He's probably buried in a National Cemetery - Chalmette, Port Hudson, Alexandria, or Vicksburg would be the most likely. An attempt was made after the war to move all of the dead Union soldiers from their graves where they were originally buried to National cemeteries, and those mentioned above are the most likely to have received soldiers from Louisiana. If you want to narrow it down, you would need to find out what unit your ancestor served in, and that would give you a pretty good indication of where he was when he died, and from that the nearest of the above cemeteries that he would most likely have been moved to. Be aware that the vast majority of soldiers were buried as "Unknown." Between original burial and re-interment, grave markers rotted away, got knocked down, were never put up, poor record keeping, etc etc. So although you may not find his actual grave, if you find his unit you can probably get a good idea of where he is, to within a few hundred yards or so. And you may get lucky and actually find him. But you need his unit.