Some examples and possible explanations for oil generation from coals and coaly sequences

Abstract

Abstract Coals and associated shales are important oil source rocks in certain deltaic environments, in which selective processes of degradation or reworking increase the content of hydrogen-rich kerogen. The formation of hydrogen-rich kerogen in a deltaic setting is either by preferential concentration of plant cuticles and spores after reworking of delta top freshwater peats, or, by accumulation of delta margin peats under saline conditions. In each case the source rocks and derived oils may be expected to show distinctive indicators of the depositional environment in the associated alkane content, sulphur content, biomarker and carbon isotope signatures. Coals in the Oligo-Miocene Talang Akar Formation of South Sumatra and Northwest Java Basins contain perhydrous vitrinite and are the source of isotopically relatively light waxy oils which contain biomarkers derived from tree resins. Waxy oils are also produced in the Sunda Basin, between Sumatra and Java but in this case the upper (coaly) part of the Talang Akar Formation is immature and the oils are derived from algal kerogen in the older, lacustrine Banuwati Formation shales; these oils are distinguished by low contents of land plant biomarkers and by a markedly heavier carbon isotopic signature. In the Western Desert of Egypt, coals of the mostly marine Jurassic Khatatba Formation have sourced oils with a relatively heavy isotopic signature. It is proposed that the isotopically heavy character of the coals and oils is due to a combination of microbial contribution, and raised salinity (and sulphate) levels due to marine influence. In northern England, Namurian (Pendleian) marine shales are generally distal turbidites derived from pro-delta sediments but mostly without oil source potential. Rich oil source rocks occur as basinal shales at the extreme margins of or interdigitating with the turbidites or as condensed sequences on structural highs. They contain kerogen derived by reworking of peats of a deltaic-coastal plain environment, and may be correlated with many of the oil shows and produced oils of the area. The examples demonstrate that the recognition of source rock facies aids exploration by guiding the prediction of the location and potential of oil-prone facies, but that this is only possible if all facets of oil and source rock geochemistry are integrated with a knowledge of sedimentary processes.