Thermal maturity and molecular transformations

3 I Nickel and vanadium distributions and sedimentary redox conditions 

The chemical form of sulphur is important in determining the availability of metal ions in marine environments. In area I of Fig. 5.34 sulphur is generally in the form of S042 and nickel is available for bonding (as Ni2+), but vanadium is unavailable. Oils from source rocks deposited under these conditions, such as those from Mesozoic and Tertiary reservoirs in the Uinta Basin (USA), have V/(V + Ni) values 0.1 and S content 1%. In area II sulphate is again the major form of sulphur, while Ni2+ and V02+ (vanadyl) are available for bonding, but the vanadyl species may 

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. 

Studies of the changes in absolute abundance of sterane and hopane isomers during oil generation under differ- 

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, 

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