Gas washing of oil along a regional transect, offshore Louisiana

Gas chromatogram data for 219 oils in a 190 km N–S transect offshore Louisiana reveal a spatially coherent pattern of compositional change which is caused by gas washing. Near the Louisiana shoreline, as much as 91% of the original n-alkanes have been removed from the oils. The maximum intensity of depletion decreases southward in a nearly regular fashion to nil at the Jolliet field 190 km offshore. The oils show a parallel change in the maximum carbon number of the removed n-alkanes, implying that the pressure at which gas washing took place also decreased in the offshore direction. The systematic change in maximum extent of depletion crosscuts tectonostratigraphic boundaries as well as oil source provinces. Models of gas washing suggest that the maximum depth of washing reflects the distribution of deeply buried continuous sands, suggesting deep sands may have provided sites for efficient gas–oil interaction.