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Coal-Type Hopane: Geologists Need to Know the Details

What is “Coal-Type” Hopane?

A high moretane/hopane ratio commonly occurs in the immature organic matter regardless of the lithology of sediments when accompanied by the high maturity stage, it usually indicates terrigenous plant-derived organic matter. This pattern of hopane distribution is called ‘‘coal-type’’ hopane.

The high abundance of moretane is most probably related to acidic and oxic conditions in peat or coal-enriched sediments. “Coal type” Hopan is called because the ratio of Moretane and Hopan is much higher and this type of hopane anomaly is usual in the coal forming depositional setting in marine sediments, hence the name.  

Interpretation of “Coal-Type” Hopan and other related Biomarkers;

Oxygen-containing aromatic compounds (such as dibenzofuran). These compounds represent the land-derived diagenetic products of polysaccharides and may be indicative of massive soil erosion (Sephton et al., 2005). The C31 hopanes can be formed by the decarboxylation of the C32 hopanoid acids. The C32 Hopanoid acids are formed in the peat deposition environment or Oxic environments and during diagenesis, it becomes the C31 Hopane.

Very high Tm/C30 hopane and Tm/Ts are found in coals or coaly shales both in the immature and mature stages. Usually Tm (17a(H)-28,29,30-tris-norhopane) is less stable than Ts (18a(H)-28,29,30-tris-norneohopane) . If we found High Tm than Ts even in the mature stage it indicates the source input might be peat-forming processes because acidic clay may be the source of Tm and peat are the source of acidic clay.

The hopane/sterane ratio (Sum of C27–C35 hopanes/ Sum of C27–C29 steranes) is usually used to indicate the input of bacteria versus algae (phytoplankton) in sediments. High hopane/sterane ratios usually occur in terrestrial sediments and rocks where bacteria are dominant.