it more than likely is your rear main seal.
check if you're leaking from the back of the head too. your half moons or rocker seals could be worn too.
to get to your rear main seal you'll need to take your gearbox off.
the rear main seal sits in the rear main seal housing, not all cars have this housing but this is how it was on my car (rb25det).
i had to undo bolts off the sump then some 10mm bolts holding the housing in. you wont need to undo your sump completley, those four or so bolts should do.
make sure you use some gasket maker like ultra blue or ultra black when putting the new main seal in, one of them is the higher oil resistant one, so use that. use some more on the bottom of the housing to seal it between the sump.
the main seal cost me around $20 with the sau discount from nissan.