Terrestrially sourced oils: where do they exist and what are our limits of knowledge? — a geochemical perspective

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Abstract
There are relatively few well-documented cases where significant amounts of oil (> 500 million barrels) have their origin from coals or their associated carbonaceous shales. The best-documented cases are from Australia and Indonesia and possibly the North Sea. The extensive debate as to whether coals as such or their associated carbonaceous shales are the source for oil has been essentially sterile: it has failed to take into account the heterogeneous nature of coals, the variety of their floral origins and the continuity of mineral dilution from pure coal to carbonaceous shale. Each case must be considered on its merits. The overall petrographic composition of coal is a poor guide to its petroleum potential. Sub-microscopic lipids of bacterial and also plant origin occur in vitrinite and inertinite and contribute significantly to petroleum potential. Suberinite and cutinite are the major source of waxy hydrocarbons. Hydrogen Indices measured on the immature versions of documented terrigenous source rocks are surprisingly low falling in the range 200–350 and occasionally lower. There is not a simple relationship between the elemental composition of terrigenous kerogen, the gross pyrolysis yield by Rock-Eval and the yield of normal hydrocarbons in pyrolysis gaschromatography. Our assessment of liquid potential in marginal source rocks, wherein most terrigenous sediments lie, is far from perfect. A combination of the mass balance approach in conjunction with pyrolysis studies has been shown to be the most effective way of demonstrating source potential and hydrocarbon products (gas vs. oil) expected from these marginal source rocks. There is clear evidence that significant amounts of gas are generated in the conventional oil window, and that high gas to oil ratios facilitate migration from the leaner source rocks. Comparative maturation studies on oils and gas-condensates show that they can be generated at similar maturation levels. The majority of terrigenous oils and condensates are paraffinic and the oils may have a high wax content. Exceptions occur when resin is a significant source in which case there is an increased proportion of naphthenic and aromatic hydrocarbons. Oils derived from coal-bearing sequences have high pristane to phytane ratios (>4.0); they have a high proportion of iso- and anteiso-alkanes derived from bacterial sources; hopanes predominate over steranes which are dominated by the C 29 member. A variety of diterpenoid hydrocarbons may be found which relate to the nature of the parent flora and increasingly can be used for oil-source correlation in non-marine sequences.

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