Most of a 450 foot (150 m), gas-bearing Niagaran reef was cored by the Shell, St. Union 1-8. The well is located approximately in the center of the pinnacle reef belt (Fig 1). Three distinct facies described by Sears and Lucia (1979) are present in the core. The facies are, in ascending order, mud mound, coral-stroinatoporoid, and restricted marine.

Crinoid bryozoa mudstone-wackestone (Fig. 2) which contains stromatactis-like vugs is typical of the mud mound facies. A more diverse fauna including corals and stromatoporoids (Figs. 3 and 4) colonized the mound as it accreted upward from deeper water. Immediately prior to post-pinnacle evaporite deposition in the Michigan Basin a restricted fauna, mostly comprised of pentamerid brachiopods, inhabited the reef.