OT21565 | Investment Allowance: Meaning of 'qualifying oil field'
From HM Revenue & Customs · Oil Taxation Manual
CTA10\S332B
A qualifying oil field is any oil field which is not wholly or partly included in a cluster area. Expenditure incurred in relation to an oil field which is part of a cluster area may generate cluster area allowance. Guidance on cluster area allowance can be found at OT21590.