Fracture patterns and their origin in the Upper Devonian Antrim Shale gas reservoir of the Michigan basin: A review

Robert T. Ryder

Open-File Report 96-23

UNUSUAL STRATIGRAPHIC FEATURES UNDERLYING THE ANTRIM SHALE GAS ACCUMULATION

Abrupt stratigraphic changes caused by facies changes, diagenesis, irregular erosion, and truncation in units underlying or equivalent to the Antrim Shale may have contributed to the development of flexures and fractures through differential compaction. Several unusual stratigraphic features in the vicinity of the Antrim Shale gas accumulation are identified here as possible causes of differential compaction in the Antrim Shale.

The oil- and gas-bearing Upper Silurian (Niagaran) pinnacle reef trend that crosses northern Michigan directly underlies large areas of the Antrim Shale gas accumulation. Decker and others (1992) suggest that differential compaction over the 400- to 500-ft-thick, 3,000- to 3,500-ft-diameter pinnacle reefs (Gill, 1979) may have created some of the flexures and fractures in the Antrim Shale. However, given that the pinnacle reefs are 4,500 to 5,000 ft beneath the Antrim Shale, this hypothesis seems rather improbable.

Halite units in the Upper Silurian Salina Group have been differentially leached along several margins of the Michigan basin and areas having the greatest dissolution are commonly the sites of salt-collapse in overlying carbonate units (Mesolella and Weaver, 1975). In northern Michigan, the lowermost salt beds of the Salina Group pinch out northwestward against a carbonate-shelf margin that borders the shoreward limit of the pinnacle-reef tract. The 150- to 200-ft-thick lower salt bed (A-1 evaporite) occupies the inter-reef areas whereas a slightly younger 150- to 300-ft-thick salt bed (A-2 evaporite) occupies inter-reef areas of the pinnacle reef tract as well as forming a continuous seal over the entire reef tract (Gill, 1979). To date there have been no reports of Salina Group salt dissolution in the pinnacle-reef trend of northern Michigan.

The Ellsworth Shale is a greenish gray shale that intertongues eastward with the upper member of the Antrim Shale (fig. 2). An isopach map by Fisher (1980) shows the Ellsworth Shale to be restricted to the west side of the Michigan basin and that its pinch-out edge trends northward through the Antrim Shale gas-producing trend in western Otsego County. Moreover, the thickest part of the Ellsworth Shale (greater than 900 ft ) is in adjoining Antrim County. Conceivably, fractures in the Antrim Shale may be concentrated along this facies change because of the contrast between the ductile Ellsworth Shale and the brittle Antrim Shale. This mechanism, however, cannot easily explain the well-fractured Antrim Shale east of Otsego County.

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