The SESSION statement option is unique because it does
 not generate an audit record when a particular type of 
statement is issued; this option generates a single audit record
 for each session created by connections to an instance. An
 audit record is inserted into the audit trail at connect time 
and updated at disconnect time. Cumulative information about a 
session such as connection time, disconnection time, logical and 
physical I/Os processed, and more is stored in a single audit 
record that corresponds to the session.
To audit all successful and unsuccessful connections to and 
disconnections from the database, regardless of user, BY SESSION
(the default and only value for this option), enter the 
following statement:

AUDIT SESSION;