THE LOST DEVONIAN SEQUENCE - SEQUENCE STRATIGRAPHY OF THE MIDDLE BAKKEN MEMBER, AND THE IMPORTANCE OF CLASTIC DYKES IN THE LOWER BAKKEN SHALE, NORTH DAKOTA, USA

The main target for hydrocarbons in the Williston basin is the middle Bakken member that represents a mixed carbonate-siliciclastic ramp system, sandwiched between the two organic-rich lower and upper Bakken member shales. For extracting oil from the middle Bakken member, this units with low porosities has to be fractured, and the fracture behavior depends heavily on the composition of the unit. Current sedimentological models envision the entire Bakken Formation to represent one large fluctuation of sea-level with the two shales reflecting the transgressions and highstands, and the mixed siliciclastic-carbonate middle member the lowstand.

However, the transition from the lower shale to the middle mixed carbonate-siliciclastic member does not represent a gradual coarsening but contains several carbonate units that are mudstones to packstones, and all of these carbonates contain glauconite to varying degrees. These partly coarse-grained carbonates are present in many of the cores close, but not directly in the basin center in North Dakota. The carbonate packstones require a significant drawdown of sea-level to be deposited around the center of the basin. In a mixed carbonate-siliciclastic system, however, carbonates without much siliciclastic content can only form if the siliciclastic input is turned off which is typical for transgressions, as is the presence of glauconite. It therefore seems reasonable to assume that sea level fell at the boundary of the lower to middle Bakken member transition, deposited packstones during the initial transgression, and subsequently fine-grained carbonates with glauconite. This scenario would also explain the presence of both carbonates and siliciclastics sedimentary dykes in the lower Bakken shales: while sea-level was experiencing a fall and subsequent rise, earthquakes lead to the rupture of the shales, and the fill reflects the sediment overlying these dykes at the time of their formation – coarse siliciclastics during the fall, and carbonates during the rise of sea-level. So both the thin layer of carbonates as well as the fill of the clastic dykes in the lower Bakken shale member document a drawdown of sea level during Bakken deposition that is otherwise not reflected in the sedimentary record.