From what I can tell, practically all stocks’ oil and gas reserves will have a Net Present Value of roughly 2-7 times the trailing twelve months’ cash flow. One relevant article is titled: “The Valuation of Oil and Gas Properties: Are They Really Worth 3x Cash Flow?“. For conventional wells, the 3X rule of thumb should come fairly close to the NPV in most cases.
I use this shortcut to get a quick sense of whether or not an E&P is massively overvalued. Currently I only short E&Ps and am not seeing any undervalued E&Ps even though these stocks have fallen a lot.