My personal opinion is that it was the latter... A more or less "straight" historical account.

This is not to say that there was no oral tradition prior to the writing of Genesis, but tradition is a mixed bag. I personally think that while the Genesis account may contain elements of oral tradition, and may have "literary" and "story" elements to it, the core of the presentation is historical.

Genesis is most definitely not apocalyptic in nature as are, for example, Isaiah and Revelation.