Now beginning with a high-level overview of our financial performance. Distributable cash flow was $91.2 million during the second quarter, an increase of 10% compared to the second quarter of 2011. Adjusted EBITDA was $130.5 million, and segment operating income was $146.3 million. In July, we announced a second quarter distribution of $0.80 per common unit, an increase of over 14% compared to the second quarter of 2011 while maintaining a distribution coverage ratio of 1.03x.
Now moving to the operational update, we continue to focus on expansion projects in rich gas resource plays, and throughout our discussion today, you'll hear how gas volumes continue to grow in our core areas. So let me begin with our Southwest business unit, which includes Texas and Oklahoma, and contributed 50% of our total segment operating income during the second quarter.
In Western Oklahoma, which includes both our Foss Lake and Granite Wash operations, gathered volumes increased 13% and processed volumes increased almost 40% when compared to the prior-year quarter. Primarily, this was driven by strong performance of our producer customers in the Granite Wash. This increase was supported by the recent expansion of our Arapaho plant in Oklahoma, which is operating near capacity.
In Southeast Oklahoma, gathered volumes remain strong at more than 500 million cubic feet per day. And while gathered volumes were generally flat compared to the first quarter, processed volumes were nearly 120 million cubic feet per day, an 18% increase from the first quarter, and a 9% increase from the second quarter of last year. This significant increase is the result of the recent drilling success of our producer customers including PetroQuest Energy. We expect processed volumes in both Western and Southeast Oklahoma to continue to grow as rich gas wells are brought online and offset the decline of dry gas production in those areas.