Seismic Duck is freeware by Arch D. Robison. The documentation in the download describes the distribution rules.
Your task is to find and extract oil from underground anticlines, curved "hills" of sandstone. You find these anticlines by sending sound waves into the ground and recording the echos on a seismogram. The anticlines have characteristic echos that hint where to drill for oil. The difficulty increases as you play. See how far you can get before going broke!
Here's a screenshot of the game in training mode:
The game comes with documentation that explains the basics of oil exploration and reflection seismology. Even children can figure it out.
Author's Games Page