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Migration of Gas and Condensate in Delta Systems

K. F. M. Thompson

A suite of oils reservoired adjacent to a Louisiana salt dome was analyzed. Multiple traps at increasing depths occur as a direct consequence of the nature of deltaic sedimentation and as a prerequisite of the phenomena described. The oils are interpreted as having been originally identical; however, they are presently of diverse compositions, particularly in the gasoline range. The oils exhibit varying degrees of light-end loss, the material lost in forming migratory gas condensates, some of which accumulated in shallower traps. The losses are accompanied by fractionation so that the remaining (residual) oils show progressive changes in certain intermolecular ratios. The extent of these changes is proportional to the extent of light-end loss. Residual oils show three typ s of compositional change: (1) increase in aromaticity (in aromatic hydrocarbons relative to normal alkanes of similar molecular weight); (2) increase in normality (in unbranched alkanes and cycloalkanes relative to branched isomers); and (3) decrease in paraffinicity (paraffins relative to naphthenes).

Experiments with systems of gas and oil at elevated pressures and temperatures reproduced in detail the extraordinary range of gasoline compositions observed in the salt dome oils, as well as in a further suite of 70 Gulf Coast oils. The experiments involved only fractionation attendant upon phase separation in the pressure vessel, followed by selective removal of the vapor phase.

Approximately 75% of analyzed Gulf Coast oils are substantially or severely altered by light-end loss and fractionation. The same phenomena were found in other basins, and are a key to understanding major aspects of petroleum variability.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.