MEXICO CITY/HOUSTON, Jan 20 (Reuters) - Royal Dutch Shell (RDSa.L) will supply crude to the 340,000-barrel-per-day Deer Park refinery for at least 15 years, as part of the sale of its stake in the Texas facility to Mexico"s Pemex, according to a source and document on the transaction. The handover, expected to happen on Thursday, will also include a labor agreement allowing Pemex to absorb most of the refinery"s employees for two years, the source and the document said.