Event log permissions while impersonating

[Guest]

Hi,
I'm trying to write to the event log while in
WindowsIdentity.Impersonate code segment.
I get this exception:
"A first chance exception of type 'System.InvalidOperationException'
occurred in system.dll
Additional information: Cannot open log for source {0}. You may not
have write access."
It is a Asp.Net 1.1 application configured with <identity
impersonate="false" /> and anonymous access only in the IIS.
The service account is granted TCB (Act as part of OS) privilege.
The event source was created manually in the registry.
Running ProcMon.exe (sysinternals) reveals no access denied events.

Any ideas what are the permissions required to write events to the
event log?

Thanks,
Yossi