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Map of Michigan Basin, USA, showing outcrop of Antrim Shale, and main area productive of natural gas in the Antrim Shale, from Ryder (1996) US Geological Survey.

Antrim Shale Play

The Antrim Shale Play is a Late Devonian black shale shallow gas play that lies in the northern portion of the Michigan Basin. Covering 39,000 sq. miles, it serves as the 13th largest producer of natural gas in the United States. As of 2009, it is estimated that between 35 and 76 TCF of natural gas is present in the play.

Michigan Basin

Map of the Great Lakes, showing the location of the Michigan Basin, from Wikipedia media database.

Basin Characteristics

The Michigan Basin is a shallow, interior cratonic basin that is believed to have been developed by the tectonic activity relating to the Appalachian orogeny. The oldest rocks are dated back to the Cambrian and the majority of the younger rocks are glacial deposits.

Subsidence History

Throughout its history, the basin has alternated through periods of significant subsidence activity and periods of little to no activity. Six different geological sequence patterns have been identified, with the Antrim Shale being located towards the bottom of the most recent sequence. This sequence of subsidence that contains the Antrim Shale lasted from the Late Devonian through the Mississippian, is categorized by it's eastward tilting, and contains some of the most enriched organic rocks in the basin.

Petroleum Geology

Stratigraphy/Lithology

Chart showing stratigraphic position of the Antrim Shale in the Michigan Basin, USA. Taken from USGS Open-File Report 96-23.

The Antrim Shale is located below the Ellsworth formation and Traverse formation of the Michigan Basin. Stretching across the lower peninsula of Michigan, the Shale is divided into four different members, Upper Antrim, Lachine (75-100 ft thick), Paxton (20-40 ft thick) and Norwood (15-25 ft thick). These members are located between 600 and 2200 feet deep and have an average thickness of 95 feet with a permeability of 9%.

Within this area, there are two different types of shales present, highly organic black shale and a low organic gray/green shale. The majority of the light gray/green shales can be found in the Upper Antrim and Paxton members, as indicated by their low gamma ray readings. The Norwood and Lachine members, on the other hand, represent the largest portions of black shale and contain very enriched organic material, comprised of up to 16 wt. % total organic content. This makes these two members far and away the most popular in terms of hydrocarbon production, as the black shale is the primary natural gas bearing zone.

Mineralogy

In addition to a significant content of organic material, the Antrim shale also has a surprisingly high quartz content, making up over half of the total mineral composition. The remaining composition consists of 20-35% illite, 5-15% kaolinite, 0-5% chlorite, and 0-5% pyrite.

Scheme of petroleum or natural gas extraction. Conventional vs. unconventional deposits. Taken from Wikipedia media database.

Hydrocarbons Source

Source

As previously stated, the source of hydrocarbons are the organic black shales located primarily in the Norwood and Lachine members. In regards to the reservoir rock, the Antrim shale is a unique because these rocks serve as both the source and the reservoir rock.

Hydrocarbon Migration

In conventional natural gas reservoirs, there is typically a considerable amount of hydrocarbon migration from your source to your reservoir. However, in this case, these black shales differ because there is little to no migration of the hydrocarbons so there is no separation between the source and reservoir rock The unique attributes of the shale and the shallow location of the gas make it one of the few major gas plays to require the use vertical drilling as opposed to horizontal.

Natural Fracturing

One of the most important reservoir characteristics of the Antrim Shale is the presence of significant natural fracturing. Primarily, fracture sets are oriented either northwest or northeast, and the majority of them are sub-vertical. One feature that stands out about the fractures networks in the Antrim shale is that they tend to be more common at shallow depths. This is believed to have been a result of the glaciation periods during the Pleistocene. The frost wedging and glacial scouring in northern Michigan during these times helped further expand previously present fractures.

The reason that these fractures are so significant is because they form channels for gas and water to being able to flow through the black shale from the borehole, which allows hydrocarbon migration and the subsequent concentration of gas. If it were not for these complex networks of fractures, this area would be otherwise very difficult to produce due to the low permeability of the shale. The fractures cause the shale to be more brittle, which provides an avenue for hydraulic fracturing techniques to be implemented. Therefore, commercial production is generally restricted to members of the Antrim that are significantly fractured, such as the Lachine and Norwood members.

Production History

Natural gas production in the Antrim Shale play began in the 1940's, with some of those wells still producing today. It was not until the 1980's, however, that production really began to take off. With the help of new completion and production technologies, wells could be produced more economically efficiently and this led to the peak of the Antrim production in the 1990s. Since the first 8 gas wells well drilled until today, over 9000 gas wells have been drilled, ranging from 150 to 2200 feet in depth.