ABSTRACT

Sandstone cores from two wells in Statfjord Nord Field have been studied to evaluate a local correlation problem. In well 33/9-14, cores have been taken in sandstones of both the Upper Jurassic Draupne Formation and the Middle Jurassic Brent Group. In well 33/9-13, a 67 m cored interval was originally identified as Brent but was subsequently suggested to be Draupne Formation. It has not been possible to confirm the correlation of this interval by biostratigraphic study or conventional heavy mineral analyses. In order to determine the feasibility of identifying the sandstones in well 13 on the basis of their bulk chemical composition, bulk chemical data from the known stratigraphic intervals in well 14 were examined using the statistical technique of discriminant analysis. Consistent c mpositional differences were found between the Upper Jurassic and Middle Jurassic sandstones, and the sandstones from the disputed interval in well 13 were found to match the Middle Jurassic compositions. The most significant compositional difference is higher phosphorous content in the Upper Jurassic sandstones. This difference is consistent with the observation of glauconite pellets (a feature commonly associated with phosphorous mineralization) throughout the Upper Jurassic interval in well 14 and the complete absence of glauconite in both the Brent in well 14 and the disputed interval in well 13. The approach used in this study--including both the use of bulk chemical analyses as a correlation parameter and the use of discriminant analysis for differentiating between stratigraphic un ts--has potential application to correlation problems in a wide variety of rock types.