Pressure significance of gas chimneys

• Published in: Marine and petroleum geology 2017-09, Vol.86, p.402-407
• Main Authors: Bello, A., Heggland, R., Peacock, D.C.P.
• Format: Article
• Language: English
• Subjects: Fluid pressure; Gas chimney; Overpressure; Seal
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2017.06.005

Analysis was carried out of part of the northern North Sea to test what the presence and style of gas chimneys indicate about fluid pressure (Pf) within hydrocarbon reservoirs. Previous results suggest that broad chimneys above a trap and thin chimneys on the flanks indicate the presence of hydrocarbons, whilst thin chimneys in the crest suggest the hydrocarbons have escaped. Each type of gas chimney is usually associated with overpressure within Mesozoic reservoirs, but the water leg is hydrostatically-pressured in most Cenozoic reservoirs. This indicates: (a) gas leaking from a trap does not necessarily cause Pf to become hydrostatic; (b) overpressure may not be necessary for the expulsion of gasses through seal units to create the chimneys; (c) although gas chimneys indicate the existence of an active hydrocarbon system, their presence does not appear to indicate anything significant about present-day Pf. •Though the presence and style of gas chimney do not appear to give information about Pf within traps.•Reservoir age and depth, however, correlate with Pf.•Gas chimneys do not mean that there has been a total failure of the seal, or that leakage has caused hydrostatic Pf.•Pf measurements may only represent a point in time in a transient system.