Steranes 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. 

At temperatures above c,100°C steroidal aromatization occurs at an increasingly faster rate than sterane isomerization. As a result, in young extensional basins in which there is a high heat flow, such as the Pannonian Basin in Hungary, aromatization can be virtually complete while sterane isomerization is at an early stage (Fig. 5.53a). In older basins with more moderate heat flows, such as the East Shetland Basin (northern North Sea Fig. 5.53b), the steroid isomerization and... 

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. 

Fig. 5. Sterane isomerization ratios (Table 2) indicate that Oued Zenani and Zarzaitine oil samples are below the peak oil window, while the other oil samples have higher thermal maturity (diagonal arrow). Ratios are insensitive to further maturation in stippled areas (Peters and Moldowan, 1993).

Only the 20.R isomers of the regular steranes and 4-methylsteranes exist initially, but they undergo isomerization to form an equilibrium mixture containing approximately equal amounts of 20.R and 20 S isomers (Figs. 5.15 and 5.16). This process appears to require higher temperatures than the isomerization ofpristane. Similar temperatures to those at which the 20S/R isomerization occurs are required for another isomerization process of the regular steranes that affects the C-14 and C-17 cyclic positions in concert. The 14a,17a isomer is converted into an equilibrium mixture of 14a,17a and 14p,17(3 isomers (Fig. 5.15) that contains rather more of the latter. 

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. 

Fig. 5.53 Plots of depth versus isomerization of steranes at C-20 and aromatization of C-ring monoaromatic steroids in the Pannonian Basin (Pliocene deposits, geothermal gradient >50°Ckm-1) and East Shetland Basin (Jurassic, geothermal gradient c.30°C km-1).The broken line represents the approximate depth of onset of oil generation. (Data after Mackenzie McKenzie 1983 Mackenzie 1984.)...

G. c. conditions have been established for the separation of 24R- and 245-isomers of 24-methyl-steranes and -stanol acetates, and are applied to the analysis of steranes in a sedimentary rock and in petroleum.Isomeric 24-ethyl-steranes were sufficiently separated under the same conditions to allow a rough analysis of 24i -245 mixtures. Liquid crystalline cholesteryl cinnamate has proved effective as a stationary phase for the capillary-g.c. separation of insect pheromones e.g. tetradecen-l-yl acetates differing in the position of unsaturation). 

Figure 2 Conversion of sterols under thermal stress. With increcising thermal stress, steranes possessing the immature fictaR (not shown) and actaR configurations are thought to he isomerized to otaotS as well as afifiS and afifiR epimers.

The Ring System and Its Possibilities tor Isomerism. According to systematic chemical nomenclature, cholesterol is a derivative of cyclopentanoperhydrophenanthrene. This alicyclic, saturated hydrocarbon (without double bonds or aromatic character) bears the trivial name sterane. The C atoms are numbered in the manner indicated. 

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