A study of natural and artificially generated light hydrocarbons (C4–C13) in source rocks and petroleum fluids from offshore Mid-Norway and the southernmost Norwegian and Danish sectors

The “free” or “natural” light hydrocarbon composition obtained by thermal extraction-GC of source rock samples is compared with the light fraction generated by pyrolysis products of the kerogens. Even though there are large differences between the composition of the “free” C₄–C₁₃ hydrocarbon fraction and the same fraction generated by pyrolysis, some characteristics have been detected which can be used interchangeably for both data types. Visual inspection of gas chromatograms from thermal extracts and pyrolysates indicates that in particular the relative content of m+p xylene corresponds well between these two analytical methods. The source rock samples used are Upper Jurassic marine shales and Middle and Lower Jurassic coals and coaly shales from offshore Mid-Norway and Denmark. More detailed analysis of the data shows that the most effective parameter which can distinguish between different source rock types in both thermal extracts and pyrolysates is the m+p xylene/nC₈ ratio. This parameter has been used to derive classification diagrams for interpreting the source of light hydrocarbons of both natural petroleum fluids analysed by gas chromatography and the same fraction generated by pyrolysis of asphaltenes from the fluids.The model was first tested on 17 natural petroleum fluids from Mid-Norway since a comprehensive study of light hydrocarbon distributions already has been published. Further, the parameter was applied to correlate with asphaltene pyrolysates of the fluids from Mid-Norway and a total of 22 natural oils and condensates from the southernmost Norwegian and Danish sectors.