Corrected entry: When the away team goes down to the planet, the oily blob blocks their path, like a moving moat, and Riker gets sucked into that. But later, the blob returns to covering the shuttle craft like it did before they arrived. But Riker is still inside it, he is released only later. So the moat is temporarily gone. Where is he supposed to be during the time that the black blob covers the shuttle?
Corrected entry: In the episode "Skin of Evil", right after Commander Riker gets pulled into the sludge type black substance watch the left side of the screen. When the away team looks into it after Riker's face comes out of the sludge you will see something plop into the goo, that was Geordi's phaser falling out of his holster when he leaned over the pool of goo.