The Michigan Stray sand, from which most of Michigan's gas is produced, consists of sand bars formed on offshore shoals in a shallow Mississippian sea. The shoals that caught the bars are on pre-Mississippian anticlinal trends, the sea-floor topography having been determined partly by structure and partly by erosion during a previous period of emergence. Contour maps of the sand bodies are strikingly similar to those of present-day offshore bars. In a field where the pre-depositional sea-floor topography has been worked out, the sand bars were accurately modelled to the shoals that caused them, and the sand piled up behind a small subsea hill or island and built up on the top of a subsea ridge, and a smaller bar formed on a lower shoal across a narrow channel, through whi h passed enough current to keep the channel almost, but not quite, free from sand. Isopach and isopore maps show that the cleanest sand was deposited in the thickest parts of the main bar, with muddier sand on the lagoonward side. In another field, successive bars were built across a wide shoal area, each bar inshore from its predecessor, and the channels between them were partially filled to form the whole into a continuous sand body.

The sand bodies are of some magnitude and are important gas reservoirs. The largest so far explored is about 9 miles long and 3 wide, and originally held about 50 billion cubic feet of gas. Twenty-three of these shoestring gas fields have been found, with a total original gas content approximating 150 billion cubic feet. None of the shoestring reservoirs of Kansas and Oklahoma shows more clearly its origin as an offshore bar.