You do have a point...the only person who does in this thread.
You are right that there isn't only one format for logs. From personal experience only, it seems there are two kinds of logs (I learnt that during the time I made my own log viewer). In both types, the log file is a series of chunks of text separated by empty lines.A chunk of the first type, which is possibly created when you just check the "Logging..." item, is like this:
Session Start: <fulldate>
[color:blue]Session Ident: <channel/nickname>[/color]
[color:green]Session Ident: <channel/nickname> (<network>, <your nick>) (<address>)[/color]
<conversation>
.....
.....
<conversation>
Session Close: <fulldate>
<fulldate> is the full date, in the exact format returned by the $fulldate identifier.
<channel/nickname> is the channel name that's logged. If it's a query, it's the person's nick.
<address> is the nick's address (if you're logging a query), in the format "user@host.domain"
The second "Session Ident" line (green part) is often found in my logs. There may be logs that only have the blue line, others
that only have the green line and others that have both lines (most of my recent logs are of the 3rd type)
The other type of logs, which is probably what you get when you "save the buffer" (from the "Buffer..." menu item) is this:
Start of <channel/nickname> buffer: <fulldate>
[color:blue]Session Ident: <channel/nickname>[/color]
[color:green]Session Ident: <channel/nickname> (<network>, <your nick>) (<address>)[/color]
<conversation>
.....
.....
<conversation>
End of <channel/nickname> buffer <fulldate>
Notice the multiple spaces in the last line, it seems mirc does that to align the first <fulldate> with the second.
I hope this info helped, although I'm not sure it's 100% accurate or complete.