Petroleum type assignment

The term "organic facies" has been used in a broad variety of contexts and connotations, but these boil down to two basic usages. One seeks to relate variations in organic matter compositions and concentrations to specific biological inputs and depositional processes, and thence recognise "anoxic and oxic facies", "marine and deltaic facies", and the like. The second usage, and the one focused on here, is to define organofacies according to criteria that allow the transformation of the static kerogen phase into petroleum to be defined qualitatively and quantitatively.

We have used the classification scheme of Horsfield (1989), based on petroleum composition to define the petroleum type organofacies gas/condensate-, paraffinic-naphthenic-aromatic petroleum- (high and low wax varieties) and paraffinic petroleum-generating (high and low wax varieties). Similarities exist between these divisions and those of naturally occurring crude oils.

In this, the first PhaseKinetics step, available source rocks, stains and seeps are analysed by pyrolysis gas chromatography to assign petroleum type organofacies. If no samples are available, the nearest PETROLEUM FACIES is selected.

The interpretation scheme is calibrated from petroleum systems in North America, Asia and Europe (see yellow data points below).

Using this method in combination with a related one based on sulphur-compounds (di Primio and Horsfield, 1996), oil-prone coals and gas-prone coals can be distinguished from one another, and marine source rocks which generate high sulphur petroleums (usually carbonate or siliceous) are distinguished from those generating low wax low sulphur crudes or gas-condensate (siliciclastic). Similarly, high wax oil potential is delineated for lacustrine and fluviodeltaic sources.