Hopanes isomerization

The kinetic data in Table 5.4 show that steroidal aromatization is much more sensitive to temperature than the isomerization reactions (i.e. it has larger Eact and A values). This is more obvious from the plot of loge(k) versus 1/T (i.e. a log plot of the Arrhenius equation) for these steroid and hopane reactions in Fig. 5.54a. Hopane isomerization is faster than sterane isomerization at all temperatures. In most oils the level of maturity is such that equilibrium has been reached in the isomerization at C-22 in hopanes, while isomerization at C-20 in steranes may not quite have reached equilibrium. 

Figure 2. GC-FID chromatograms for the sulfide fractions from different Alberta petroleums. The peaks labeled B13 and B20 correspond to the bicyclic terpenoid sulfides with 13 and 20 carbons, respectively. The peak labeled T23 corresponds to the tetracyclic terpenoid sulfide with 23 carbons and peaks due to the hopane sulfides are indicated at the end of the chromatograms. The clusters of peaks spaced one carbon apart on the Bellshill Lake trace correspond mainly to complex mixtures of isomeric monocyclic sulfides possessing a linear carbon framework. These sulfides have been removed by biodegradation from the upper two samples. For more complete peak identification see References 9, 10 and 35. (Reproduced from Reference 34. Copyright 1989, American Chemical Society.)...

The apparent isomerization at C-14 and C-17 in steranes occurs over a higher maturity range than the isomerization at C-20 in steranes and C-22 in hopanes. It may be at least partially controlled by differences in thermal stability towards the higher maturity limit,... 

The transformations reviewed in Section 5.5 are potential maturity indicators.The most useful reactions are those in which only one of the pair of components is present initially in immature sediments, so that the extent of the transformation can be attributed entirely to thermal maturation (the kinetics of the transformation are also simpler Box 5.4). Such reactions include isomerization of pristane at C-6 and C-10, of steranes at C-20 and of hopanes at C-22, and also the aromati-zation of C-ring monoaromatic steroidal hydrocarbons. A number of molecular maturity parameters are shown in Fig. 5.47, together with some bulk maturity measurements.The correlation of values is approximate and varies with the type of organic matter present, its potential for generating petroleum and its heating rate. 

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. 

Tables 5 and 6 must be considered together with Figures 27 and 28, which show the chiral centers. The triterpanes of the hopane family have the m/z 191 fragment and thermal structural changes occur at carbon positions 17,21 (rings) and 22 (side chain). The natural tetrahydroxyhopane (see Figure 28) has the conformation 17j5(H), 21jS(H) and 22R. With increase in temperature the 22R 22S isomerization occurs, as well as...

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