Abstract: Based on the analysis of the condition and mechanism for forming abnormal pore fluid pressure in undercompacted mudstone, two methods for calculating current abnormal pore fluid pressure and for buildup of the ancient abnormal pore fluid pressure of undercompacted mudstone by acoustic transit time are developed in this paper. Also, an approach for determination of the formation period and end period of abnormal pore fluid pressure is presented, which applied results in Qingshankou mudstone of northern part of Songliao basin show that it is practical for systematic study of abnormal pore fluid pressure in under compacted mud- stone.

Key words: compaction, mudstone, abnormal pressure, Songliao basin