Steranes kinetics

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. 

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. 

Abbott G.D., Wang G.Y., Eglinton T.I., Home A.K., Petch G.S. (1990) The kinetics of sterane biological marker release and degradation processes during the hydrous pyrolysis of vitrinite kerogen. Geochim. Cosmochim.Acta 54, 2451-61. 

However, larger alkanes with more than two carbon atoms which have kinetic diameters of more than 4.3 A, cyclohexane with a kinetic diameter of 6.0 A, and even larger and more viscous oil constituents like polyaromatic compounds, steranes, hopanes, resins and asphaltenes, are unable to move through the caprock, partly clogging the available effective permeability, and hence remain in the trap (cf. Karlsen 1987 ... 

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