Stillatit, this pipe is probably a cleanout riser on the sewer pipe. I would recommend that you obtain an as-built plan of your septic system. The drawing of the system location, components, and dimensions may be on file with the on-site system regulatory agency in your area.
It is important to have the indigestible sludge removed from the septic tank on a regular basis (every 3 to 4 years) to insure the proper function of the system. It is impossible to do this if you do not know where the septic tank is located. Out of sight is out of mind, and if you don’t provide regular maintenance of your system, it is doomed to failure. I recommend that you install risers from the access holes in the top of the septic tank to the ground surface to facilitate easy access to the tank for this important maintenance function. JWA