Abstract: 11 samples of Tertiary argillaceous hydrocarbon source rocks in the Dongying Depression were selected and separated into three fractions according to density difference (＜1.6 g/cm3, 1.6-2.2g/cm3 and＞2.2 g/cm3) after crushing them in order to extract the free organic matter, inorganic minerals and organo-clay complex from the argillaceous hydrocarbon source rocks (labeled as 1, 2 and 3, respectively). The quantities and TOC of different fractions were tested and observed by scanning electron microscopy to explore the differences of the occurrence forms of organic matter and the modes of organic carbon distribution in different density fractions. After the density separation, the recovery rate of the quantity and the organic carbon of different fractions approximately reached 100%. These features show credibility of the density separation method and less destruction to hydrocarbon source rocks. Through the observation of each fraction by scanning electron microscopy, we found existence of a large number of biological debris in the fraction l. After hydrochloric acid treatment, the surface of the biological debris became clearer in the fraction l, while in the fractians 2 and 3, a large number of mineral grains can be observed. The energy spectrum analysis shows existence of element carbon. These features indicate that the fraction 1 is mainly the organic matter of biological debris, while the fractions 2 and 3 are the organic matter mixed with the inorganic minerals. Through analysis of the TOC and the amount of organic carbon of different fractions, we found that the average TOC content of the fraction 1 is about 14.24 %, but it occupies only about 1.11% of the total amount of organic carbon in the wholerock samples. The TOC values of the fractions 2 and 3 are 3.26% and 2.77 %, respectively, totally occupying more than 90% of the tatal amount of the organic carbon in the whole rocks. From this point of view, the mode of combination with the minerals is the most important occurrence form of organic matter in the hydrocarbon source rocks. This part of organic matter for the hydrocarbon-generating capacity of whole rocks can not be ignored.