Complex sentence: Approximately 90% of North Sea oil fields are located in Scottish territorial waters, although at present the tax revenue generated from offshore sites are not counted within the nation or region nearest to it, but is instead allocated to the UK Continental Shelf.
Split into simpler sentences: Approximately 90% of North Sea oil fields are located in Scottish territorial waters. The tax revenue generated from offshore sites are not counted within the nation or region nearest to it, but is allocated to the UK Continental Shelf.