That’s Ok noutram. I’ve been a petroleum geologist for almost 34 years and I’m sure I understand them either. I can offer a couple of points though. Enhanced recovery techniques do require a minimum mass to be practical. Very small fields might not have such measures taken. But I suspect the real basis for this “relationship” is that mega fields are just so big that there are an insufficient number of wells to deplete them as fast as smaller fields.

But the decline rate of any field is a function of the energy driving the oil to the producing wells. Two main types of drive: water drive (water pushing out the oil that floats on top of it) and pressure depletion (like shaking warm bottle of soda and cracking it open real fast). In water drive reservoirs like Ghawar the KSA has been pumping billions of barrels of water into it for decades in order to improve recovery and maintain high flow rates. Thus a field may show little of no decline for many years. But eventually the water level will reach the perforations in the producing wells and a very rapid decline will begin. This is the natural process and will occur whether a field produces 1 million bo or 10 billion bo. That’s not just my opinion: its physical law taught in every reservoir engineering school in the world.

And it happened in another mega field (Cantarell in Mexico). A huge pressure depletion reservoir which maintained a high flow rate thanks to PEMEX pumping huge amounts of nitrogen into it. But it is now showing a 35%+ decline rate according to PEMEX. And that’s how the late stage depletion grows in all such projects regardless of the original reserve size.