Gerald G. Calhoun
The aromatic component in oils may prove to be the key to defining oil traps in mature basins as well as pioneer areas. This is a direct indication of vertical oil migration and can precisely duplicate the oil signature in soil samples.
This study traces the changes that occur in San Andres oils over the Permian basin. The sequence of sync-scans along the Central Basin platform pictures the gradual changes in the mix of lighter aromatics in the north toward predominately heavier aromatics in the south. This change parallels decreasing depth, freshening of formation waters, and decreased gravity of the oil.
A comparison of oils and their source rocks is presented for the Woodford, Pennsylvanian, and Wolfcamp generation-migration systems. The sieving effect of the source shales is evident in the higher concentrations of complex molecules retained in the source rock as compared to the resultant oils trapped in conventional reservoir rocks. Movable oil retained in the source rock is also evident using synchronous scans. Productive areas are reflected at the surface with oil to soil correlation coefficients up to 90%. Barren areas are often evident in soil samples by their close resemblance to the source rock signature. The contrast between productive and barren areas is also evident in a 100+% increase in the concentration of the various families of compounds-Benzenes, Naphthalenes, Phenant renes-Anthracene, and compounds containing five or more aromatic rings.
Any surface geochemical evaluation should include no less than three techniques, and one of these should be soil fluorescence. No other geochemical technique can identify the source of microseeping oil. Only fluorescence cannot only say "yes" or "no," but can also name the objective horizon.
AAPG Search and Discovery Article #90980©1994 AAPG Southwest Section Meeting, Ruidoso, New Mexico, April 24-26, 1994