Here's why:

* Sedimentary rocks are formed from the accumulation of sediments like sand, silt, and organic matter. These layers build up over millions of years, compressing the organic matter.

* Organic matter (like ancient plants and animals) decomposes under heat and pressure, releasing hydrocarbons, including natural gas.

* Porous and permeable rocks: These layers allow for the accumulation and storage of natural gas. These rocks often contain layers of shale, sandstone, and limestone.

So, while natural gas is not technically found *in* a specific layer, it's formed and trapped within the sedimentary layer of the Earth.