UPPER BAKKEN MEMBER SHALE FACIES ARCHITECTURE, NORTH DAKOTA, USA – A TALE OF RADIOLARIANS AND CLAY CLASTS

The main target for hydrocarbons in the Williston basin of North Dakota, USA, and a prolific oil reservoir in parts of southern Canada is the Upper Devonian to Lower Mississippian middle Bakken member. This unit consists of mostly fine-grained siliciclastics and minor carbonates deposited on a mixed carbonate-siliciclastic ramp. The middle Bakken member is sandwiched between two organic-rich shales, the lower and the upper Bakken members. Even though all hydrocarbons have been generated from the shales, very little is known about their internal architecture and facies.

Detailed sediment analysis of 37 cores along three transects shows that the upper Bakken shale can be subdivided into a lower unit containing abundant radiolarians, high TOC values, and a significant amount of siliciclastic detritus, whereas the upper part of the succession shows low amounts of radiolarians, low TOC values, and high amounts of clay. It is interpreted that the lower part represents the transgressive systems tract, and the upper part the subsequent highstand systems tract.

Within the lower part, the abundance of radiolarians varies significantly through the basin: it is lowest in the depocenter and towards the edge of the basin. In-between, radiolarian abundance is highest. It is assumed that radiolarian blooms that are responsible for the radiolarian distribution increased in frequency towards shallow water – nevertheless, in the shallowest portion of upper Bakken shale deposition, reworking was also high leading to the observed distribution of radiolarians.

Clay clasts are rip up clasts that consist of siliciclastic mudstones. They are abundant in the eastern and northern parts of the basin but largely absent towards the west and south. Only locally in the south of the basin, detrital quartz, often concentrated in laminae, becomes more frequent. Clay clasts are generally interpreted to reflect sediment starvation. It is therefore likely that the eastern and northern portion of the Williston basin experienced little sediment input during upper Bakken times whereas the west and south showed higher sediment supply. This is also reflected in the in places abundant siltstone laminae. The sediment source therefore must have been located to the south to southwest during upper Bakken times, and not to the north as previously assumed.