Palaeozoic petroleum systems of the central North Sea/Mid North Sea High

摘要

This report synthesises the results of the 21CXRM Palaeozoic project to describe the Carboniferous and Devonian petroleum systems of the Central North Sea/Mid North Sea High area (Quadrants 25–44).udFocusing on frontier areas to the north of the Southern North Sea gas fields and west of the Auk-Flora ridge, integration of a large volume of seismic, well, geophysical, organic geochemistry, maturity and reservoir property data at regional scale has established:ud Extensional to strike-slip Devonian and Carboniferous basins cutting across the Mid North Sea High on orientations strongly controlled by basement inheritance, granites and a complex Palaeozoic stress field. Varsican orogenic transpression and inversion was superimposed resulting in a variety of structural trapping styles and burial/uplift histories, and a complicated pre-Permian subcrop map.ud A widespread spatial and temporal extent of oil and gas mature source rock intervals within the Carboniferous succession particularly;udo lower Carboniferous (Visean) coals and mudstones of the Scremerston Formation, dominantly fluvio-deltaic and lacustrine with some marine influence, dominantly gas prone. Gas mature in Quadrant 41 and central-southern Quadrants 42-44 and oil mature in the Forth Approaches and North Dogger Basinudo lower-mid Carboniferous (Visean-Namurian) coals and mudstones of the Yoredale and Millstone Grit formations in fluvio-deltaic to marine cycles, gas prone with oil prone intervals. Gas mature in central Quadrant 41 and southern Quadrants 42-43, oil mature across northern Quadrants 41-44, Quadrant 36, 38 and 39.udo Lower-mid Carboniferous (Visean-Namurian) mudstones and siltstones of the Cleveland Group, over 1 km thick, deposited in dominantly marine environments. Gas mature to overmature in southern Quadrants 41-44 and modelled as having generated oil and gas.ud Potentially widespread reservoir intervals of varying reservoir quality. Favourable intervals include the Upper Devonian sandstone of the Buchan Formation expecially where fractured, channels within the fluvio-deltaic lower-mid Carboniferous (Visean-Namurian) Scremerston, Yoredale and Millstone Grit formations, the laterally extensive, high net:gross Fell Sandstone Formation, and possibly turbidites or shoreface sands within marine mudstones/siltstones in southern Quadrants 41-44 (likely tight gas unless early hydrocarbon charged)ud Widespread opportunities for structural (fault/fold/dip) traps utilising a Silverpit mudstone, or Zechstein evaporite seal as in the Breagh Field. Intraformational Carboniferous seals are documented widely in onshore Carboniferous fields and in some offshore fields and should be further investigated, particularly in mudstone/siltstone-dominated basinal successions with modelled Carboniferous and recent hydrocarbon generation, along with possibilities for stratigraphic traps.ud Basin modelling predicts oil and gas generation at a variety of times (Carboniferous, Mesozoic and Cenozoic dependent on the well) from lower-mid Carbonferous (Visean-Namurian) strata in Quadrants 41-44. In the Forth Approaches, Quadrant 29/North Dogger basins and on the poorly constrained Devonian-Carboniferous Mid North Sea High, oil window maturity levels are modelled at selected wells in a largely gas-prone sequence, though basinwards gas maturity may be achieved. It is recommended that the contribution and volumetrics of relatively thin oil-prone intervals within the Carboniferous succession be further investigated.