1. There is a seal mark on the piece of paper,for I pressed the seal on the piece of paper.

2. There is a seal on the piece of paper,for I pressed the seal on the piece of paper.

(Which one is correct? Do I have to use seal or seal mark about the mark of the paper?)

The second is better than the first. You don't need to use the word "mark."

(And don't forget to put a space after the comma!)