<p>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 (CH<sub>3</sub>/CH<sub>2</sub> 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>
Chemical composition and molecular weight distribution of dissolved organic matter produced by bacterial degradation of green algae
