APPLICATION OF PYROLYSATE CARBON ISOTOPE AND BIOMARKER TECHNOLOGY TO ORGANOFACIES DEFINITION AND OIL CORRELATION PROBLEMS IN NORTH-SEA BASINS

Detailed sampling of apparently homogeneous potential source rock intervals frequently indicates unsuspected variations in organic facies which may or may not be reflected in the wireline logs. Carbon isotope analyses of pyrolysates provide a useful technique in these investigations which are essential for valid oil to source correlations. In the Jurassic of a North Sea Viking Graben well the rich source shales at the top of the Draupne Formation are significantly depleted in C-13 and correlate with the oils from this area, thus suggesting that this interval has acted as the primary source of the Viking Graben crudes. A quite different pattern is evident in a well from the Central Graben and the data not only explain why the oils of the Greater Ekofisk area are relatively enriched in C-13 but also demonstrate that the variations within these oils are facies controlled. Pyrolysate carbon isotopes provide an excellent correlation between the Middle Devonian Upper Caithness Flagstones of the Inner Moray Firth and the Beatrice crude oil. This correlation is supported by the biomarker data and there is no need to appeal to a contribution from either an Upper or Middle Jurassic source to achieve a match with the Beatrice oil.