Chemical heterogeneity among adjacent coal microlithotypes — implications for oil generation and primary migration from humic coal

Abstract

Abstract Laser radiation focused onto a coal polished perpendicular to bedding has been used to thermally extract and pyrolyse the organic matter from three adjacent microlithotypes in a Tertiary, autochthonous, hydrogen-rich, sub-bituminous (early catagenesis/bituminization stage) humic coal from Indonesia. The distribution of hydrocarbons released during laser irradiation are shown to vary on a millimetre scale between adjacent humotelinite-, liptodetrinite-, and resinite-dominated microlithotypes. This variability is not due to varying thermal maturity but rather to (1) the inherent chemical variability of the macerals comprising each microlithotype; and/or (2) the presence of migrated, or non-indigenous, bitumen/oil. Hydrocarbon yields from each microlithotype suggest that primary migration between microlithotypes has occurred. Specifically, long chain n -alkanes and isoprenoids, which were most likely generated in the cutinite-derived liptodetrinite-dominated microlithotype (liptoclarite), are much more abundant in the overlying liptinite-free, humotelinite-dominated microlithotype (telite) and in the underlying resinite-dominated microlithotype (resinoclarite). These observations suggest that while the type of macerals plays an important role in the generation of oil within coals, it is the physical association between macerals (i.e. the microlithotypes present) which significantly influences the primary migration of oil within, and expulsion of oil from, coal seams. Thus since coals are extremely variable, both chemically and physically, generalizations suggesting all liptinite-rich humic coals can act as oil source rocks may be misleading. Liptinite-rich humic coals with high concentrations of finely-admixed liptinite and huminite macerals (e.g. liptodetrinite in a humodetrinite and mineral matrix, as would occur in microlithotypes with a detrital origin) may permit both oil generation and expulsion. Characteristics of the telite suggest, due to their high absorptive capacities and (micro)-porosity throughout most of bituminization/catagenesis, coals which contain an abundance of massive huminite macerals (e.g. humotelinite or gelinite) may tend to trap hydrocarbons generated from nearby or encased liptinites within their networks. Such macerals (the precursors to desmocollinites in bituminous coals) may become relatively hydrogen rich and often fluoresce under uv light and may not expel hydrocarbons until they are subsequently cracked to gas.