British Oil and Gas giant Shell Plc. (NYSE:SHEL) needs an exploration breakthrough or a big merger after its oil reserves fell to the lowest levels since 2013, exposing the company to a production shortfall in less than a decade. Shell's so-called 'reserve life'--denoting how long its proven reserves can sustain production at current levels– has dropped to less than 8 years, significantly lower compared with Exxon (NYSE:XOM) and TotalEnergies (NYSE:TTE), each with reserve lives exceeding 12 years. Shell is now facing a production shortfall…