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Four generalized cave levels of the Trenton-Black River in south-central Michigan are most notable because of lost circulation zones in productive field areas. The levels are herein referred to as uppermost, upper middle, lower middle, and lowermost.
The uppermost cave level extends about 250 ft (76 m) into the Trenton. It is best developed in the widest, most productive areas of Albion-Scipio, where the drill bit dropped as much as 62 ft (19 m). About 59 productive wells in Albion-Scipio lost circulation at this level.
The upper middle level includes the lower, slightly argillaceous Trenton, and the uppermost Black River formation. This level is developed to some extent in all fields, and is a common zone of solution-collapse brecciation. Because this level is collapsed, lost circulation is less common.
The lower middle level below the "Black River shale" is not penetrated by many wells in Albion-Scipio. Where penetrated, it is developed in all fields, and is the main producer in most smaller fields. About 26 wells in Albion-Scipio lost circulation at this level, with the drill bit dropping as much as 80 ft (24 m).
The lowermost cave level extends into the Glenwood Shale. It is the most widely developed level, but is intersected by few productive holes. Only seven wells lost circulation at this level in Albion-Scipio, but the drill bit has dropped as much as 8 ft (2.4 m) in the Glenwood. This cave level effectively drained some oil from reservoirs where the oil-water contact intersects the Glenwood.
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