Oil & gas companies typically use a Net Asset Value (NAV) calculation to derive the value of all their reserves. As you mentioned, reserves are typically categorized as 1P, 2P, or 3P, representing roughly 90%, 50%, and 10% recovery probabilities, respectively. However, a number of geological and legal factors affect the exact classifications and probability estimates.
Oil & gas assets are split by business segment (upstream, refining/marketing, etc.) and then by geographic region, which may include specific countries or merely continents. The value of each asset is then adjusted individually for risk (i.e., uncertainty in actual recoverable
reserves) to arrive at Risked Value. This all sums up to Total Enterprise Value, which is finally adjusted for net debt, corporate expenses, etc.
The greatest variability in oil & gas modeling stems from estimating the Risked Value of both proven and probable/possible reserves; each firm and its jurisdiction will have their distinct guidance. On an actual deal, financial analysts would find much more information in production-sharing contracts (PSCs) and regulatory guidelines. PSCs contain all the restrictions and procedures on how a given government and resource company split profits, ranging from how to handle any production set aside to recoup the company's costs ("cost oil") to bonuses that come into effect past certain levels of production.