The Marine pool, Madison County, Illinois, was discovered in 1943. It yielded the first known production of oil from a Silurian reef in Illinois. The principal producing zone is a coquina-like detrital limestone which forms the mantling deposit of a Niagaran reef.

The reef is horseshoe-shaped with a subsidiary fore-reef belt east of the main reef. There is 120 feet of closure over the reef area. The reef topography, although reflected in the structure of the post-Silurian strata, is less pronounced in successively higher beds. The Ordovician conforms to the regional dip in the two deep tests that were drilled through the flanks of the reef. The Marine pool structure is, therefore, interpreted as due to the local increase in thickness and to the rigid unyielding frame of the Niagaran reef deposits in the surrounding compactable silty and argillaceous normal extra-reef strata.

As many as four porous discontinuous streaks have been reported in the principal producing zone of the Silurian limestone that caps the reef. The oil production appears to be out of proportion with respect to the storage capacity of the discontinuous streaks. Secondary porosity zones in the wall-rock adjacent to sand-filled and clay-filled fissures indicate a porous network that connects the discontinuous streaks and extends into the underlying reef core.

The solution-enlarged fissures extend in great numbers from the post-Wapsipinicon unconformity of Middle Devonian age, downward into the Silurian deposits. Devonian production of a few wells over the southern margins of the Silurian reef is best explained as fissure production of Silurian oil.

The stratigraphy of the Silurian reservoir rocks and of the reef-capping Devonian limestones is presented in detail.

By January 1, 1947, there were 108 producing wells in the pool, with an average daily production of 3,200 barrels and a cumulative production of 2,528,000 barrels. The pool is still in the development stage.