with TOC larger than 0.5% may be effective source rocks.