These are basically shale produced from oil shale mining, 19th century as well.

这些基本上是由19世纪开采产生的。

At the same time, the U.S., having edged closer to energy independence with natural gas and oil shale, no longer needed Riyadh the way it once did, and was noticeably less attentive.

与此同时，随着天然气和的开采，美国已经逐渐接近能独立，已经不再像过去那样需得了，对沙特也明显不那么在意了。

And that's a level that doesn't allow US oil shale producers to make a lot of profit and incentivise extra drilling for the new oil and gas reservoirs that they need to boost production.

而这个价格水平不足以让美国生产商获得大量润，也不足以激励他们为需增加产量的新气储层进行额外钻探。