Chemical fossils: trends in organic geochemistry

It is now clearly recognized that many organic compounds which were the complex biochemical products ofancient organisms have survived in geological sediments for relatively long periods, often with little or no transformation of their basic skeleton. Other constituents, however, may be altered severely under these conditions and retain little which indicates their original structure. All these chemical fossils are, of course, far more numerous than their morphological counterparts, due to the multifarious chemical processes which occur in the subsurface environment. In recent years, application of modern analytical techniques has Ied to a greater knowledge of the discrete molecules, especially those related to the porphyrins, which can be isolated from oil and sediments, and has thrown much light on diagenetic processess including the rate and mechanisms of several geochemical reactions. Such knowledge has, in turn, been applied to the elucidation of the more difficult problern of the origins of diverse geopolymers. The processes outlined above and the conclusions which can be drawn from the results obtaincd are comprehensively reviewed.