So, mostly abiotic. The next question is whether new methane is still being generated? If so, then it is a renewable energy source.
Time and migration are your enemy. It would need to migrate into some sort of trap for it to be easily produced. I can go into any river in Colorado or California and find a few flakes of gold. But unless it accumulates into something of significance, it is worthless. Igneous and metamorphic rocks by their very nature are imporous and impermeable. They have no storage capacity.
Either way, there is no shortage of methane. We have discovered huge quantities in these shale plays. But with low gas prices, they are not commercial. What we need is oil to make gasoline. And it ain't coming out of the mantle...