User:Jschiff11/sandbox

Medina Group

Medina Group
Stratigraphic range: Early Silurian
This is a photo of the Medina Group outcrop in Upstate New York. Person for scale.
Type	Geologic group
Lithology
Primary	Sandstone
Other	Shale
Location
Region	Mid-Atlantic
Country	United States
Type section
Named for	Medina, New York

A map of the Medina Group depositional history in the Niagara Region of New York. This was used by R.R. Hettinger in a USGS article, in 2001.

Introduction

The Medina Group in New York State includes (ascending) the Whirlpool Sandstone Formation, Power Glen Shale Formation, Devils Hole Sandstone Formation, Grimsby Formation, Thorold Sandstone Formation, Cambria Shale Formation, and Kodak Sandstone Formation. The Medina is of early Silurian age and consists of 25 to 35 m of white, green, red, barren to moderately fossiliferous sandstone, siltstone, and shale. It overlies the Ordovician Queenston Formation and underlies the Silurian Clinton Group. The Medina Group occurs in West-Central New York state. The Power Glen to Thorold Formations extend to central Ohio and the Whirlpool Formation extends to eastern Ohio. The (ascending) Whirlpool, Power Glen, and Grimsby Formations extend to western and central Pennsylvania. The Medina group was named by Lardner Vanuxem in 1840. It was then renamed the Albion group in 1942 and then USGS returned the name to Medina in 1954.

Stratigraphy

The Medina group overlays the Queenston Formation, which was deposited as a deltaic unitduring the Ordovician. The basal stratigraphic unit of the in the Medina Group is the Whirlpool Formation, with a maximum thickness of 7.5 m. It is a very fine to coarse- grained white quartz sandstone with scattered clasts of shale and accessory minerals. There is also evidence of secondary silica as well as small areas of calcareous cement. The Power Glen Formation is deposited on top of the Whirlpool, with a maximum thickness of 10.5 m. This is a muddy shale with thin interbeds of grayish-white fine grained sandstone, with hummocky cross-stratification and bioturbation. It also contains interbedded dolomite, calcareous siltstone, and sandy limestone. On top of this is the Grimsby Formation, with a maximum thickness of over 30 m. The Grimsby consists of interbedded red and green sandstone, siltstone, and shale, and tends to be more of a silty mudstone toward the base. The basal 1.5 to 3 m of the Grimsby is typically intensely burrowed greenish-gray and maroon shale. Fossils are common in the basal beds and include pelecypods, cephalopods, ostracodes, bryozoans, and brachiopods. The remainder of the Grimsby consists of red and white mottled, fine- to medium-grained sandstone and conglomerate, with shale interbeds. The ratio of sandstone to shale increases upward to the contact with the overlying Thorold Member. Common sedimentary structures in the Grimsby Formation are crossbedding, ripple marks, and mud cracks. There are also minor thrust faults in the Formation that are thought to have occurred due to deformation in the late Paleozoic. The Thorold Formation, with a maximum thickness of 3 m, is a grayish-white, argillaceous, sanstone in the Niagara Region of New York and cross-bedded, channel sandstone with pink and red mottling and trace fossils to the East. This is overlain by the Cambria Formation, with a maximum thickness of 4 m. It consists of red to reddish-green, slightly fossiliferous, fine-grained sandstone with interbedded shale and siltstone. The lower portion is shale and contains an abundance of ostracodes and bivalves. The middle interval is mottled red and green, bioturbated, sandstone with thin shale interbeds. The upper interval is again shale that is similar to the lower interval. The uppermost stratigraphic unit within the Medina is the Kodak Member, with a maximum thickness of 3.5 m. It is white, argillaceous, quartz sandstone.

Depositional History

The Medina Group was deposited in fluvial, tidal, and marine environments. The lower part of the Medina Group is considered to be a sequence with lowstand fluvial and transgressive marine deposit and the upper part is a progradational marine and coastal plain deposit. This sediment was derived from an eastern and possibly a northeastern source. The lower part of the Whirlpool is characterized as being a braided stream system, as evident by the cross-stratified sandstone. This is overlain by a marine transgressive units in the upper part of the Whirlpool, Power Glen, and Devils Hole, which is evident from shallow shelf sands and deeper shelf muds. Although the Power Glen has sediment consistent with a deeper shelf and Devils Hole is consistent with a shallow shelf, these units were deposited in an overall marine transgression. The Grimsby coarsens upward, related to progradation of shoreface and marine tidal flat sand over marine mud. This is consistent with shallow shelf and tidal flat environments. This regressive interval extends into the Kodak, as evident from the channel sands and abundant marine fossils.

Economic Value

The Medina Group is currently developed for its natural gas reservoirs. The bulk of the gas is held in the Grimsby Formation where the bodies of sandstone are separated by thin beds of shale. The main gas zone in this formation is held in the upper section of sandstone. Since the sandstone in the Medina group has low porosity and low permeability, the gas in this reservoir rock is referred to as tight gas. This therefore requires hydraulic fracturing to make drilling economic. Due to the Federal Energy Regulatory Commission Tight Gas Sand program, there was extensive exploration of the Medina Group that took place in the 1980's. Peak production was reached the mid-1980s. In 2005, 5,223 active wells produced 8.5 Bcf, which accounted for 15 percent of New York’s annual gas production. A typical well from the Medina Group in New York produces 80 to 100 MMcf. It is estimated that there is more than 4 Tcf still undiscovered in the New York portion of the Medina group. The source bed for the Medina Group is the Utica Shale Formation.