An assessment of fluid flow and overpressure modelling in selected North Sea and Laramide basins
The occurrence of overpressure observed in petroleum bearing basins must be understood in terms of generation and distribution in order to build up a fluid flow history. Overpressure influences expulsion and migration of hydrocarbons from source rocks. This thesis details and interprets observations and results of a case study into aspects of overpressure distribution, fluid flow patterns, causative mechanisms, palaeo- pressure and subsequent pressure history, the surface expression of overpressure and the mechanical strength relationships of sealing rocks. One of the principle study areas was the Alwyn field in the Northern North Sea. The overpressure distribution over the field area was seen not to be uniform. Organic geochemical data indicated that the source of reservoired hydrocarbons and associated fluids was the Viking Graben depocentre. Fluid inclusion data recorded in specific diagenetic mineral phases interpreted fluid flow conditions of hydrocarbons into the reservoir under normal pressures but elevated temperatures. When combined with computer generated pressure models, these diagenetic events were exclusive of modelled overpressure periods. The greatest contribution of overpressure was modelled as being a result of compaction disequilibrium but withalikely contribution from the thermal cracking of oil to gas which would also account for the present day distribution of overpressure across the Alwyn field area. The Uinta Basin with its relatively simple burial and thermal history allowed the production of a model . : involving a temporal history of overpressure generation, fracture development, regional tectonism, hydrocarbon maturation and expulsion and the process of Gilsonite emplacement. It is inferred that initial hydraulically induced fracturing of the Green River and overlying formations was a result of combined overpressure due to disequilibrium and regional extension with a possible contribution from the maturation of the source rock. This study recognised that the hydrocarbon was emplaced under a high pressure regime with evidence provide. by the existence of forcibly injected hydrocarbon sills. Hydrocarbons fractionated in the pre-existing vertical fractures to leave residual highly viscous and immobile hydrocarbons in veins seen at the present day. The third major component of this study detailed results from an assessment of the mechanical capacity of sealing rocks with respect to specific composition and mineralogy. It was found that increasing organic carbon decreased the compressive strength of the tested shale specimens. This relationship was interpreted as a possible result of the interaction of alkaline fluids to produce a dispersant which acts to reduce the cohesion of the organic rich shale.