Silica diagenesis and its pore-scale influence on the characteristics of the upper and lower Bakken shales, Williston Basin, North Dakota and Montana

Abstract

The Bakken Formation has been a major producer of oil and gas in North Dakota and Montana since the advent of horizontal drilling, reaching peak production of over 1.2 million barrels of oil per day. It is Late Devonian and Early Mississippian in age and consists primarily of a dolomitic reservoir member stratigraphically bounded by an upper and a lower organic-rich mudrock member. These bounding shales have been interpreted as depositional sequences of transgression and regression related to eustatic sea level variations and serve as important source rocks in the Williston Basin. The lower and upper Bakken members were deposited in euxinic bottom-water conditions linked to transgressive system tracts and contain a significant portion of biogenic silica in the form of radiolaria. Biogenic silica dissolution and reprecipitation is known to contribute to the development and/or preservation of pore space, ultimately affecting source rock quality and reservoir recoverability. In general, silica diagenesis occurs in multiple stages, beginning with amorphous silica (opal-A) and followed by a sequence of metastable intermediate polymorphs opal-CT, then chalcedony, and finally microcrystalline quartz. The transition from amorphous silica to progressively more orderly crystallographic stages in this sequence is largely controlled by an increase in temperature and a decrease in silica saturation. Recent studies of mudrocks in other high-profile unconventional plays have highlighted the importance of silica diagenesis in the post-depositional development and preservation of porosity, but this sequence remains understudied in the setting of the Bakken Formation’s radiolarian-rich lower and upper shale members. This study describes and characterizes silica in the lower and upper Bakken shales to estimate the extent to which its diagenesis has altered pore networks within the shales. This study incorporates data from three core samples drilled in northwestern North Dakota and one core sample drilled in northeastern Montana. Data was collected via X-ray fluorescence (XRF), X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and nitrogen physisorption. XRF analysis provided a compositional framework to assist in selecting ten sample locations within the cores. FE-SEM and XRD analysis paired with quartz crystallinity index calculations assisted in determining the nature and extent of silica diagenesis in each sample. Finally, the nitrogen physisorption technique was used to measure differences in pore volume and surface area between samples. XRF and XRD results suggest that the intervals of each core with the highest concentration of silicon contain a significant amount of biogenic, amorphous silica that is associated with recrystallized radiolarians, which are observed with optical light microscopy and FESEM analysis. However, no amorphous silica phases were observed with the FESEM, so these phases may occur at a scale below the instrument’s resolution. XRF data also indicates that the upper shale intervals sampled in this project contain a higher concentration of silicon on average than the sampled lower shale intervals. Nitrogen physisorption results indicate that intervals in the shales that contain the highest concentration of silica and radiolarian tests have a slightly larger surface area and pore volume than samples with low silica concentration and no radiolarians. These results tentatively suggest that radiolarians contribute a minor amount of surface area and pore space to the lower and upper Bakken shales through the dissolution and reprecipitation of silica in the most heavily siliceous intervals of the shales. Similar future analysis at a regional scale would assist in determining the extent or existence of this trend throughout the Williston Basin.