Geology of Peters Reef, St. Clair County, Michigan

Abstract

The Silurian reefs of the Michigan basin occur in the tectonic hinge belt of the basin. They were discovered by recent drilling in southeastern Michigan and southwestern Ontario. The probable presence of such reefs can be predicted by a detailed study of regional geology, paleoecology, and depositional history. The Peters Reef in the Engadine Dolomite is one of a series of patch reefs arranged in a circular row, and grew in the southeastern marginal area of the Michigan basin. It is formed primarily of organisms which grew in place, in addition to a bewildering mixture of detrital, chemical, and biochemical deposits. The Peters Reef grew in a subtropical, humid climate, and its growth was stopped intermittently by the receding waters and increased concentration of salt in the basin. An intricate balance of influx of fresh sea water, evaporation, and sinking of the basin triggered and maintained deposition of evaporite deposits around the reef. The anhydrite deposits, generally found near the marginal areas, were precipitated in warm shallow water where evaporation was greatest. This increased the salinity of the water with the result that it could not hold the sulfate ions. Correlation between the reef dolomite and the surrounding strata is difficult. The destruction of identifiable fossils by subsequent dolomitization complicates the task. The Peters Reef is completely dolomitized. Magnesium-rich water circulating through the reef was the main factor in this post-depositional alteration. Dolomitization did not increase porosity appreciably in this particular reef. Leaching, which wa the main cause of the Peters Reef porosity, took place during the erosion of the reef. Fractures also add to the porosity. The known extent of the Peters Reef is about 700 acres; it is being drilled actively. Total initial oil production amounted to more than 2,500 barrels per day. The only source rocks adequate to provide the hydrocarbons that fill the scattered reefs in the Silurian section are the underlying shales, the Clinton and Cabot Head, of Middle and Early Silurian age, and the A-1 dolomite (Cycle II carbonate). During compaction of the shales, the hydrocarbons were squeezed into the overlying Engadine Dolomite, through which they passed into reef traps. The ultimate and closest available seals are anhydrites in the overlying Salina Group of Late Silurian age.