Ann: Quarterly Activities/Appendix 5B Cash Flow Report, page-40

Petroleum assets are GCU (cash generating Units.
Under normal operating conditions, the value represents the remaining historical costs as you suggest, and it is not (generally) revised upwards to reflect the true value of the asset.
A few decades ago, accounting rules changed to require companies to value assets on their balance sheet at the lower of
a) (remaining value) of historical cost
b) NPV of future cashflows of each asset
OEL's Board revalued the future value of GC-21 p[roduction at $0.
This is a legal obligation.
You should read the Annual Report, where BDO, the Auditor, went into some detail regarding GC-21 write-off as at June 30th, 2023.
Perhaps, BDO thought that OEL should have gone further, and the Board stood their ground (their methodology could be defended). Hard to tell.

Fairly detailed information is in the Note of the Annual and Half Yearly: this supports what I am saying.