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The Middle Ordovician Trenton Limestone has produced over 350 million bbl of oil and an unestimable amount of gas from reservoirs in northwestern
Ohio since the 1880s. Owing to unchecked production methods and inadequate plugging procedures by the early drillers, sufficient reservoir pressures no longer exist in the once prolific Lima-Indiana fields.
In northwestern Ohio, the Trenton was deposited as primary limestone, ranging from mudstone to grainstone. However, most early production came from dolomitized zones within the upper part of the formation. Many past workers have attributed this dolomitization to an unconformity at the top of the Trenton. Current work indicates the dolomitization to be secondary, possibly caused by solution and precipitation by mobile, concentrated brines. Dolomitization is not restricted to the upper portion of the formation. In many wells, a thick limestone section caps porous dolomitized hydrocarbon zones. Along or near fractures or faults, it is not uncommon for the entire Trenton-Black River sequence to consist of dolomite. Much of the current exploration within the Trenton is being focused on loc ting such fractures.
In the northwest corner of the state, the Trenton is overlain by the gray Utica Shale. To the south and east, the Trenton is overlain by brown to black, highly calcareous shales commonly referred to as the Cynthiana formation. These two shales provide the seal for most Trenton reservoirs and are the probable source rock for the hydrocarbons.
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