Shallow Gas in the Eastern North Sea - Unravelling a Complex Fault- and Salt-controlled Fluid Migration System

Subsurface fluid migration is an essential process in sedimentary basins and is critical for the charging of hydrocarbon reservoirs. Observations of direct hydrocarbon indications and other geological and geophysical features indicative of fluid migration such as shallow gas, pockmarks, gas chimneys, pipes and velocity effects, all record past and/or present fluid migration. The specific migration routes are however often difficult to address in detail and thus typically represent a key unknown factor in studies of hydrocarbon plumbing systems. In this study, we present a detailed 3D-seismic analysis of shallow gas anomalies occurring in relation to a listric fault detaching on top of an elongated salt structure in the eastern North Sea. A neural-network-trained chimney-cube has been applied on the dataset allowing us to unravel the migration routes for the shallow gas anomalies and to understand the development of the gas accumulations in relation to the structural evolution of the listric fault and underlying salt structure. The shallow gas is thus suggested to be linked with deep and possibly thermogenic fluid migration and to be highly dependent on the sealing properties of the listric fault which is observed to be permeable in its lower part and sealing in its upper part.