The Bakken and Three Forks Formations of the Williston Basin have been targeted by over 10,000 unconventional style wells (horizontal drilling coupled with hydraulic fracturing). Exploration and developmental drilling initially focused on the Middle Bakken but later expanded into the underlying upper Three Forks (located directly below the Bakken) and eventually the middle and lower Three Forks, which are positioned 10s to 100 + feet below the Bakken. Hydrocarbons (oil and associated gas) within both formations are believed to be sourced from the upper and lower Bakken shale members, which are both highly organic-rich (>10% TOC), thermally mature petroleum source beds. However, minimal literature exists examining the relationship between hydrocarbons generated by the lower Bakken shale and expelled downward into the various reservoirs of the Three Forks. Core-plug oil and water saturation data from 65 partial to complete Three Forks cores were utilized to map out fluid saturation trends within the upper, middle, and lower Three Forks. Over 500 core chip analyses from 52 cores were used to spatially delineate the present day average hydrogen index (thermal maturity) of the lower Bakken shale while a lower shale isopach map was created from wireline logs of 240 wells. Lastly, the geochemical and standard core-plug data from four complete Bakken-Three Forks cores were used to calculate and compare the volumetric amount of oil generated and expelled from the lower Bakken shale with the original oil in place (OOIP) of the entire Three Forks section. The results show a strong spatial relationship between increased thickness-thermally maturity of the lower Bakken shale and increased core-plug oil saturations across the upper to middle Three Forks members, and a similar, but less pronounced relationship with the lower Three Forks. The calculated lower Bakken shale expelled oil volumes ranged from 13 to 51 million barrels of oil (MMBO) per section (640 acres) which overall equaled to exceeded the OOIP of the entire Three Forks section (9–25 MMBO/section). These spatial and volumetric relationships indicate that local hydrocarbon generation and expulsion from the lower Bakken shale is the primary control for hydrocarbon charge within the underlying Three Forks Formation, where in some locations hydrocarbons have been pushed downwards 200 + ft. Understanding downward hydrocarbon charge has implications in exploring for and developing hydrocarbon resources within the Bakken-Three Forks as well as numerous other unconventional petroleum systems. • The lower Bakken shale (source bed) overlies the Three Forks Formation (reservoir). • Thickness and thermal maturity control source bed hydrocarbon expulsion volumes. • Three Forks oil saturations are a function of overlying source bed properties. • Downward hydrocarbon charge in low permeability rock can extend over 200 ft. [ABSTRACT FROM AUTHOR]