UKOGL resources, our models and re-interpretations have led us to conclude that Carboniferous structure interpretations to date for onshore UK have important shortfalls, for two main reasons:
Firstly, for decades the Lower Carboniferous has been recognised as a post-orogenic rift sequence. That means major extension, and the Lower Carboniferous sequence is going to be characterised by numerous, large growth faults. But the mainstream published mapping and the reports filed by the various operators since the 1980s, don’t reflect this. Just one publication does: by far the most important contribution is the 2011 British Geological Survey memoir “Structure and Evolution of the East Midlands region of the Pennine Basin”, covering tectonics and sedimentary history of East Midlands, this has an intra Lower Carboniferous regional map. Its purpose is to be a reference for new exploration models, in which it succeeds and we strongly recommend review of that work.
In fact most operators didn’t map below the top of Dinantian, at all. There are maps published showing main structural trends, but the detail needed for exploration target definition just isn’t there. Definition of structural styles developed in extension and modified by subsequent basin inversion, has been at best limited, and often poor. It is clear that potentially large footwall traps remain undrilled. Footwall modification by Variscan inversion faults, which are commonly imaged on seismic but never mapped, is a critically important process to trap formation, leaving the rift-phase Lower Carboniferous as prime acreage to focus on for gas. Several large structures which have been tested unsuccessfully to date were poorly mapped, and they remain attractive targets for re-evaluation and further drilling. They include significant oil plays.
Secondly, adding to the structure opportunity, footwall collapse on inversion created natural fracture systems. These, particularly with karst development on platform margins, mean that the reservoir situation is much more encouraging for Lower Carboniferous than previous accounts would suggest. And the onshore Lower Carboniferous has an important, untested hydrocarbon system for gas. In addition to the documented Upper and Lower Hodder oil and gas source rocks widespread in base Namurian and Mississippian, major volumes of gas source rocks exist in Tournaisian successions, too.
Our conclusion is, explore the Lower Carboniferous. If it makes sense to explore in the southern gas basin, why not drill similar structures onshore?
Having shown the basic approach we’ve taken to onshore northern basin exploration possibilities, we should like to invite persons interested in purchasing our report to contact us.