Another example of a stratigraphic trap developed in fractured secondary dolomites is the Adams N. Adams Field in Arenac County, MI. Adams lies slightly west of Deep River. Here the main reservoir is a diagenetic dolomite trending NE – SW, in contrast to the NW – SE trend observed at Deep River Field. In the accompanying figure, the dolomite trend (color) is again superimposed on the structure contours of the Dundee Formation (lines) and show the initial production (IP) for each well as a white circle. The diameter of each circle is proportional to the IP. As with Deep River, several deep "bulls eyes" occur in the field. The 3D diagram in the figure gives another perspective on the dolomitization pattern (deep red colors) and suggests that the dolomitization occurred along intersecting fractures. Two prominent dolomite lineations are apparent, but only one, the main NE - SW, trend was developed.
The main dolomite zone in the Adams Field is approximately 3 miles in length and strikes N 30 W. It is about the same width, ½ miles as Deep River but here the dolomite lies more on top of a structural anticline. The dolomite is finely crystalline and vuggy. The dolomite zone appears to be vertical and continues downward at far as the drill penetrated. Oil pay was encountered below the Bell shale. The early wells were drilled with cable tools and generally flowed oil on the first few feet of penetration into the dolomite.
The model for the dolomitization is the same as Deep River: hot (?) brines ascending from below along joints and fractures. The Dundee at Adams may have been subaerially exposed prior to the deposition of the Bell Shale and may have had a extensive karstic topography developed on it, as evidenced by the deep sinkholes (?) on the flanks of the field.
In general, N. Adams follows the same recipe as Deep River: a stratigraphic trap developed in porous, fractured secondary dolomite that cuts across structure. The main productive dolomite trend was developed at bout 90o to the trend at Deep River, which would be expected if the conjugate joint set was developed. These two reservoirs show that elongated zones of secondary dolomite probably produced along a common preexisting fractures/joint system were the main control on reservoir development.