Critical point missed.
Oil companies (and mining companies for that matter) do not throw effort at keeping their book value up with reserves. In fact they do the opposite. (Somehow. perhaps the idea that modern CEOs are best served by a high share price is translated into companies keeping the value of reserves high - but this is a thin argument).
Resources are subject to the three Ps - not two. proven, probable, possible (with a fourth as producing).
Not only do you need to be able to show that the oil or gas is there, but that you have a viable way of getting it. Actually currently producing is even better.
But, the economics of how oil and gas (and minerals) does not favour aggressive exploration for probable reserves. The way the system works is geared to effectively limit exploration to provide for just about the right speed of exploration to meet projected needs.
Oil companies do not get to explore for oil for free. Countries auction off exploration rights, and they do this is a highly controlled manner, in order to maximise the amount paid for these rights. Rights to explore time out. There is no value in buying them if you don't intend exploring. Indeed, usually you will forfeit the rights if you don't actively explore. Exploration isn't cheap. This is especially true now that the easy oil and gas has been found. 3D seismic can be astoundingly expensive. Many millions. Sometime a great many millions. Exploratory wells, especially in deep water are silly money. A deep sea exploration rig costs about $1million a day to run. The moment you purchase an exploration lease you are committed to your exploration programme.
Now, what if you find oil or gas? Well then you need to buy a production lease. And these are not going to be cheap. Secondly, the lease will involve a percentage cut for the country you are working in. Since this is a serious money making effort for the country, they are not interested in you sitting on your production lease. Once you have bought it you must start producing, or you may forfeit the lease.
The bottom line is that it is a significant liability to a company to have leases that they are not currently in the process of exploiting. The industries (both oil and gas, and minerals) have always operated this way. All the other economic arguments are a waste of oxygen. The bottom line is that the proven reserves that a company controls via its leases and the prospective reserves that are controlled via its exploration leases are balanced to address the expected demand within the lifetime of those reserves. To add new reserves that will not be added to this pipeline, but are somehow just a book asset is a significant and unwelcome cost, and all companies avoid it.