Drilling for Niagaran (Middle Silurian) pinnacle reefs in northern Michigan dominated the petroleum exploration scene in Michigan.for 19 consecutive years (1969-1987). By the end of 1986. cumulative production.from the northern reefs reached 268 MM brl (million barrels) of oil and 1.4 Tcf (trillion cubic feet) of gas. This study presents quantitative and graphic summaries of several attributes of exploration and discovery data for this period, which together provide a concise ＇＇portrait＇＇ of the exploration history of the play. Data subsequently available confirm these trends for more recent years. The success ratio of exploration wildcats peaked at 51-53% during the years 1971-73, after a ＇＇learning period＇＇ of three years. Thereafter, it declined gradually to around 16%, which is similar to the average of the US oil industry in the last decade. The success ratio for development drilling peaked at 68% during 1971-2, and it has fluctuated between 40 and 50% thereafter. The frequency distribution of field sizes is log-normal. However, since very small uneconomic or marginally-economic prospects and fields have either not been drilled or developed, the observed distribution represents only the density distribution of economic fields. The reef fields are rather small. Reef-substrate areas range from about 40 to about 840 acres, and recoverable reserves range from 30,000 to 22 MM brl of oil equivalent (o.e.). The ten largest fields in the play were found by drilling the first 25% of the exploration wells. The discovery per wildcat peaked at 1.55 MM brls o.e. after 500 wildcats, and declined steadily thereafter to a level of 0.02 MM brls o.e. per well after 1,900 wildcats, indicating that the play had reached a state of maturity. About 25% of the discovered reserves were found in 5% of the fields, indicating a more even reserve distribution patten than in many other plays. Consequently, the relative rate of growth of the discovered reserves has been slower than in other plays.