Organic Material in Sedimentary Rock Sequences

The information gathered from natural silicate-organic systems is obviously not adequate to establish definite limits on the stability of the different forms of organic materials found in sediments. The data points are too few and the geologic history in most of the instances is too complex to decide whether time, temperature or pressure are effectively controlling the rate of decomposition of the organic molecules. 

A very interesting beginning has been made in experimental determinations of the behavior of fatty acids in a water-montmorillonite system at one atmosphere pressure (Johns and Shimoyama, 1972). The basis of the study is the transformation of a long chain molecule to smaller units by breaking specific carbon-carbon bonds. The energy necessary to do this is estimated to be about 46 Kcal/mole in a montmorillonite system, whereas calculations put the value at 56-58 Kcal in a uniquely hydrocarbon 

The available data indicate that, in shaley layers, petroleum or petroleum-like substances will be formed near the temperature of montmorillonite instability. At great depths, allevardite or the regular 

In summary there is not enough information published regarding the production of light hydrocarbon molecules (petroleum) or very heavy ones (coal) in geologic time. It is not possible at present to determine whether time, temperature or pressure plays a role, predominant or minor, in the evolution of the organic debris incorporated in sediments. It is certain that the interaction of silicate equilibria and transformations 

A large part of the difficulty is due to the complexity of the reactions in the organic system, their inherent instability after long periods of time and the influence of various catalytic agents on various reactions. This will undoubtedly be a fruitful field for further research. 

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