Bhai jaan, I am no Statistician. I am just putting down my analysis using factors that looks fit to me. 
Anyhow, when we predict, it is called probability or likely chance of something to happen, that can never be certainty. Anyhow, prediction can be ‘speculation without any reason’ and that is questionable. When there are valid reasons for speculation than even though there is no certainty, one can say that speculation is fair.
Price of any commodity: Depends on supply, demand, and alternatives.
For Oil:
1: Supply is increasing and chances are that it would keep increasing due to new discoveries, as well as technology to harness Shale oil and Sand oil is getting cheaper, plus Iranian oil would enter the market too.
2: Alternative energies (Solar, wind, gas) is getting cheaper and use is
getting widely spread.
3: Scientific advances has made alternative energy cheaper, plus green house effect is making countries give incentives to alternative energy, means lower demand for oil. Harness of energy from water flow, that is cheapest way to get electricity is going to get more popular with time.
4: Use of energy is becoming efficient.
4a: Oil is used in producing electricity and today turbine is becoming more efficient, hence less oil could produce same amount of electricity.
4b: One of the major use of oil is running vehicles. Today, vehicles are becoming more efficient. Car giving ‘miles per gallon’ today is almost twice as much as it was for similar ‘cc’ car built 15 years ago.
4c: One can see increasing hybrid cars, that uses much less oil than non-hybrid cars.
etc …
All that means, not only increase of oil production due to shale and sand oils in market, new oil production from existing wells and newly found wells … but at the same time efficient use (higher percentage conversion rate from oil to needed energy) is making demand of oil lower.
Since efficiency in use of oil is increasing fast and would keep increasing, plus use of alternative energy increasing, especially renewable energy (nuclear, solar, wind, water, etc), I believe that demand of oil in future would decrease further and all that is bad news for oil price.
A lot also depends on growth rate of China and India, but that is stagnating anyhow. Another factor was how oil producing countries would cooperate with each other, and that is unlikely too.
Further, new developments in energy sector has put ‘price lid’ over oil price, that is cost of alternative energy to replace oil, and value of that ‘price lid’ is coming down fast with time, as technology is getting better.
So … Looking at all these factors (plus few more I am avoiding to keep things short), I believe that it would be an achievement if oil would ever reach $50 a barrel. I believe, present price lid to keep shale and sand oil at bay is around $50 a barrel, that could go down with time. I also believe that USA would keep producing shale oil to stay self-sufficient in oil, so government may subsidise this sector, that would eventually going to make harnessing ‘shale oil’ more cheaper). In my opinion, $20 to $30 is reasonable stabilising price.
[Waisay … I could be wrong too. :)]