You're right that it's rock salt, and while some of it is mined as coal/metal ore is mined, the salt can also be extracted by gently washing it out with water,

You're right that it's rock salt, and while some of it is mined as coal/metal ore is mined, the salt can also be extracted by gently washing it out with water, as is done near Freeport, TX. The oil doesn't do that because while salt dissolves readily into water, it does not dissolve in oil.