The occurrence and microscopic features of micrinite and exudatinite in some Australian coals are reported. The origin of these macerals and also the significance of their occurrence are discussed in connection with hydrocarbon genesis. In Australia, micrinite occurs in bituminous and sub-bituminous coals which are very rich in inertinite, and also in brown coal rich in inertinite. One of the possible progenitors of micrinite is oxidized porigelinite. There is little reason to conclude that micrinite was formed from resinite and other macerals at an early stage of coalification and that liquid hydrocarbons were formed during this process. Exudatinite occurs in sub-bituminous and high-volatile bituminous coals in the Gippsland Basin. There is no positive evidence of a genetic relation between liquid hydrocarbons, exudatinite, micrinite and liptinite macerals. The formation of liquid hydrocarbons from solid liptinite etc. may take place just before and during Teichmiiller's so-called '2nd coalification jump'.
It is generally believed that oil is generated from organic material finely dispersed in a sediment at a certain stage of its geothermal alteration. However, not all authors are in agreement on the actual stage at which this occurs. Some suggest that a fairly low degree of organic diagenesis is sufficient for oil generation. Recently, M. Teichmuller132 stated that the appearance of two microscopic constituents of coal, micrinite and exudatinite, is indicative of the generation of oil. However, in Australia, micrinite commonly occurs in low-rank coals, and even in lignites. If petroleum is generated from organic matter of such a low degree of diagenesis, then some ideas on petroleum prospecting, particularly in the Australian context, should be revised. Several shallow basins, for example, could be reinvestigated.
The purpose of this paper is, in the light of new evidence presented, to reconsider the genesis of micrinite and exudatinite and to assess the significance of their occurrence in the generation of petroleum.
An explanation of the terms appearing in this paper and a summary of the macerals found in bituminous and brown coals are given in the Appendix. Following the recommendation of the International Committee for Coal Petrology3, the term 'micrinite' is used throughout the paper instead of 'fine micrinite'. The term 'exudatinite' is used as defined by Teichmuller'.