Structural order and disorder in Precambrian kerogens

High-resolution transmission electron microscopy (HRTEM) has been used to examine the structures of a wide range of Precambrian kerogens from rocks with ages between 0.9 and 3.8 billion years. We find recognizable structural ordering in samples that show little or no evidence of crystallinity by powder X-ray diffraction measurements. A wide range in degree of ordering is evident in the HRTEM images. A rough correlation exists between the ordering displayed in the HRTEM images and both the sample ages and their H/C ratios. Many kerogen samples are structurally heterogeneous, possibly reflecting a variety of precursors and source regions. The observed structural heterogeneities probably extend to other parameters; when isotopic and X-ray measurements can be made on the same scale as HRTEM images, similar scatter presumably will also be evident.