Origin and accumulation mechanism of shallow gas accumulations in the SW Barents Sea

Abstract

Events associated with shallow gas accumulations are reported all over the Norwegian continental shelf. These accumulations are potentially dangerous geo-hazards related to drilling activity, but they can also be of economic interest if the accumulated volumes are large. Petroleum in the Barents Sea has been influenced by the late periods of uplift and erosion creating a complex province. The exact distribution and formation mechanism of shallow gas accumulations have yet to better understood. This thesis focus on mapping the distribution of shallow gas accumulations located within the Tromsø Basin and adjacent areas in the SW Barents Sea. Data used consists mainly of publicly available 2D seismic data within the study area. 3D seismic data (surveys EL0001 and LN09M01) focus on specified amplitude anomalies associated with shallow gas accumulations, possibly linked with the formation of diagenesis-related BSR. Mapped seismic evidence of shallow gas is associated with bright spots, zones of acoustic masking and chaotic reflection pattern, chimneys and leakage along faults. The main stratigraphic units of this thesis are defined above Torsk fm., below Torsk fm. top and below Kolmule fm. and the upper Kolmule fm. Overall, the results give a general overview of the location of potential shallow gas accumulations. The general shallow gas origin is most likely of thermogenic generation from sources located at depths below the Kolmule fm. The source is most likely a mixture of source rocks, including the major Hekkingen fm. Major migration pathways within the study area bypass zones of deeper faulting, mainly the Ringvassøy-Loppa Fault Complex to the shallower levels. Migration pathways are also identified towards structural highs along the Tromsø basin border (Loppa High, Veslemøy High and Senja Ridge) and towards salt diapirs within central parts of the basin. Accumulation and migration occur mainly related to the interpreted intra Torsk fm. and below the sealing upper regional unconformity (URU). Accumulation mechanisms are related to structural and stratigraphic trapping and the development of traps associated with diapirs and BSR (both gas-hydrate and diagenesis-related BSR).