I believe they most likely did have security; however, CW was an employee, so he had access to the oil fields. I am imagining a badge or something like that he could scan to gain entry.
As a field operator for Anadarko, his job responsibilities were to "perform operational duties in accordance with operating procedures and guidelines, including executing start up, shut down, and maintenance of field equipment, performing troubleshooting, repairs, inspections, and replacements of equipment as necessary..." (from Anadarko website, job description). Basically he checked on the oil wells, so perhaps nothing strange at all about him being out there, on the surface.
Per the affidavit, police knew which site he had been at that morning BEFORE he gave them that information. I'm guessing that is because he had to gain entry through some kind of security measures, such as an employee ID/badge.