Hydrocarbon Charge and Leakage in Deep North Sea Geopressured Sandstones: Calibrated by Diagenesis

R. Stuart Haszeldine and Mark Wilkinson
University of Edinburgh, Edinburgh, United Kingdom

Drilling of a regional structural high in the North Sea Central Graben, found a water-wet Fulmar Formation High Pressure High Temperature sandstone. Petrographic textures and diagenetic minerals in the sandstone record two phases of hydrocarbon filling and leakage. The oldest charge is predicted from 2-D basin modelling. The mineral record is a vertical series of radiometric K-Ar ages from minor grain-coating fibrous illite clay. This shows late Cretaceous filling of a 200m oil column, lasting 25 Ma, at a palaeo-burial of 1.5km. Shallow oil–fill may have preserved primary porosity. Leak-off occurred at 60Ma, so that cementation by ankerite and quartz pore-filling cements could then commence. Pore-lining bitumen coats these cements, and so post-dates them. Release of geopressure at 4 Ma permitted high salinity, isotopically high δ¹⁸O, deep basin waters from evaporites to displace oil. This produced high temperature fluid-inclusion trails (140 C) in healed fractures within quartz, with salinities comparable to present-day pore fluids. This demonstrates that a reservoir can fill and empty several times during burial. A detailed record of mineral and organic cements, combined with accurate petrography, must be used to calibrate regional basin models, so that correct predictions can be made.