The Paleogene lacustrine marlstone is an important source rock for giant oil fields found in Eastern China, which presently has become a main exploration target for tight oil resources. However, seldom studies have been conducted on possible variations in geochemical features of lacustrine marlstones during on-going burial process as well as the thermal evolution. Hydrous pyrolysis experiments were performed using organic-rich thermally immature marlstone samples from the Shulu Sag, Bohai Bay Basin, in order to have a better understanding in the formation process and accumulation mechanism for tight oil retained within the source rock. Laboratorial data of samples including solid residues, retained hydrocarbon (extracted bitumen) and expelled hydrocarbons were analyzed. The results indicate that quantity and composition of retained hydrocarbon are controlled by thermal maturity of the source rocks. Mass balance of organic matter provides a clue for recovering the amount of gaseous/light hydrocarbon retained in the solid residues. The formation and accumulation of tight oil mostly takes place in the oil window-wet gas window, which consists of four stages: (1) In the thermal maturity range of 0.5-0.8% Ro, the amount of retained and expelled hydrocarbons appears to increase synchronously; (2) At 0.8-1.0% Ro, the retained hydrocarbons reach maximum amount while the expelled hydrocarbons keep constant; and the gas to oil ratio of retained hydrocarbon (GORR) in the above two stages keeps at a low level; (3) Over the maturity of 1.0-1.1% Ro, the amount of retained hydrocarbons decreases while expelled hydrocarbons increases, during which GOR(R) increases significantly and retained hydrocarbons characterized by more lighter hydrocarbon composition; (4) At about 1.3-1.5% Ro the amount of retained hydrocarbons is less than that of expelled hydrocarbons. (C) 2015 Elsevier Ltd. All rights reserved.