Controls on structural styles, basin development and petroleum prospectivity in the Midland Valley of Scotland

Integration of well-calibrated seismic data from the Firth of Forth with field and subsurface data from neighbouring onshore areas provides important new insights into the structural evolution and petroleum habitat in the Midland Valley of Scotland. The results demonstrate that the NNE–SSW striking Midlothian–Leven Syncline, and its flanking Burntisland and D’Arcy-Cousland anticlines, developed as major growth folds during the Late Palaeozoic under a predominantly dextral strike-slip regime. These syn-sedimentary folds controlled depositional thicknesses, facies and dispersal patterns in Early to Late Carboniferous (Viséan-Westphalian) times. Structural geometries suggest that the folds experienced tightening during Late Carboniferous (Variscan) deformation events before being transected by E-W striking dykes and extensional faulting during the Late Carboniferous (Westphalian D) and Early Permian. Although direct evidence for the basin's post-Permian burial history is no longer present, it can be inferred that the area had at least 1 km of cover prior to Paleogene uplift. The new understanding of the eastern Midland Valley's structural and subsidence history provides the means by which the occurrence of petroleum in the basin can be explained. Burial of Lower Carboniferous (Mississippian) oil-shale source rocks beneath the Midlothian–Leven Syncline now appears to have been the primary driver in the maturation and up-dip migration of waxy crude oil into age-equivalent clastic reservoirs located in the adjacent anticlines, rather than oil generation by direct contact between the source rocks and igneous intrusions. Initial oil charging is thought to have taken place during the Late Carboniferous, was renewed during Mesozoic reburial and appears to continue locally today despite the detrimental effect of significant Early Cenozoic uplift. It is now evident that the basin's limited exploration success has been almost entirely governed by the presence of unbreached palaeostructures lying directly up-dip from an oil kitchen, developed in the core of the Midlothian–Leven Syncline during Late Carboniferous and Mesozoic-Recent burial