Ya' think? And just where were you planning on looking? Deeper? I guess the first two things that come to mind when considering that are: porosity (the bubbles in the Swiss cheese) and permeability (how well said bubbles communicate with each other). In order for there to be oil and gas in a layer of rock that CAN be recovered, you have to have ENOUGH porosity and permeability. The... uhh... deeper you go, the greater the pressure and, therefore, the less actual open volume (bubbles) there is in said rock. That the temperature of the actual rock actually goes up significantly as you go closer to the core plays in with the actual density and, therefore, those two aspects of rock. That said, if you're going to be seeing any appreciable evidence of ongoing abiotic methane production, you'd be more likely to see it aplenty near active fault lines like the San Andreas. If that's not working out for ya' then you'd be better to give up on the idea and try to think of better ways to spend what energy we have left
as we descend into the Olduvai Gorge.