Perfectly safe. Many oil fields have gas sitting on top of them either as virgin fields or develop gas caps over time if the reservoir pressure is not maintained on extraction of the liquids.
3D Surface Seismic once calibrated to depth and rock response can be used to provide an indication of hydrocarbon type and oil/water contacts but are not that reliable - otherwise there would be no exploration drilling and everything would be declared commercial on the seismic alone.
Did they stop early and if so why? Possible reasons - we know they set the 13 3/8" casing early. The main reasons for doing this are hole instability problems or mud losses - if that was the problem in this case then they may not have had the mud to continue. Or there could have been other technical difficulties with the rig. Another reason could be that they have hit all their main targets geologically speaking and drilling deeper would not add anymore to the picture at this point. I think this is more likely IMO.