THE ROLE OF CHEMOSTRATIGRAPHY IN DEVELOPING A SEQUENCE STRATIGRAPHIC SCHEME FOR THE MIDDLE TO UPPER JURASSIC SUCCESSIONS OF THE UK CENTRAL GRABEN

In the Middle to Upper Jurassic of the UK Central Graben, the key to exploration lies in the mapping of the distribution of turbidites, shallow marine and shoreface sandstones, which are proven reservoirs. However, drilling occurs in a HPHT regime and the establishment of a reliable chronostratigraphic framework is hampered by moderate to poor biostratigraphic recovery and inconsistent wireline correlations. To counter these issues an independent sequence stratigraphic model for the Jurassic successions, based on the integration of chemostratigraphic, sedimentological, biostratigraphic and heavy mineral data, has been established.

This paper reports on Phase 1 of an ongoing basin wide study over 100 wells. This multidisciplinary approach has led to the establishment of a detailed chemostratigraphic correlation framework, which in selected type wells can be tied to existing biostratigraphic zonations and associated sequence stratigraphic schemes. The corroboration of the chemostratigraphic scheme in the type wells enables the chemostratigraphic methodology to be applied basin wide with the creation of a new chronostratigraphic framework to constrain paralic, shallow marine and basinal facies within the Jurassic succession. This chemostratigraphic scheme reflects changes in mineralogy, depositional environment, climate and provenance as recognised by changes in the elemental concentrations. A sedimentological model developed as part of this study for the Pentland/Fulmar Formations also suggests the successions may be subdivided into highstand and lowstand systems tracts on the basis of sedimentary facies and evidence for paralic/marine environments. From this model, it is evident that while many of the reservoir sandbodies lie within lowstand systems tracts this is not universally the case and episodes of increased sediment flux resulted in forced regressions such that fluvio-distributary channel sandbodies occur interbedded with shallow marine facies.