Chalk reservoirs of the North Sea.

Abstract

Chalk in general is a micropore reservoir. North Sea Chalk hydrocarbon fields are all located over thick areas of Kimmeridge and Oxford Clay source rocks on structures which grew during the Tertiary. Structural growth caused fracturing allowing hydrocarbons, which were generated from as early as Oligocene times onwards, to build up in the fracture systems within structural closures in the Chalk. Chalk burial diagenesis does not start until approximately 1000m below surface. In the case of North Sea Chalk reservoirs, diagenesis, which will normally reduce porosity from approximately 50% at the sea bed to 10% at between 3000 and 4000m burial depth, is arrested by three factors: the pressure, which supports the overburden; oil or gas in the pore space; magnesium ions, which poison sites of nucleation of calcite. As a result, all Chalk fields show anomalously high values of porosity. Chalk reservoir quality is controlled by four predominant factors: the purity in terms of calcium carbonate of the sediment; the rate of deposition of the Chalk which in turn determines the degree of early frame-work cement; the tectonic setting of the field area during Chalk deposition; and the size distribution of the coccoliths being deposited.-from Author