A regional geochemical study of the Slave Point Formation within an area of 35 000 square miles (~90 000 km2) in the subsurface of western Canada was made to relate chemical variation to facies change. Core and cuttings samples were obtained from 90 wells drilled for petroleum. The Slave Point Formation is a relatively uniform and pure calcitic limestone of Middle Devonian age. A reef facies, dolomitized in places, is developed along the margin of the carbonate shelf with a shale basin. These dolomites produce natural gas from several fields.R-mode factor analysis methods have been used to help interpret the element distribution. There are slight, but significant, chemical differences between limestones occurring close to dolomites and gas discoveries and limestones distant from these features. The principal differences are a smaller content of magnesium and strontium held in solid solution in the calcites and less clay minerals and pyrite in limestones occurring close to gas discoveries. These differences, which are related to dissimilar conditions during deposition and diagenesis, are used to form a multivariate discriminant function separating the two groups of limestone. This discriminant function is used to classify the different limestone sections. Sphalerite (with galena and quartz), a prominent feature of the unit, occurs principally in the dolomites along the margin of the shale basin.