Which means that any speculation right now is likely to be proved to be just guess work but nothing ventured…
I suspect that the seismic data was so noisy that the true nature of the fractured granite could only be confirmed by actually drilling it. When they did, the fractures found were either not well interconnected with each other and/or the porosity of the granite was low i.e. not enough minor cracking.
However the other possibilities are that there was either not enough source rock to provide the oil, or not a trap with a good enough impermeable cap to prevent the hydrocarbons leaking to the surface. In other words, FB is not a license to print money; it is difficult to work out what might be down there on the basis of the poor 3D seismic surveys but expensive to drill speculatively. Every successful well has to support not only its own overhead costs but a share of the cost of the unsuccessful drills.
Dr T had to be an optimist to even consider FB in the UK; Lancaster gave him encouragement, Halifax confirmed that the resource was vast, so perhaps Warwick Deep will bring a dose of realism. There is oil in the FB, but only in places where there is a lucky combination of source rock, well fractured granite moved to a position above the source, and a stratigraphic trap below an impermiable capping layer.
WD confirmed the presence of hydrocarbons but the crestal drill has the best chance of finding a decent oil