The biologicals trapped in the sediment were laid down the same way all that sediment that formed the rock was laid down. It took a lot of time to build up that much sediment. The Bakken exists in the Devonian and early Mississippian period of sedimentation, about 400 million years old. You need to remember the earth didn't look like it did today in that time period.
Are you familiar with Van Flandern’s exploding planet theory?