--> Pore Characterization of the Organic-Rich Devonian-Mississippian Bakken Shales in the Williston Basin

AAPG ACE 2018

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Pore Characterization of the Organic-Rich Devonian-Mississippian Bakken Shales in the Williston Basin

Abstract

The hydrocarbons produced from the Bakken unconventional play are mainly from the middle dolomitic member, which was sourced by the adjacent lower and upper shales. While numerous unconventional plays produce hydrocarbons from the shale interval, the lower and upper shale intervals lack attention as reservoirs and their reservoir quality have been poorly understood. In this study, a series of measurements, including field emission scanning electron microscope (FE-SEM) imaging, N2 adsorption, anhydrous pyrolysis, X-ray diffraction (XRD), and total organic carbon (TOC) content, were conducted to understand the nano- and micrometer-scale pore structure characteristics. The collected Bakken shale samples have a wide range of thermal maturity, TOC, and different mineral composition.

The SEM imaging of the ion-milled Bakken shales indicates that organic-matter (OM) pores and mineral matrix pores are present in all Bakken shale samples. The size of these pores varies from 10 nm to no more than 8 μm. The OM pores are predominantly preserved in the amorphous OM, and they are most abundant in the immature and late mature shales. On the other hand, structured kerogens (e.g., Tasmanites and terrestrial phytodetritus) are typically non-porous regardless of thermal maturity. This study also suggests a decreasing organic porosity from immature to early mature stages, followed by a general increase in the peak and late mature stages. Additionally, an increase of mineral matrix porosity appears to be related to higher clay and dolomite content. The quantitative investigation of SEM images shows the dominance of pores with the diameter smaller than 300 nm.

The pore structure parameters from N2 adsorption, including BET surface area, DFT and BJH pore size distribution, and total pore volume, were used to quantify the pore characteristics. In contrast to the SEM observation of dominant macropores (pore width larger than 50 nm), the N2 adsorption mainly characterizes the mesopores with pore width of 2-50 nm. All shale samples show a general bimodal pore size distribution: prevailing pores with the size of larger than 25 nm and secondary pores with a size of 6-20 nm. The relatively OM-rich (TOC>3 wt.%) shales, regardless of clay content, show a generally increasing pore volume contribution as the pore size increase. The most mature shale sample lacks pores of smaller than 6 nm, which is consistent with the SEM foundings of pervasive migrated oil and bitumen occluding porosity.