btw let me now miss lead you.
NOT EVERY single well had those rocks.
The ones in the middle of the fields is were you would see those type of filler rocks on a pad. The ground was just soft and when it rained you wouldnt be able to get in and out with out some support under you.
You wouldn't see those rocks unless you were a belly dump driver bringing those in before the top layer was put in or if you were like me and they sent you out to wells to check the place out and take pics for training dept and saftey...They used those pics for office folks to understand what the field is actually like. Since they would sit inside all the time. I ran to a well when they were cutting the ground and watched what was going on and thats were I say those size rocks. That rock is an abnormality its so large. The majority would be basketball sizes.
Lots of pads at this crushed red clay type rocks. Not sure the exact type. But when those got crushed under weight and rain they would turn to soft Georgia type clay. An absolute nightmare to drive over when wet and soggy. You would be chaining in the middle of Summer after a rain storm....fun times