Middle Devonian Sylvania Sandstone, Michigan Basin, USA

The Middle Devonian Sylvania Sandstone in the Michigan Basin, United States, is noteworthy for its mixed, cherty dolomitic carbonate, limestone, and sandstone composition and excellent reservoir quality and fluid-flow properties in many areas of the basin. Substantial commercial brine production was initiated in the middle twentieth century, and liquid waste disposal continues today in the Sylvania Sandstone, although little or no hydrocarbon production is known from this unit. The Sylvania Sandstone pinches out to the south and west in the basin and overlies either the Bass Islands Group at the regional base Kaskaskia unconformity to the south or the Bois Blanc Formation, with which it is proposed to be in facies relationship to the northeast. The Sylvania Sandstone is overlain by and apparently interfingers with the Meldrum Member of the Amherstburg Formation throughout the basin. The Sylvania unit predominantly consists of siliciclastic rocks in the southeastern Michigan Basin and in outcrop in Ohio, but it is transitional to predominantly cherty carbonate in the northwest along a depositional hinge striking from southeast to northwest through the central basin. This hinge zone is dominated by mixed carbonate and siliciclastic strata deposited in normal-salinity, tidally influenced paralic and shallow-marine environments. Excellent reservoir quality is present in quartz sandstones and, especially, mixed sandy dolomite lithofacies in the central basin. Cherty facies may also possess significant porosity, but typically with low permeability. Limestone lithofacies are commonly non-reservoir-quality facies. Multiple high-frequency, low-magnitude relative sea-level cycles within the Sylvania Sandstone are suggested from regional stratigraphic analysis. The Sylvania Sandstone in the Michigan Basin is interpreted as a mixed carbonate and siliciclastic, basin-margin facies assemblage deposited in an overall transgressive systems tract above the base Kaskaskia unconformity and in conformable relationship with more basinal facies of the Bois Blanc Formation.