**5.4.2 Homohopanes**

The homohopanes (C31 to C34) are believed to be derived from bacteriopolyhopanol of prokaryotic cell membrane. C35 homohopane may be related to extensive bacterial activity in the depositional environment (Ourisson et al., 1984). Homohopane index can be used as an indicator of the associated organic matter type, as it can also be used to evaluate the oxic/anoxic conditions of source during and immediately after deposition of the source sediments (Peters and Moldowan, 1991). Low C35 homohopanes is an indicator of highly reducing marine conditions during deposition whereas high C35 homohopane concentrations are generally observed in oxidizing water conditions during deposition, consistent with the oxic conditions (Peters and Moldowan, 1991).

#### **5.4.3 Gammacerane**

Gammacerane, originally thought to be as hypersalinity indicator (Sinninghe-Damste et al., 1995), is associated with both marine and lacustrine environments of increasing salinity (Waples and Machihara, 1991; and Peters and Moldowan, 1993).

Fig. 5. Gammacerane chemical structure.

Biomarkers 175

Fig. 6. Structures of different types of metalloporphyrins.

The low biomarker concentrations in oils (often in the range of several parts per million) in the presence of a highly complex petroleum hydrocarbon matrix especially weathered oils, the variety of chemical classes present in oils and the possible co-elutions in conventional chromatographic separations make the identification of biomarkers a more difficult task.

The development of more reliable, highly selective, fast and sensitive separation and identification tools for biomarker analysis purposes can be considered as one of the most

important research points in this field for a meaningful biomarker analysis.

**6. Developments in GC-MS instrumentation** 

### **5.4.4 Ts/Tm**

The ratio of Ts (trisnorneohopane) to Tm (trisnorhopane) more than (0.5) was found to increase as the portion of shale in calcareous facies increases (Hunt, 1996). Van Grass (1990) stated that Ts/Tm ratios begin to decrease quite late during maturation but Waples and Machihara (1991) reported that Ts/Tm ratio does not appear to be appropriate for quantitative estimation of maturity.
