What's the password to the System keychain?
The login (user account) password isn't stored in keychain access, but keychain access automatically adopts the user account password as the keychain access password by default.

So, your password for keychain access is most likely the same as the login password. 

There is no default password in mac, the user creates one on install. If your mac was provided already installed, there was likely an assigned password created by the admin, perhaps associated with AD. If you reinstalled OSX, it would not have wiped the password, but if you formatted the harddrive, then reinstalled OSX, you would have created a new password.