I think it means basically just a map of the field, drawn up by the state, "utilising the geological information that is available to it at that time."

I think you are on the right track, actually, now you can re-determine fields based on economical grounds.

2. Prior to May 2008, fields had been determined only on the basis of geological criteria. In May 2008, the Department for Business, Enterprise & Regulatory Reform - now succeeded by the Department of Energy & Climate Change (DECC) - and HM Treasury (HMT) agreed that the Secretary of State may determine a currently unexploited part of a previously determined PRT-liable field to be a new, separate field which would be "non-taxable" i.e. exempt from PRT. Simultaneously, the boundaries of the existing PRT-liable field would be reduced. In such cases there could, therefore, be more than one oil field determined in a single petroleum geological structure. For the purposes of unitisations the combination of determined fields in a single petroleum geological structure would be taken to be a single field.

3. It is up to licensees to make a case for a change of field determination on economic grounds. Any such case will be scrutinised by DECC, Treasury and HMRC officials to ensure there is a genuine case for change and that it satisfies economic, geological and wider fiscal requirements. The decision to make a change of field determination on economic grounds shall be at the discretion of the Secretary of State if he considers it expedient to do so for the purpose of facilitating or maintaining the development of the petroleum resources of the United Kingdom or if he considers that it is in the national interest in order to secure the maximum ultimate recovery of petroleum. Such a change would need to be approved by both DECC and Treasury Ministers.