Diagenesis in North Sea HPHT elastic reservoirs - consequences for porosity and overpressure prediction

Upper Jurassic elastic reservoirs of the Fulmar formation in the Central No rth Sea possess anomalously high porosities for their present day depth of burial. Reservoirs with the highest overpressures have the highest porositi es, possess less macroquartz cement, and have significant secondary porosit y. Quartz cementation in HPHT (High Pressure High Temperature) reservoirs h as been inhibited by a combination of factors: high overpressure, limited f luid movement, presence of early grain coating cements, high pore fluid sal inity, and possibly petroleum migration. Secondary porosity has contributed to reservoir quality, with an average of 4vol% extra porosity created. Qua ntitative prediction of porosity would require an improved depositional mod el for the Fulmar, accurate thermal and pressure modelling, and detailed kn owledge of field filling and leakage histories. Theoretical calculations in dicate that diagenetic reactions occurring in the Fulmar formation (smectit e illitisation and quartz cementation), did not generate significant overpr essure, because seal permeabilities were too high and the rate of volume in crease associated with the reactions too small. Therefore diagenetic reacti ons can effectively be ignored when modelling overpressure generation in th e Central North Sea, although cementation will affect rock permeability and rates of fluid dissipation. (C) 1999 Elsevier Science Ltd. All rights rese rved.