The objective of this paper is to outline the issues that should be considered and provide guidance on the assessment of reserves and resources in a variety of scenarios where petroleum accumulations straddle licence boundaries.

Petroleum accumulations straddling boundaries are common. Global estimates of the numbers of fields which straddle boundaries vary but run into hundreds or thousands with several countries/basins reported to have up to half of discovered fields straddling at least one boundary.

It is widely recognised that development of oil and gas fields straddling boundaries can be optimised through unitisation. However, less attention has been paid to the assessment of reserves and resources in such situations and how the nature and status of unitisation, or lack thereof, impacts reserves and resources assessment using the Petroleum Resources Management System (PRMS). Even in ideal situations where unitisation has been agreed, the methods and objectives of unitisation and reserves assessment are quite different. Unitisation is typically: based on in-place volumes, considers a single deterministic interpretation, follows specified technical procedures, without consideration of economics and is done at a specified time during a field's life, and may be followed by subsequent redeterminations. Reserves are: remaining recoverable volumes, consider a range of outcomes, do not follow specific procedures, do consider economics and are typically assessed at regular intervals, often annually, throughout a field's life.

The paper will examine a range of scenarios from those where unitisation has been agreed, through those where unitisation is mandated but not yet in place, to those where no unitisation is either in place or mandated. Several factors influence how such situations should be assessed. The status of the field and data on either side of the boundary will be considered including: have wells been drilled on one or both sides? Is production occurring on one or both sides? What consideration should reserves assessors give to scheduled redeterminations where agreements, formulas and outcomes are unknown. There are many cases where production has occurred on both sides and unitisation is required retroactively. Regulatory requirements regarding unitisation vary throughout the world. Where present, regulations often require unitisation to occur, but the practice lags behind the legislation. There are also situations where production has occurred on one or both sides in "rule of capture" scenarios without any requirement or plan for unitisation. Accumulations straddling international boundaries introduce additional factors that should be considered.

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