Organic Petrography and Geochemistry of the Bakken Formation, Williston Basin, ND USA

Author

Zain Abdi, Southern Illinois University CarbondaleFollow

Date of Award

5-2023

Degree Name

Doctor of Philosophy

Department

Geosciences

First Advisor

Rimmer, Sue M.

Second Advisor

Rowe, Harry D.

Third Advisor

Anderson, Ken B.

Abstract

The environmental processes and conditions controlling productivity and organic matter (OM) accumulation/preservation as well as bottom–water redox conditions in the lower black shale (LBS) and upper black shale (UBS) members of the Devonian-Mississippian (D–M) Bakken Formation were evaluated utilizing trace metal (TM) concentrations, degree of pyritization (DOPT), enrichment factors (EF) of TMs, bi–metal ratios (V/Cr, V/(V+Ni), Ni/Co, U/Th), total sulfur (ST) vs. iron (Fe), total organic carbon (TOC), carbon–sulfur–iron relationships (C–S–Fe), as well as Mo–TOC and Mo EF–U EF relationships. High-resolution (1- to 3-cm scale) chemostratigraphic records were generated for twelve drill cores, four of which closely flank the N–S-trending axis of the Nesson Anticline, proximal to the center of the Williston Basin in northwest North Dakota, USA. Furthermore, five of the twelve drill cores were selected (sample selection was based on down–core spacing and TM concentrations) for petrographic and Rock-Eval analysis to assess variations in kerogen type, quantity, quality, and thermal maturity (based on solid bitumen reflectance (%SBRo), vitrinite reflectance equivalence (%VRE), Rock–Eval Tmax–derived vitrinite reflectance (%Ro)) from immature to condensate, wet gas hydrocarbon generation windows.

Degree of pyritization (DOPT) values (0.25 to 1.0) indicate that bottom waters were frequently dysoxic (> 60%) with intermittent aerobic and anoxic/euxinic conditions which is in agreement with C–S–Fe and total ST vs. Fe assessments of paleoredox conditions and sedimentological evidence. Furthermore, using published Mo–TOC relationships from modern anoxic-euxinic basins, it is estimated that renewal time of the sub-chemoclinal water mass during accumulation of the LBS and UBS approximated 10 and 30 yrs., respectively. Agreement is also seen between Mo/TOC and Mo EF/U EF where both suggest the Bakken shales were deposited under relatively unrestricted water mass conditions resulting in consistent renewal of TMs into the basin. However, bi–metal ratios suggest > 80% of samples were deposited under suboxic to anoxic/euxinic conditions. Trace metal concentrations for the Bakken Fm. show considerable range for Co (0–10324 ppm), Mo (0–2018 ppm), Ni (0–1574 ppm), U (0–1604 ppm), and V (0–3194 ppm), and bi–metal ratios for the Bakken Fm. are up to 5x greater than those reported for other D–M black shale formations.

The Bakken black shales represent a unique sedimentary system where the EF of various TMs such as Cu (6.2–7.7), Mo (219.7–237.8), Ni (9.4–10.2), U (20.6–29.3), V (9.9–14.2), and Zn (10.4–12.2) as well as total organic carbon contents (LBS = 10.80 and UBS = 11.80 avg. wt.%) are considerably higher than other Devonian–Mississippian black shales. In this study, raw distributions of elemental concentrations combined with bivariate and principal component analysis (PCA) were used to elucidate the processes that could have contributed to the high EF of TMs in the Bakken shales. Total organic carbon shares heavier PCA component loadings (>0.445) and stronger correlation coefficients (r) with Cu, Mo, Ni, U, V, and Zn rather than with pyrite-associated (As, Co, Fe, and S) elements, suggesting that TOC played a primary role in the scavenging and accumulation of TMs in the sediments. Reducing conditions within bottom waters or sediment pore waters may have accelerated the accumulation of redox-sensitive Cu, Mo, Ni, V, and Zn introduced into the sediments via primarily an organic matter (OM) detritus host and most likely played a secondary role in the enrichment of TMs. The high EF of TMs observed in the Bakken shales may be the result of the frequent resupply of TMs into basin waters, enhanced primary productivity that is necessary in scavenging TMs from the water column, the presence of H2S within sediment pore or bottom waters, or possibly secondary processes associated with basin-wide fluid and hydrocarbon migration. Factors controlling TM accumulation during time of deposition (e.g., TM availability, bottom-water redox conditions, adsorption onto organic matter) and during diagenesis and catagenesis (e.g., alteration and break down of organic matter, movement of fluid hydrocarbons or other basinal fluids) likely contribute to the lack of agreement between redox proxies, and subsequently, the lack of applicability of bi–metal ratios (i.e., V/Cr, V/(V+Ni), Ni/Co, U/Th) in assessing bottom–water conditions for the Bakken shales.

Solid bitumen (SB), a secondary organic matter formed as a residue after hydrocarbon generation (through either sufficient thermal maturation or microbial degradation) and expulsion, is primarily dispersed within the mineral matrix and increases in quantity with increasing thermal maturity. Rock-Eval II and HAWK analyzers were used to measure and estimate the hydrogen index (HI; avg. 201 mg HC/g TOC), oxygen index (OI; avg. 7mg CO2/g TOC), S1 (free hydrocarbons; avg. 8.0 mg HC/g rock), S2 (hydrocarbons generated after cracking kerogen; avg. 24.3 mg HC/g rock), and %Ro (0.60–1.03%; estimated from Tmax). The HI and OI values are calculated from TOC as well as S2 and S3 (oxygen bonded to hydrocarbons). Plots of HI vs. Tmax (ºC) and HI vs. OI as well as S2 vs. S3 ratio were utilized to determine the type of kerogen, primary OM that is insoluble in organic solvents. However, these relationships are not in agreement with kerogen typing based on petrographic observations, where samples from more thermally mature cores plot as Type III (vitrinite) kerogen instead of observed Type I/II (marine algae) kerogen. This is largely due to the abundant presence of SB in the more thermally mature section of the Bakken (Rock-Eval Ro = 0.83–1.03%) as SB is known to have a lower HI content than Type II kerogen. Petrographic evidence shows greater abundance of alginite and amorphous organic matter (AOM or bituminite) in the thermally less mature (Rock-Eval Ro = 0.60–0.83%) section of the Bakken compared to the greater abundance of dispersed SB in the more thermally mature section where AOM is absent.

Early research on the Bakken Fm. reported lower than expected vitrinite reflectance values attributed to vitrinite “suppression". The overall lack of vitrinite and abundance of solid bitumen in these shales suggests that these early attempts likely reported solid bitumen reflectance rather than vitrinite reflectance. More recent attempts to assess the thermal maturity of the Bakken Fm. black shales have measured and converted SBRo to vitrinite reflectance equivalent (VRE). However, samples selected for SBRo by some previous workers have included heterogenous, granular as well as high reflecting SB samples, which introduce error in the measurements. As such, reported reflectance values are most likely lower than they would be if smooth, homogenous solid bitumen with no inclusions were measured. For this project, smooth and homogenous SB was measured to produce consistent and reliable VRE values to assess the thermal maturity gradient from the Bakken Fm. basin margins to the depocenter. Blue-light fluorescence petrography was done to support thermal maturity assessments. Results from SBRo, Rock-Eval Ro, VRE, and blue-light fluorescence observations suggest that cores from the current study range from early oil window into condensate, wet gas.