SemGroup Corporation (NYSE:SEMG) is working with an adviser on options to raise capital, according to people familiar with the matter, as much of the public equity market remains effectively closed to the so-called midstream sector. SemGroup has hired an adviser to look at options that could include project joint ventures, non-core asset sales and preferential equity offerings, said the people, who asked not to be named because the discussions aren’t public. Tom Droege, a spokesman declined to comment on specific efforts by Tulsa, Oklahoma-based SemGroup.