The evolution and hydrocarbon potential of the Northumberland–Solway Basin

Abstract

Seismic and well data establish the three-dimensional geometry and structural evolution of the Northumberland–Solway Basin. The basin-controlling southerly bounding normal faults, the E–W-trending Maryport–Stublick–Ninety Fathom fault system, formed in early Carboniferous times by extensional reactivation of a major basement thrust zone. The faults are sub-planar with a present-day throw (at the base of the Carboniferous) of <4 km. Other important Dinantian syn-sedimentary normal faults trend roughly NNE-SSW and suffered dominantly oblique-slip displacements. Extension and syn-sedimentary faulting accompanied deposition of the Lower and Middle Border groups. Thereafter, until late Westphalian times, sedimentation was mainly in response to regional thermal relaxation subsidence. The maximum thickness of Carboniferous rocks ranges from around 5000 m in the Northumberland Trough to about 7000 m in the Solway Basin. At the end of Carboniferous times, the basin was partially inverted by Variscan transpression, with preferential reversal of the NNE-trending faults and the development of major monoclinal and anticlinal folds. Locally, there is evidence of embryonic syn-depositional folding in late Namurian times. In the Solway area, a Permo-Triassic to Jurassic succession, locally over 1.5 km thick, rests with marked angular unconformity on the Carboniferous strata. The argillaceous rocks of the basin commonly have high TOC values but are dominantly gas prone. The reservoir potential of the interbedded limestones and sandstones is poor. Most strata at outcrop are in the oil window, but in some boreholes, or where the rocks have been metamorphosed by the Whin Sill, overmature values have been recorded. Peak hydrocarbon generation was in late Westphalian times, prior to inversion, although some post-Variscan generation is possible in the Solway area, where there are oil and gas shows.