Steranes source indicators

The steranes are a class of biomarkers containing 21 to 30 carbons that are derived from sterols, and they include regular steranes, rearranged diasteranes, and mono-aromatic and tri-aromatic steranes. Among them, the regular C27-C28-C29 homologous sterane series are the most common and useful steranes because they are highly specific for correlation. The concentrations of C29 steranes (24-ethylcholestanes) compared to the C27 and C2g-steranes may indicate a land plant source. 

Sterane isomerization ratios indicate that the Assekaifaf, Dome, Hassi Messaoud, and Zemlet oil samples experienced thermal maturities near or past the peak of the oil window, while the Zarzaitine and Oued Zenani samples are less mature (Fig. 5). This interpretation is supported by terpane data, such as the Ts/(Ts + Tm) and tricyclics/(tricyclics + hopanes) ratios (Table 2), which are lower for the Zarzaitine and Oued Zenani samples than the other oil samples. Both of these terpane ratios depend partly on organic facies, but the data in the table suggest that maturity rather than source input is the main control on high tricyclic terpanes and Ts in these oil samples. 

To assess applicability of the pyrolysis results to natural systems, C4-benzenes were quantified and thermal maturity ratios calculated for oils and condensates from the Fort Worth Basin (Fig. 5). The Fort Worth Basin hydrocarbons in this study were generated from a single source, the Mississippian Barnett Shale. The only molecular maturity indicator available for use for these oils and condensates was TAS. The steranes and hopanes had reached equilibrium and even TAS was no longer useful for the highest maturity condensates (Table 2). 

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