RECOGNITION AND QUANTIFICATION OF THE EFFECTS OF PRIMARY MIGRATION IN A JURASSIC CLASTIC SOURCE-ROCK FROM THE NORWEGIAN CONTINENTAL-SHELF

A detailed quantitative geochemical study was made of a mature Jurassic clastic source rock sequence in order to attempt to recognize and quantify petroleum migration effects. Two different quantiative approaches were followed to monitor primary migration effects on both the bitumen-1 (e.g. whole rock extract) and the bitumen-2 (e.g. isolated kerogen extract) fractions. These methods were (A) comparisons of TOC-normalized extract yields and quantified molecular compositions of petroleums from both tightly carbonate cemented zones and those from less cemented zones; and (B) comparison of petroleums from shale laminae vs those from interbedded siltstone laminae. On this basis it was possible to assess apparent primary migration efficiency. Relative expulsion efficiencies in certain shale samples and relative import efficiencies in siltstone bands ranged from 11% (total extract) to 85% (saturated hydrocarbons and individual n-alkanes). No carbon number dependent fractionation effects were observed for n-alkanes of bitumen-1 in the C-15 to C35 range (i.e. expulsion efficiencies were independent of carbon number) suggesting bulk oil flow with chromatographic modification as the predominant transport mechanism.