MOLECULAR CHARACTERIZATION OF LIGNITE DEPOSITS IN THE UPPER CRETACEOUS MCNAIRY FORMATION OF THE GULF COASTAL PLAIN

The Upper Cretaceous McNairy Formation in the Mississippi Embayment is composed of fluvio-deltaic strata containing dispersed lignite bodies. In this study, the molecular structure of the solvent soluble and insoluble organic matter (OM) composing lignite deposits in the McNairy Formation is characterized using solid state cross-polarization magic angle spinning ¹³C-NMR (CPMAS-¹³C-NMR) (bulk lignite), gas chromatography-mass spectrometry (GC/MS) (soluble OM), and pyrolysis-GC/MS (insoluble OM). One outcrop sample from western Kentucky and two subsurface lignite samples from southern Illinois are examined. The subsurface samples were dispersed in silty to sandy clay and were concentrated using heavy liquid (sodium metatungstate) prior to analysis. CPMAS-¹³C-NMR analysis revealed 38-46% aliphatic carbon (0-50 ppm spectral region), 26-30% aromatic carbon (112-145 ppm spectral region), and 7-9% phenolic carbon (145-163 ppm spectral region). Soluble OM ranged from around 8000 to 14000 ppm in the subsurface samples up to about 50000 ppm in the outcrop sample. Aliphatic hydrocarbons in the soluble OM fraction are dominated by long chain n-alkanes maximizing at C27 and having high odd-over-even carbon number predominance that are derived from leaf waxes, and pentacyclic triterpenoids composed primarily hopanoids ranging from C27 (17β(H)-trisnorhopane) up to C31 (17α(H)21β(H)- and 17β(H)21β(H)-homohopanes), and C29 to C32 hop-17(21)-enes. Bacteria is the likely source of hopanes in the soluble OM.

Pyrolysis of solvent extracted lignites was performed at 610°C for 20 seconds using a platinum coil under helium. The pyrolyzates are dominated by alkylphenols which is consistent with pyrolyzates of organic matter rich in vascular plant debris. Metoxyphenols (guaiacyl type) were detected and were more abundant in the outcrop sample. The presence of guiacyl-type methoxyphenols indicate contribution of lignin from gymnosperms. Aliphatic compounds are also abundant in the pyrolyzates and are composed primarily of n-alkene/n-alkane pairs. Pyrolysis was also performed in the presence of tetramethyl-ammonium hydroxide (TMAH) to provide online methylation of carboxylic acid and phenolic functional groups.