HYDROCARBON GENERATION - A POSSIBLE CAUSE OF ELEVATED PORE PRESSURES IN THE OSEN-ROA THRUST SHEET, NORWAY

The Osen-Roa thrust is very large (275 km wide in the thrust transport direction), thin (1-4 km thick) and internally shortened by 270 km. Over much of its extent the thrust sheet has moved over autochthonous or para-autochthonous Alum Shales (Cambrian). Such a large and thin thrust sheet is difficult to move as a coherent unit. High pore-fluid pressures are commonly invoked to help explain such problems. The Alum Shales are very rich in organic carbon (5-20 weight percent). They are capable of generating large volumes of hydrocarbons which could have periodically caused elevated pore-fluid pressures. The first event that was likely to have buried the Alum Shales to oil-generating depths was overthrusting during the Caledonian orogeny. Consequently, overpressuring in the Alum Shales due to hydrocarbon generation may help to explain the size of the Osen-Roa thrust sheet.