Relationships between organic maturity and inorganic geochemistry in Upper Jurassic petroleum source rocks from the Norwegian North Sea and the United Kingdom
The aim of this study was to. examine the relationship between organic maturity and trace element geochemistry of organic rich mudstones, to assess their behaviour as sources or sinks of metals during diagenesis, and their role in mineral deposit genesis. The suite studied consisted of 193 samples from the Draupne, Heather, and Kimmeridge Clay Formations from the Norwegian North Sea and onshore UK. All three formations had above average contents of C and S, and the Draupne and Heather Formations had a low carbonate content. The Draupne Formation was particularly enriched in a number of trace elements but only Cr, Mo, Nb, and U were enriched in all three formations. After transformation of the data to avoid difficulties in interpretation, correlation analysis allowed the trace elements to be divided into three groups on the basis of their mineralogical residences. The use of a battery of geochemical and mineralogical indices demonstrated that variation in sedimentological and environmental factors were significant within the formations studied, and that the relationship between organic maturity and trace element content could not be investigated in isolation. The technique of path analysis was used to assess the strength and nature of this relationship relative to the variation due to other causes. Pb and Zn were found to increase with increasing maturity in both the Draupne and Heather Formations, and Mo was found to decrease. U was seen to decrease, and Cd to increase, in the Draupne Formation only, where they were especially abundant. It is believed that Pb and Zn were supplied by migrating basinal brines which may have acted to remove Mo and U. Other processes which may have mobilised these elements are the in situ generation of organic acids and hydrocarbons.