Characteristics of High-quality Source Rock in Member Liu-2 of D Sub-sag in Wexinan Sag

The test results of samples demonstrate high-quality source rocks with abundant content of organic matters are developed in some intervals of Member Liu-2 of D sub-sag. In order to study the distribution of these intervals and their geochemical characteristics, the authors, firstly, determined TOC=2% is the lower limit of TOC for the high-quality source rocks based on the relationship of TOC versus "A". Secondly, the TOC curves of Liu-2 source rocks were fitted by ΔlogR method. The results indicate that the contents of organic materials are overall high and meet the standard of the high-quality source rock, and two intervals with abnormal high contents of TOC, up to 10%, were recognized. The kerogen types of high-quality source rocks classified by kerogen elements and rock pyrolysis parameters are type Ⅰ and Ⅱ₁. Threshold depth of oil generation and expulsion were determined by R_o values and evolution characteristics of soluble organic matters, they are 2 600 m and 3 100 m, respectively. The authors also studied the genesis of high-quality source rocks, it is thought that the paleo-productivity derived from the algal blooming and reducing environments of deep lacustrine facies are main controlling factors.