To anybody else, the shallow circle might have looked like one more patch of boring sedimentary rock.

For 'shale' read 'fracking', hydraulic fracturing, firing highpressured jets of water into small cracks, or fractures, in underground rock formations — generally shale, a soft, mud-based sedimentary rock — to extract gas.

You need sedimentary rock, such as sandstone or slate: something soft enough to have been ground into an appropriate shape by the sea.

Instead, it lies trapped within a type of organic-rich, porous sedimentary rock.

These deposits probably consist of both sedimentary rock and poorly indurated or unconsolidated sediments.