SYNCHRONOUS OIL MIGRATION AND CEMENTATION IN SANDSTONE RESERVOIRS DEMONSTRATED BY QUANTITATIVE DESCRIPTION OF DIAGENESIS

We present a model that explains the patterns of sandstone burial, dia genesis in certain oil reservoirs, in which petroleum migration and bu rial cementation were synchronous. The coincidence of these two proces ses controls the chemistry and distribution of major burial cement pha ses across the field, which in turn controls the distribution of reser voir quality, causing a rapid decline of porosity and permeability wit h depth. Such a rapid poroperm deterioration is observed in many North Sea sandstone oilfields; we highlight the Magnus Sandstone Member of the Magnus Oilfield, northern North Sea as a type example of such a re servoir. The two most significant elements of the synchronous cementat ion and migration model are that burial cementation in the reservoir o ccurs over a restricted time interval, probably less than 10 Ma and th at rapid and widespread fluid circulation is not invoked to explain th e concentrations of cements observed. We speculate that cementation ta kes place at, and in a series of zones below, the oil-water contact wh ich descends as oil fills the reservoir, with little change to the bul k chemistry of the reservoir formation waters through time.