2014 Rocky Mountain Section AAPG Annual Meeting

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Effects of Upper and Lower Bakken Source Rock Differences on the Charge of Middle Bakken and Three Forks Reservoirs in the Williston Basin


Observation of the Bakken-Three Forks petroleum system in the Williston Basin reveals that, in areas of relatively low source rock maturity, Three Forks reservoirs can yield economic volumes of petroleum, while Middle Bakken reservoirs primarily produce water. This study uses an integrated approach to link the observed charge trends to geologic and geochemical properties at the locations where the Three Forks is the more productive reservoir. Prior studies of Bakken source rock properties have sought to explain production sweet spots in the Williston Basin by treating the Upper and Lower Bakken units as layers of identical source rock. This study, of more than 2600 pyrolysis analyses, reveals that subtle differences between the two source beds, along with reservoir quality differences cause preferential charge into Three Forks reservoirs at early maturation stages in the basin. Specifically, the geochemical data show that the Lower Bakken source beds are of higher quality and generate petroleum sooner than the Upper Bakken source beds. The lower Bakken source beds, while lower in original total organic carbon, have higher original hydrogen indices. Curve fitting of the hydrogen index vs. Tmax plot shows that lower Bakken source beds reach petroleum generation and expulsion at lower total thermal stress.