Abstract: This work used organic geochemistry and organic petrology to study the depositional environment, organic matter characteristics, and thermal maturity of the Cambrian-Ordovician organic-rich marine shales in the Baltic Basin. The main macerals in Cambrian samples include alginite, bituminite and solid bitumen, while zooclastic macerals become the major proportion of organic matter in the Ordovician samples. As the maturity increase, solid bitumen becomes abundant and dispersed. Semifusinite-like maceral was observed only in Furongian of well DBH15/73, which probably indicates the local intrusion of Permo-Carboniferous dolerite dikes. The programmed pyrolysis results show that immature, early mature, and over-mature samples are developed. However, the data of high-uranium Furongian samples were greatly altered by igneous intrusives from local dikes. HIo calculation model is simulated based on pyrolysis data and fitted according to the least-square fitting method. The simulated fitting HIo: 400 mg HC/g TOC (369.5 mg HC/g TOC, 430.5 mg HC/g TOC as 95% confidence bounds) is within the worldwide marine shale HIo and indicates a marine anoxia and transgressive sea with shallow water column (organofacies B).