My understanding is. It's cheaper to drill a basic hole to the...

My understanding is. It's cheaper to drill a basic hole to the target depth and find out what if nothing is there. If there is oil down there and once they know what grade it is they can drill a second hole and case it accordingly to suit testing/ extracting what they're going to get out.
No point spending more than they need to find out it's a duster. They won't have trouble raising capital for a second hole if it's neccessary.