key points.....
“What not everyone knows is that horizontal drilling techniques and hydraulic fracturing currently recover less than 20% of the oil in place, and in some places even less than that.
Total production costs have fallen from around $80 a barrel three years ago to around $50 to $60 a barrel today at the most advanced producers’ sites. As WTI prices continue to slide, wringing out production costs will become more important than ever. Energy consulting firm Bentek has said that even if oil prices fall to around $60 a barrel production will continue to grow.”
Production cost do not include exploration cost, ie, reserve replacement cost.
This may help explain the difference and why one cost is discussed separately from the other.