Multi-Scale Organic Material Characterization Of The Bakken Source Rock

The Bakken Shale samples were analyzed by utilizing several techniques including AFM-based Nano-IR spectroscopy, Rock-Eval 6, XRF, NMR spectroscopy, VRO, and organic petrography using reflected white light and UV light for understanding chemistry of organic matter (OM). Organic petrography showed that OM consists of various types of solid bitumen (SB), matrix bituminite, acanthomorphic acritarch, marine alginite, micrinite, and inertinite macerals. Liptinite fluorescence color was used to confirm the thermal maturity level. The results indicate that kerogen is mainly marine Type II. The original hydrogen index was restored using various mathematical/empirical methods. It was found that organofacies ‘B’ is the most abundant organofacies present in the Bakken Shale. The Bakken was missing an independent correlation of Tmax with VRO. This study showed that linear trends cannot accurately represent the relationship between these two parameters, considering the kerogen kinetics and non-linear relationship between transformation ratio and Tmax. Therefore, a polynomial relationship was proposed to accurately represent the Bakken Shale maturity. Tmax, liptinite fluorescence, SBRO, and NMR as thermal maturity indicators were utilized to establish a reliable database to compare redox-sensitive trace metals (TM) concentration to maturity variations. Comparing TMs concentration with TOC confirmed the presence of anoxic/euxinic conditions in the depositional environment. A relative enrichment in TMs was detected with all utilized indices, confirming that thermal maturity has played an important role in TMs concentrations. OM particles were found that are evolving in-situ into solid bitumen. This in-situ bituminization allows examination of a continuous transformation in OM molecular structure at micron-scale using AFM-IR spectroscopy applied at the transition zones. The OM chemical heterogeneity was examined at the nanoscale. Significant heterogeneity was observed within unaltered telalginite and bacterial degraded Tasmanites, and also between two separate solid bitumens that are next to one another and at the same stage of thermal progression. While thermal maturity progression was found to reduce molecular heterogeneity in the organic matter particles during the maturation pathway, on the contrary, during the bacterial degradation, the Tasmanites has lost its fluorescence and the relative heterogeneity was increased compared to the unaltered telalginite

Molecular Weight Distribution of Kerogen with MALDI-TOF-MS

Kerogen is an amorphous organic matter (AOM) in fine grain sediments, which produces petroleum and other byproducts when subjected to adequate pressure and temperature (deep burial conditions). Chemical characteristics of kerogen by considering its biogenic origin, depositional environment, and thermal maturity has been studied extensively with different analytical methods, though its molecular structure is still not fully known. In this study, conventional geochemical methods were used to screen bulk rock aliquots from the Bakken Shale with varying thermal maturities. Organic matter was isolated from the mineral matrix and then a mass spectrometry method was utilized to quantify molecular weight distribution (MWD) of four different kerogens at various thermal maturity levels (immature to late mature). Furthermore, to complement mass spectrometry, Fourier transform infrared (FTIR) spectroscopy was employed as a qualitative chemical and structural investigation technique. The MWD of four samples was obtained by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, and the results are correlated with the absorption indices (CH3/CH2 ratio and aromaticity) calculated from the FTIR attenuated total reflectance (ATR) method. The results showed when the degree of maturity increases, the aliphatic length shortens, and the branching develops, as well as the aromatic structure becomes more abundant. Moreover, based on the MWD results, higher maturity kerogen samples would consist of larger size molecular structures, which are recognized as more developed aromatic, and aliphatic branching stretches. The combination of infrared spectroscopy (AFT-FTIR) and mass spectrometry (MALDI-TOF) provided MWD variations in kerogen samples as a function of maturity based on varying absorption indices and revealed the rate of change in molecular mass populations as a function of thermal maturity.</p

Backtracking to Parent Maceral from Produced Bitumen with Raman Spectroscopy

In order to assess a source rock for economical exploitation purposes, many parameters should be considered; regarding the geochemical aspects, the most important ones are the amount of organic matter (OM) and its quality. Quality refers to the thermal maturity level and the type of OM from which it was formed. The origin of the OM affects the ability of the deposited OM between sediments to generate oil, gas, or both with particular potential after going through thermal maturation. Vitrinite reflectance and programmed pyrolysis (for instance, Rock-Eval) are common methods for evaluating the thermal maturity of the OM and its potential to generate petroleum, but they do not provide us with answers to what extent solid bitumen is oil-prone or gas-prone, as they are bulk geochemical methods. In the present study, Raman spectroscopy (RS), as a powerful tool for studying carbonaceous materials and organic matter, was conducted on shale and coal samples and their individual macerals to show the potential of this technique in kerogen typing and to reveal the parent maceral of the examined bitumen. The proposed methodology, by exhibiting the chemical structure of different organic matters as a major secondary product in unconventional reservoirs, can also detect the behavior of solid bitumen and its hydrocarbon production potential for more accurate petroleum system evaluation