Mapping of Gas Migration into, and out of, the Tyra Field in the Danish North Sea

The paper presents various pieces of evidence telling a story of gas migrating into – and out of – the Tyra gas field in the Danish North Sea. Seismic observations of gas charge include a (1) gas chimney from the Jurassic into the Cretaceous chalk reservoir along major faults, (2) low stacking velocities in the lower chalk, coincident with a (3) push-down of the base chalk reflector, and (4) low reflectivity spots in the base chalk reflector, thought to be gas entry points. Above the reservoir, low seismic velocities in marine shales are coincident with top reservoir relief, and these low velocities are supported by well data. Low seismic velocities coincide with higher gas readings in the mud logs. In the same area, both well data and bright seismic amplitudes indicate the presence of Eocene and Oligocene limestones in an otherwise shaly marine sequence. Stable isotope analysis of cuttings show very depleted 13C levels of these limestones, suggesting a carbonate partially sourced by thermogenic methane. In this scenario, methane vents would support colonies above the leaking reservoir. Only later would the reservoir be effectively sealed.