If Luke was able to select from a wide variety of sources in putting together an accurate account of the earthly ministry of Jesus Christ, then it stands to reason that he also had access to Mark and other earlier material. He also would have had contact with direct eyewitnesses to the resurrection (1 Corinthians 15:6). One commentator put matters in this manner:
"1:1 draw up an account. In the Mediterranean world, disciples of teachers often committed the teacher’s lectures to writing; otherwise, others often did so (especially if the disciples were not very literate). Most scholars agree that at least one of the written works by Luke’s day was Mark; one of the many other sources might be a collection of material shared by Matthew and Luke (perhaps a collection of mostly sayings reported by Matthew). Sometimes students published their teacher’s sayings in ways that even reflected the teacher’s distinctive style. things that have been fulfilled among us. Reflects the kind of subject noted by ancient historians (rather than by other kinds of writers) in their prefaces."
Moreover, Luke's work demonstrates that the four gospel accounts are rooted in history. They are not fictional or mythological.