ELECTRON-MICROSCOPY AND PYROLYSIS OF KEROGENS FROM THE KIMMERIDGE CLAY FORMATION, UK - SOURCE ORGANISMS, PRESERVATION PROCESSES, AND ORIGINOF MICROCYCLES

Recent studies revealed short-term cyclic variations (microcycles) in total organic carbon (TOC) and the hydrogen index (HI)in the Kimmeridg e Clay Formation, Bn organic-rich deposit considered to be a lateral e quivalent of the main source rocks of the North Sea. In addition, thre e different types of organic matter that all appear to be amorphous wh en observed by light microscopy (AOM) were recognized. Together, these AOM types account for over 80% of total kerogen and their relative ab undances show large variations along each microcycle. In the present w ork, transmission electron microscopy(TEM) observations were carried o ut on samples (whole kerogens, kerogen subfractions only comprising a single type of AOM, selected rock fragments) corresponding to typical points within a microcycle and obtained via high resolution sampling. The nature and the relative abundances of the products generated by Cu rie-point Py-GC-MS and off-line pyrolyses of isolated kerogens were al so determined for two selected samples corresponding to the beginning and the top of the microcycle. Combination of such ultrastructural obs ervations, including some semiquantitative studies, and the analysis o f pyrolysis products allowed (1) determination of the ultrastructural features of the three AOM types thus providing what we believe to be t he first example of correlations between light microscopy (palynofacie s, in situ maceral analysis) and TEM observations on ''amorphous'' fos sil materials; (2) identification of the source organisms and elucidat ion of the mode of formation of the different AOM types in the Kimmeri dge Clay; (3) explanation of the variations in their relative abundanc es taking place along a microcycle and establishment of tight correlat ions with TOC and HI changes; and (4) explanation of the origin of the microcyclic variations in kerogen quantity (TOC) and quality (HI) occ urring in the Kimmeridge Clay Formation. Interrelationships between pr imary productivity, sulphate reduction intensity, and lipid ''vulcaniz ation'' likely played a major role in the control of such variations.