地球微生物过程与潜在烃源岩的形成:钙质微生物岩

Calci-Microbialite as a Kind of Potential Hydrocarbon Source Rock and Its Geomicrobiological Processes

钙质微生物岩是各种微生物生命活动引起的特殊的碳酸盐建造,主要集中发育在前寒武纪及显生宙等重大地质转折期,在我国华北及华南地区有着广泛的分布.微生物岩中大量微生物化石的存在反映当时的海洋具有很高的初级生产力.明显偏低的钼同位素值显示在前寒武纪相当长的时期里,海洋底部处于缺氧状态.Th/U值的急剧降低和黄铁矿在浅水台地区的广泛出现显示二叠纪大绝灭后,海洋环境也同样呈现缺氧的还原状态.高生产力和还原的沉积环境为生物有机质的埋藏及烃源岩的形成创造了条件.前寒武纪及晚泥盆世微生物岩中沥青的发现,进一步增加了钙质微生物岩作为潜在碳酸盐型烃源岩的可能性.

Calci-microbialite is a special carbonate buildup, formed due to the activities of different kinds of microbes. Most of microbialites were developed in Precambrian or at the great transitional stages in Phanerozoic. Widespread microbialites have been found both in North and South China. Abundant microfossils preserved in the microbialites show high-level productivity during deposition. The obviously lower value of Mo isotope shows that the seafloor is of anoxic condition during the long time of Precambrian. The sharp reduction of Th/U value as well as the wide spread pyrite in shallow carbonate platforms indicate that the ocean environment is anoxic after end-Permian mass extinction. High-level productivity and anoxic sedimentary environment favor the efficient preservation of organic matter and thus the formation of source rocks. Bitumen observed in many outcrops of Precambrian and Late Devonian microbialites further raises the possibility of calci-microbialite as a potential hydrocarbon source rock.