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Trace Elements in Illitic Clay Fractions - Possible Clues to the Subtle Stratiagraphic Trap
Primary stratigraphic traps are often discovered seismically by mapping velocity contrasts between reservoir and nonreservoir facies or associated positive structures and hinges, but traps poorly defined seismically could be overlooked. Comparison of B/K2O, Na/K2O and V/K2O ratios in clay fractions (adjusted concentrations) from a slowly deposited Visean section in northern England with similar data from two rapidly deposited Eocene sections in southern California suggests that geochemical methods could aid exploration.
Adjusted B is low in marine Eocene rocks but high in marine Visean. This cannot be attributed to differences in clay mineralogy because clay separation always yielded almost pure illite. Therefore, rapidly deposited marine sediments are probably B deficient because illite was buried before B could be absorbed. When time for B sorption was adequate, as with the slowly deposited Visean section, adjusted B correlates with paleosalinity. Adjusted Na correlates with paleosalinity in all three sections and must, therefore, be less influenced by deposition rate than adjusted B. However, average Eocene values are higher than in the Visean suggesting that Eocene illites were derived from a paragonite-rich source. Average adjusted V is the same in all three sections but correlation with paleosalinity is only seen in the Visean.
It follows that in rapidly deposited marine facies, proximity to traps associated with reduced sedimentation rates over weak highs should be marked by increasing adjusted B. Shoal traps should also be indicated by abrupt lateral adjusted element changes because shoals frequently separate areas of contrasted sedimentation rate, salinity or provenance. Finally, in both California sections, abrupt vertical adjusted concentrations of all these elements correlate, suggesting that weak contemporaneous highs and their associated shoals could be found by isopaching units contained between geochemical markers.
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