Distribution of sub-surface pressure in the Norwegian Central Graben and applications for exploration

Abstract

The Norwegian Central Graben contains a distinctive three-tiered sub-surface pressure system. This pressure system has an important effect on distribution of oil and gas fields, generation and migration of hydrocarbons, reservoir qualities and drilling procedures. A number of mechanisms have been proposed for the development of overpressure. In this paper, five are briefly discussed: thermal expansion of water, sedimentary compaction, rate of sediment burial, volumetric increases due to generation of hydrocarbons, and column heights of major oil and gas fields. None of the factors are mutually exclusive and all contribute to the development of overpressure in the Norwegian Central Graben. Utilizing drill-stem tests (DSTs), repeat formation tests (RFTs) and sonic and density logs, the horizontal and vertical boundaries of the three-tiered pressure system of the Norwegian Central Graben have been mapped. A shallow hydropressure system exists down to 1000–2000 m, underlain by a transition zone where pressures abruptly increase to the fracture level where overburden and fluid pressure are in balance. Under the transition zone, a geopressure zone exists to 3000–4000 m, within which pressures increase with depth at a hydrostatic rate. In the centre of the graben, this compartment is underlain by a second transition zone and a deep, high temperature, high pressure zone. The location and depth of the boundaries of the two upper pressure compartments and transition zones are more dependent on temperatures than lithologies or tectonic movements. Within the deep, high pressure zone, variations in pressure gradients indicate the presence of a number of smaller pressure cells, probably separated by faults. The identification and prediction of overpressured zones can have a significant effect on exploration programmes. From the drilling standpoint, prediction of sharp increases in pressure is a major safety factor, reducing the danger of well blowouts and allowing the operator to be adequately prepared for a number of drilling hazards. In potential reservoirs, overpressure reduces net overburden, thereby reducing porosity loss due to compaction. Sub-surface pressure variations have a significant effect upon hydrocarbon migration as oil and gas will attempt to move from high to low pressure compartments. Defining pressure variations from well to well can give indications of sealing and non-sealing faults. The pressure configuration has a significant effect upon the distribution of discovered hydrocarbons in the Central Graben. Most recoverable hydrocarbons are found in the first sealed permeable bed in the middle pressure compartment, overlying or adjacent to the transition zone above the deep, high pressure compartment. This includes the Cretaceous to Danian chalk fields in the centre of the graben and the Jurassic sand-reservoired fields on the northeast flank. Exploration in the deep, high pressure compartment has resulted in only limited success due mainly to poorer potential reservoir conditions, more difficult definition of traps and increased drilling expenses and risk.