The Devonian aged Antrim Shale is an important gas zone in the Michigan Basin. The gas production is derived from organically rich, fractured black shales within the Antrim formation. The Lachine and the Norwood are the principle gas bearing zones. On a per well basis, these two zones collectively average 110 feet in thickness.
Antrim Shale, Fractured Shale, Mudstone, well log analysis
The first Antrim gas production in northern Michigan dates back to 1940. During the 1940's, 8 gas wells were drilled south of Gaylord in central Otsego County. Five of these original wells are still producing today. However, the Antrim gas play did not gain acceptance as a viable drilling objective until the 1980's. It was during the 1970's and 80's that important advancements in both completion and production technology were made in the oil and gas industry. The Antrim shale benefited greatly from these new methods. These technologies made it possible to economically produce gas from the unconventional reservoir. The productive capacity of the shale reservoir continues to expand even today with more recent improvements in drilling, completion and production design. The initiation of the section 29 tax credits during the mid 1980's was also important in spurring drilling activity during this period.
To date, the productive Antrim gas trend extends east to west (from Alpena to Manistee) across the northern part of the lower peninsula of Michigan. The productive fairway is located immediately inside the Antrim subcrop belt. As of the end of 2003 there have been 7,700 gas wells drilled throughout the trend. Depths of the Lachine/Norwood in these productive wells range from 150 feet to 1500 feet. Cumulative gas production is 1.8 TCF. Entering 2004, the Antrim continues to be the main drilling objective in the Michigan Basin.
The main reservoir characteristic defining the Antrim producing trend is natural fracturing. The Antrim fracturing is generally most prevalent proximal to the subcrop belt where the Antrim is relatively shallow in depth. In the deeper parts of the Michigan basin, the shale is not fractured as a rule. The primary origin of the fracturing is thought to be the result of frost wedging during the Pleistocene glaciation periods. So it is logical that the superior natural fracture network would exist near the subcrop where fresh water could permeate into the shallow Lachine and Norwood zones. Another important geological feature persistent throughout the producing Antrim trend are paleo-channels which downcut into the bedrock overlying the Antrim. These paleo-channels are considered to be the result of surface erosion during the Jurassic period. Locally, they can downcut thru the overlying bedrock and into the Antrim and therefore be a likely source of fresh water recharge. The Antrim Shale is a self-sourced unconventional gas reservoir. The shale produces dominantly methane (CH4), with lesser amounts of CO2. The gas is considered to be mainly biogenic in origin. The methane and CO2 is absorbed onto the organic matter within the shale. Gas contents (methane) range upwards to 125 scf/ton. Water confined within the shales natural fracture system provides a hydrodynamic trap for gas reservoir. As a result, the wells also produce water, particularly during the initial years of production. As this water is produced off by the wells and the reservoir pressure lowered, the gas desorbs from the organic matter. Average gas in place across the trend is 11 BCF/square mile.
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