Fluid escape from reservoirs: implications from cold seeps, fractures and injected sands - Part I. The fluid flow system

Fluid escape from reservoirs can take place through (hydraulic) fracturing, sand injection and seepage. Above several Tertiary hydrocarbon reservoirs in the North Sea, substantial amounts of fractures, sand injectites and seeps occur. Petrographic observations of these features show that all have carbonate cement associated with them that contains fluorescing hydrocarbon inclusions. The petroleum fluids escaping were partially trapped during the cementation and analysis of the cements allows understanding of the fluid flow system and the fluids involved. Cathodoluminescence indicates that sand injectites and fractures have one phase of cement associated with them. Seeps, however, show zoned carbonate cement, suggesting precipitation in a varying geochemical environment. This suggests that fractures and sand injectites were short-lived fluid-conduits, whereas seeps can act as fluid escape pathways over prolonged periods of time. (C) 2003 Elsevier Science B.V. All rights reserved.