A Biomarker Review of the Palaeoecosystem and Palaeodepositional Environment of the Bakken Shales of Saskatchewan

Abstract

Summary This study presents a detailed source characterization in relation to depositional condition during the deposition of the Bakken shales, using a biomarker approach. The geochemical characteristics for the Bakken shales is investigated using core samples to conduct a depth biomarker profile, examining the organic matter input and depositional markers within the Bakken shales in the northern part of the Williston Basin. The Bakken shales are reported to consist mainly of Type II kerogen, with some distinct molecular ‘fingerprints’ associated with their organic inputs and depositional environment. These molecular discriminants distinguish the Bakken shales from other sources within the Williston Basin and also employed as a means to trace Bakken oils, identify migration pathways and also to define petroleum systems. Results from this study suggest a significant variation in concentration of steranes, hopanes and gammacerane within a single borehole and also when compared spatially. The geochemical trend is such that it can be correlated when compared for several boreholes across Saskatchewan. The results reveal the presence of more than a single source facies within the Lower Bakken shales. The top section of the Lower Bakken shale imply a dysoxic condition, hosting a number of phytoplankton communities including algae and cyanobacteria while the other half through the base of the Lower Bakken suggests the presence of water column stratification, associated with photic zone euxinia, the presence of green sulphur bacteria and ciliates. This is evident from the high concentrations of gammacerane, presence of aryl isoprenoids and isorenieratane. Similar variations were noted for the Upper Bakken shale, although these variations were irregular. Based on these analyses, a generalized characteristic is inapplicable to correlate the Bakken shale. The Type II kerogen within the Upper and Lower Bakken shales are maceral assemblages consisting of a number of biological entities and not a single organic matter Type, as defined by bulk pyrolysis parameters. This study identified a number of molecular fingerprints, associated with multiple source facies within the Bakken shale. This study not only provide an understanding of heterogeneity in hydrocarbon potential for the Bakken shales but also documents a number of characteristics useful for oil to source correlation studies as well as defining an accurate petroleum system.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016