Carbonates are dominantly sedimentary rock in the Proterozoic to the Paleozoic marine basins of China, and were put more attentions for a long time and were thought to be primary source rock in many researches, but shales/mudstones were rarely studied. Based on the statistics of more than 11 thousands of samples, according to the geological age and lithological classification, from 147 outcrop sections and 289 wells in the Yangtze area, Yunnan, Guizhou and Guangxi Provinces of the Southern-Western China, Tarim and Ordos basins of the Northwestern-Northern China, it is very clear that shales/mudstones are rich in organic matter and are the essential source rock in the Proterozoic to the Paleozoic marine basins, while the carbonates are poor in organic matter and are secondary source rock. The relation between TOC and carbonate content is weak. It indicates that muddy input is favorable for the formation of organic-rich carbonate source rock, but is not an essential, while the productivity, sedimentary and preservation environments of organic matter are the essential factors to control the concentration of organic matter in the carbonate rocks. It is not lack of organic-rich shales or mudstones in the Proterozoic to the Paleozoic marine basins of China, they occur widely in the Doushantuo formation of the Neoproterozoic, the Lower Cambrian, the Upper Ordovician-Lower Silurian and the Upper Permian in the Yangtze area, the Lower-Middle Devonian in the Yunnan, Guangxi and southern Guizhou provinces, the Lower Cambrian, the Lower Ordovician and the Middle-Upper Ordovician in the Tarim basin; the Hongshuizhuang and Xiamaling formations of the Mesoproterozoic to the Neoproterozoic in the Northern China. Carbonate source rocks, such as lime mudstone, marl and argillaceous limestone, occur locally in the Cambrian and Ordovician, Tarim basin, and in the Lower Permian, Southern China.