By Paul Farrimond
25-Norhopanes have been reported in biodegraded oils since the late 1970s, and Seifert & Moldowan (1979) proposed that the methyl group at C-25 in the regular hopanes was removed by microbial demethylation associated with the degradation at the oil/water contact of an oil accumulation. This has been further supported by mass balance approaches showing a decline in hopane concentration as 25-norhopanes increase with severe biodegradation (Moldowan & McCaffrey, 1995; Bennett et al., 2006) and matching carbon isotopic compositions of the two hopane series (Funing Sun et al., 2022).
Occurring as a series of peaks in the m/z 177 mass chromatogram, the C29 member (25-norhopane) gives a prominent m/z 191 ion as the D/E ring fragment in its mass spectrum, resulting in its presence also in that mass chromatogram where it is routinely identified and measured. 25-Norhopanes form during more advanced stages of biodegradation, after the more easily degraded n-alkanes have typically been destroyed, and their relative abundance compared with regular hopanes (typically measured using the ratio of 25-norhopane to the C30 ab-hopane) gives an indication of the severity of biodegradation. Furthermore, if both n-alkanes and 25-norhopanes are present in significant abundance in the same oil, this must indicate a mixture of severely biodegraded oil with undegraded or only mildly biodegraded oil. This in turn is often interpreted to indicate a fresh charge to a biodegraded accumulation, but mixing within the reservoir should also be considered.
References:
Bennett B, Fustic M, Farrimond P, Haiping Huang & Larter SR (2006). 25-Norhopanes: Formation during biodegradation of petroleum in the subsurface. Organic Geochemistry 37, 787-797.
Funing Sun, Wenzuan Hu, Zhirong Zhang & Jian Cao (2022). Isotopic evidence for the formation of 25-norhopanes via in situ biodegradation in the Permian Lucaogou shales, southern Junggar Basin. Organic Geochemistry 163, 104334.
Moldowan JM & McCaffrey MA (1995). A novel microbial hydrocarbon degradation pathway revealed by hopane demethylation in a petroleum reservoir. Geochimica et Cosmochimica Acta 59, 1891-1894.
Seifert WK & Moldowan JM (1979). The effect of biodegradation on steranes and terpanes in crude oils. Geochimica et Cosmochimica Acta 43, 111-126.
