While the upstream mega deals in the U.S. shale patch have been drawing the most market attention, pipeline operators are also embarking on a merger spree in a quest to add scale, optimize assets, and gain more exposure to export markets. The next big midstream deal could be one involving a major U.S. oil and gas producer that has recently announced a large upstream acquisition to add acreage and scale in the top-producing shale basin, the Permian. Occidental Petroleum, which has recently announced a $12-billion deal to buy CrownRock, is now considering…