Abstract: Based on the analysis of petroleum exploration data combined with thin-section identification and organic geochemical data, the paper studies the geological and geochemical features of shales of Bakken formation in Williston basin and Eagle Ford formation in Western Gulf of Mexico basin, and analyzes the reason causing hydrocarbon accumulation differences between the two formations. Different rock associations result in the differences of hydrocarbon accumulation patterns between Bakken formation and Eagle Ford formation -- the former is dominated by outer-source accumulation and the latter by inner-source accumulation. The overpressure caused by hydrocarbon-generation in shales of Bakken formation is larger than that of Eagle Ford formation, which allows greater hydrocarbon expulsion forces and is helpful for oil and gas expulsion from the shales. Therefore, less oil and gas retain in shales of Bakken formation than that of Eagle Ford formation (with most oil and gas retaining in shales). The physical properties of middle Bakken formation are better than those of the upper and lower Bakken formations due to more developed connecting pores and larger pore throats, and the middle Bakken formation is the main interval for oil and gas occurrence. Shales and limestones in Eagle Ford formation with well developed pores contribute significantly to hydrocarbon storage. Tight oil reservoirs in both Bakken and Eagle Ford formations are continuous or quasi-continuous reservoirs. The tight oil reservoirs in Bakken formation mainly migrate vertically, leading to the formation of continuous large-area tight oil regions in the middle Bakken reservoir. Whereas, vertical and lateral migrations occur in Eagle Ford tight reservoirs, resulting in the overlapping of shales and tight limestones vertically, which could act as the source rock and reservoir simultaneously featured by interconnection and superimposition