Apparently you can do this using MS Logparser to extract the relevant information from your IIS logs.
Download MS Log Parser 2.2 and install - download from: http://www.microsoft.com/en-us/download/details.aspx?id=24659
Grab your log files from C:\inetpub\logs\LogFiles\W3SVC1, copy them to C:\log\ and merge them into one big log file using
logparser.exe -i:iisw3c “select * into c:\log\mergedlog\merge.log from c:\log\*” -o:csv
Get OWA logins using
LogParser -i:csv “SELECT cs-username, date, time, c-ip, cs-uri-stem, cs(User-Agent) FROM C:\log\mergedlog\merge.log TO C:\log\Output.csv WHERE cs-method LIKE ‘%get%’ and cs-uri-stem LIKE ‘%owa%’
Source: http://myriadofthings.com/outlook-web-access-owa-and-activesync-reporting-using-iis-logs/