Here are estimates of the present value (as of 2020) of Lebanese Govt tax revenue from future offshore gas projects under diff price & recoverable volume scenarios. Using best info available today ($65 oil, 25 tcf), offshore gas cld be worth $8-$22 billion in Govt revenue 
We won't know how much gas we have until drilling begins. Govt tax revenue based on the Exploration & Production Agmt for Blocks 4 & 9 (royalty, profit tax, profit petroleum) over a 30-year period. All other assumptions based on similar offshore gas projects being developed today
For non-finance nerds, present value is just a way to quantify the value of a future stream of cash flow (over 30-years for example) into a single dollar figure as of some fixed date (2020 for example). A dollar today is worth more than a dollar in 30 years.
While gas price and volume are the two most important sources of uncertainty in calculating the PV of govt tax revenue, there are many other challenges in the road ahead which could cause delays
There's this idea out there that's its too early to estimate Govt taxes from a potential future oil&gas project. It's important to have a quantitative basis for discussing this sector, so let me clarify. There are 3 unknowns that actually affect the results: price, volume, timing
Price will always be uncertain so you look at a range. Timing will always be uncertain because most projects experience development and construction delays. You have to assume something. Volume is the only one for which there'll certainty later on, so today we can look at a range
Using a PV calc (over a 30-yr period) is helpful b/c there will always always be uncertainty around tax revenue in any single year. Oil prices are volatile, production can shut down temporarily for various reasons. The PV calc helps you work around the timing & price volatility.
