When the well reaches a pool, oil usually rises up the well because of its density difference with water beneath it or because of the pressure of expanding gas trapped above it.

当井和一个水池相接时，油通常会从井里上升，因为它与井下的水的密度不同，或者因为停留在它上方的膨胀气体的压力。

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That breaks up shale formations, releasing the gas trapped inside so it can be pumped to the surface.

这会破坏页岩层的构造，致使储藏其间的气体被挤出地表。

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When this happens, it can cause the removal of the hydrate layer, releasing the methane gas trapped below.

当发生这种情况时，它会引起的水合物层的消失，被困在下面的甲烷气体便释放出来。

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Companies are developing gas supplies trapped in shale rock two to three thousand meters underground.

VOA: special.2009.10.23

Shale gas is natural gas trapped within shale rock formations deeper under ground.

FORBES: China Closer To Joining Shale Gas Fracking Craze

China has some of the largest reserves in the world of shale gas, which is gas trapped in rock formations.

BBC: Shell signs first shale gas production pact in China

However, the study warned that further research is needed to determine the long-term impacts of the natural gas hydrates -- essentially methane gas trapped in ice.

CNN: Study: Tap natural gas from Alaska's frozen areas