Regional distribution of diagenetic carbonate cement in Palaeocene deepwater sandstones: North Sea

Sandstones of the Palaeocene Montrose Group were deposited in a deepwater f an environment, and form a major oil reservoir in the North Sea. Calcite co ncretions occur commonly within thick-bedded and structureless sandstones. These concretions have been identified by sonic logs and well reports, and were cross-checked with available core data. Regionally, 101 wells have bee n examined and carbonate concretions form 0.6-7.2% of the core. Concretions are most abundant along the flank of the Fladen Ground Spur, the north Wit ch Ground Graben (WGG), the east south Viking Graben and East Central Grabe n (ECG). Concretions of the ECG formed at deep burial, with C from decarbox ylation. Geochemical inheritance of Mn and Sr from Cretaceous chalk clasts may occur. Concretion growth may also have been influenced by vertical expu lsion of fluids (leak-off) localized above salt tectonics. Isotopic and pet rographic evidence indicates that much carbonate C in the WGG was derived f rom biodegradation of migrating oil in meteoric water at shallow depth. The locations of abundant carbonate with characteristic negative C isotope sig natures can be used as shallow exploration guides to leak-off points locate d above deep overpressured structures.