According to various genealogies including the Sonpi bunmyaku (Bloodlines of Noble and Base) and Utsunomiya Keizu (literally, the genealogy of the Utsunomiya clan), Soen was on the lineage of Kanpaku (Chancellor) FUJIWARA no Michikane of the Fujiwara Hokke (the Northern House of the Fujiwara clan) (i.e. a descendant of FUJIWARA no Michikane), and was allegedly the second son of FUJIWARA no Kanefusa (Chugu no suke) who was Michikane's grandson.
尊卑分脈や宇都宮系図等の各種系図上では、藤原氏北家の関白藤原道兼の流れを汲み（道兼流）、道兼の孫である藤原兼房(中宮亮)の次男とされる。