Faulting and the relationship to fluid migration and shallow gas accumulation in the Hammerfest Basin, SW Barents Sea

Abstract

This thesis has focused its efforts to identify and map faults and their relationships to shallow gas accumulations and fluid flow features. The area of this investigation lies in the hydrocarbon-prone Hammerfest Basin in the southwestern Barents Sea, above the Base Cretaceous. Nine 3D seismic datasets were used in this study, aiming to improve the knowledge of the timing of the tectonic activity and the relations and the controlling mechanisms for fluid migration and accumulation.

Based on their vertical extent two fault groups exists: deep-seated and shallow faults. Deep-seated tectonic faults are further subdivided into three groups, F1, F2 and F3, based on the formation they terminate in. F1 terminates at the URU, F2 within the Torsk Formation and F3 in Cretaceous strata. Initiation of these faults occurred in the Kimmerian tectonic phase in Middle – Late Jurassic, where the E – W and NE – SW striking F2 and F3 faults experienced a small reactivation in the Barremian and Aptian. Multiple reactivations of the N – S striking F2 faults through the Cretaceous period occurred in response to thermal subsidence of the Tromsø Basin. Tectonic readjustments related to the opening of the Norwegian-Greenland Sea reactivated F2 faults and initiated the shallow faults. Faulting of the URU suggests tectonic adjustments after the onset of the Plio-Pleistocene glaciations.

Fluid migration of thermogenic gas from deeper reservoirs is evidenced by seven gas chimneys located above deep-seated faults that represents excellent migration pathways for gas from deeper levels. High amplitude anomalies within the Torsk Formation most likely represents accumulations of free gas below a sealing layer of gas hydrates. The URU occasionally acts as an impermeable barrier and amplitude anomalies located along the unconformity probably represent free gas accumulations.

Pockmarks on the seabed and the URU indicate at least two major events of fluid flow release, where variations in the stability conditions of the GHSZ have allowed free gas to burst upwards and create circular to sub-circular depressions. Pockmark formation could be an ongoing process if the hydrate layer is currently decomposing.