Is it okay to begin a chapter without an "introduction section"?
In general, sections should be organized to create a chapter just like subsections are organized to create a section and subsubsection should be organized to create a subsection.  In other words, the important unit is the largest one, and the smaller elements (sections, subsections, etc.) are used only the assist and guide the reader in their understanding of a large chunk of narrative.

As such, I would consider it the normal and preferred case to begin a chapter with prose, setting forth an introduction to the chapter before the first section heading.  In fact, a particularly short chapter might have no section headings at all, as it is a good rule of thumb to note use any textual subdivision until you need two of them (e.g., don't divide a chapter into one section, or a section into one subsection).