海相碳酸盐岩的形成环境与有机质特征

Features of organic matter and formation environment of marine carbonate rocks

海相碳酸盐岩形成于海水蒸发量大、生物繁盛、Eh和pH值容易改变的浅海陆棚相环境。海水的动荡性与高含氧量破坏了海洋中死亡生物的软体部分,造就了碳酸盐岩有机质的低丰度和低品位,TOC低于0.2%且干酪根为Ⅲ型或Ⅱ2型的碳酸盐岩在世界范围内具普遍性。因此,纯净碳酸盐岩不能成为有效烃源岩。碳酸盐岩随含泥量增加TOC变大,但仅有少量TOC大于0.5%的泥灰岩有可能成为有效烃源岩。

Marine carbonate rocks are usually formed on neritic continental shelf where sea water evaporates furiously,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 o ganic 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.