Depositional and chemical controls on sedimentation, sequence stratigraphy, and pore development of the Pronghorn, Lower Silt, and Lower Shale members of the Bakken Formation, Williston Basin

Abstract

An integrated stratigraphical, petrographical, geochemical, and petrophysical analyses were presented in this study to characterize the depositional environments, stratigraphic framework, and pore network of the Upper Devonian Pronghorn Member, Lower Silt Member, and Lower Shale Member of the Bakken Formation. The lithological changes from the mixed clastic-carbonate deposit (Pronghorn Member), bioturbated clay-rich silty mudstone (Lower Silt Member), to homogeneous siliceous mudstone of the (Lower Shale Member) are interpreted to manifest an overall basinward shift from subtidal shoreface, offshore transition, open marine, to offshore environments on a storm-dominated clastic-dominated ramp. The sequence stratigraphic analysis suggests one 3rd-order transgression-regression cycle for the Pronghorn Member and the subsequent transgressive to the early regressive stages of the latest Devonian glacio-eustatic cycle for the Lower Silt and Shale members. The Lower Silt Member indicates a progressively deepening and more oxygen-restricted setting, followed by the rapid development of the euxinic bottom-water condition of the Lower Shale Member. The 3rd-order maximum flooding surface is characterized by the development of euxinic peak, high organic concentration, and overlying strata with an increasing detrital influx. Eustatic sea-level is suggested to be the major control over the architecture and depositional pattern of the Pronghorn, Lower Silt and Shale members. However, the tectonic process also plays a critical role in clastic supply, episodic subsidence, and salt dissolution-associated faulting events. The lower part of the Lower Shale Member is characterized by elevated primary productivity, which is evidenced by a negative δ15Norg excursion and increasing biogenic silica. This is closely related to deep-water nutrient renewal and enhanced nutrient recycling during the transgression. The redox condition and sediment flux, corresponding to relative sea-level fluctuations, exert primary controls on the patterns of organic matter accumulation, particularly in proximal and shallower regions. On the other hand, the high-level primary productivity does not serve as a dominating control on the organic concentrations. The biogenic silica, in contrast to clay, exhibits a significant dilution effect on the accumulation of organic material in the Lower Shale Member. This study characterizes the pore-network property of the Bakken shales: scanning electron microscope imaging reveals mineral matrix pores and organic-matter pores in sizes of 13 nm to no more than 8 μm; N2 adsorption measurements highlight the presence of predominant mesopores (2-50 nm) and subordinate 1-2 nm sized micropores. Although the pore network of the Bakken shales is dependent on the mineral composition (i.e., quartz and clay), it is more closely linked with thermal maturity of organic matter: an increasing amount of generated bitumen/oil displaced interstitial water and filled pore space during early-to-peak mature stage; however, sponge-shaped OM pores due to cracking of oil/bitumen to gas contributes to the pore-network improvement at the late mature stage.