ABSTRACT

Hydrocarbon production in the Michigan basin is essentially from pinnacle reefs or fractured reservoirs. The latter represents linear production zones which only recently have been demonstrated as related to shear faults and accompanying shear folds.

Dolomite distribution of the Middle Devonian Traverse, Dundee, and Detroit River producing formations in several linear fields were studied by X-ray diffraction of well samples at 20 to 60-ft (6 to 18 m) intervals. The epigenetically formed porous dolomite reservoir rock is intimately related to the shear faults (channelways for rising high Mg/Ca ratio fluids) and to the resulting shear folds, the latter showing dolomite/calcite ratios increasing generally from outer closure to the fold axes.

The Reed City field (anticline) of western Michigan represents a dramatic exception to this picture with the dolomite/calcite ratio increasing from outer closure to maximum part way up the limbs then decreasing to the axis. The Traverse Formation (highest stratigraphically) shows dedolomitization throughout 540 ft (165 m); the Dundee, throughout the entire 60 ft (18 m); and the Detroit River (the lowest target) in the upper 20 ft (6 m) but not in the lowest 30 ft (9 m). This lowest zone is the only unit not dedolomitized, a fact perhaps commensurate with its low stratigraphic position at the bottom of (and apparently beyond the reach of) the descending high-calcium, low-magnesium waters what brought about the dedolomitization.

A change from dolomite to calcite is witnessed by zonation of dolomite rhombs, which show on staining a ferroan dolomite core followed by successive layers of ferroan calcite and calcite in the outer layer. The individual calcite rhombs are usually pseudomorphic after the dolomite. Clear dolomite rhombs (dolomitizing stage) and calcite rhombs (dedolomitizing stage) often occur along microstylolites.

The presence of anhydrite and gypsum in this field likely supplied the CaSO4 considered important to the dedolomitization process with the general reaction: CaSO4 + CaMg (CO3)2 = 2CaCO3 + MgSO4. Iron found in the ferroan dolomite, ferroan calcite, and iron hydroxide in the dedolomitized rock probably came from pyrite, an important constituent of the original microcrystalline limestone country rock. The dedolomitization model would call for a shallow water to exposed oxidizing environment, possible with the position of this area astride the “West Michigan Barrier” that separates a lagoonal facies from a more open sea facies to the east. Thus, waters with a high Ca/Mg ratio passed down the same shear faults that earlier were channelways for the rising high Mg/Ca ratio waters.

On the bases of isopach, structure and dolomite/calcite (Isodol) maps, one can piece together a reasonably chronological sequence of pre-Dundee shear faulting and folding, post-Traverse upward migration of dolomitizing fluids, upward migration of hydrocarbons along the shear faults, downward-moving dedolomitizing fluids, and a later episode of faulting (especially shear cross-faults).

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