6.0 Vortec - What is this missing piece?

[JR_Denali]

Hi there. I own a 2002 denali with the 6.0 that has recently been leaking lots of oil... A quick examination reveals what appears to be a missing piece in the block ... A bright silver threaded hole where something should go. I'm not sure if this has to do with my oil leakage or not - but any help to identify what belongs here would be appreciated...
Thank you

 

[powderbrake]

May I suggest that you post this in the 2002 section, you may get a better response.